OTTER TAIL CORP, 10-K filed on 2/19/2025
Annual Report
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Cover - USD ($)
 12 Months Ended
Dec. 31, 2024
Jan. 31, 2025
Jun. 30, 2024
Cover [Abstract]      
Document Type 10-K    
Document Annual Report true    
Document Period End Date Dec. 31, 2024    
Current Fiscal Year End Date --12-31    
Document Transition Report false    
Entity File Number 0-53713    
Entity Registrant Name OTTER TAIL CORPORATION    
Entity Incorporation, State or Country Code MN    
Entity Tax Identification Number 27-0383995    
Entity Address, Address Line One 215 South Cascade Street    
Entity Address, Address Line Two Box 496    
Entity Address, City or Town Fergus Falls    
Entity Address, State or Province MN    
Entity Address, Postal Zip Code 56538-0496    
City Area Code 866    
Local Phone Number 410-8780    
Title of 12(b) Security Common Shares, par value $5.00 per share    
Trading Symbol OTTR    
Security Exchange Name NASDAQ    
Entity Well-known Seasoned Issuer Yes    
Entity Voluntary Filers No    
Entity Current Reporting Status Yes    
Entity Interactive Data Current Yes    
Entity Filer Category Large Accelerated Filer    
Entity Small Business false    
Entity Emerging Growth Company false    
ICFR Auditor Attestation Flag true    
Document Financial Statement Error Correction false    
Entity Shell Company false    
Entity Public Float     $ 3,545,374,386
Entity Common Stock, Shares Outstanding   41,827,999  
Documents Incorporated by Reference
The Registrant's definitive Proxy Statement for its 2025 Annual Meeting of Shareholders is incorporated by reference into Part III of this Form 10-K.
    
Entity Central Index Key 0001466593    
Amendment Flag false    
Document Fiscal Period Focus FY    
Document Fiscal Year Focus 2024    
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Audit Information
 12 Months Ended
Dec. 31, 2024
Auditor Information [Abstract]  
Auditor Firm ID 34
Auditor Name Deloitte & Touche LLP
Auditor Location Minneapolis, Minnesota
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CONSOLIDATED BALANCE SHEETS - USD ($)
$ in Thousands
Dec. 31, 2024
Dec. 31, 2023
Current Assets    
Cash and Cash Equivalents $ 294,651 $ 230,373
Receivables, net of allowance for credit losses 145,964 157,143
Inventories 148,885 149,701
Regulatory Assets 9,962 16,127
Other Current Assets 30,579 16,826
Total Current Assets 630,041 570,170
Noncurrent Assets    
Investments 121,177 62,516
Property, Plant and Equipment, net of accumulated depreciation 2,692,460 2,418,375
Regulatory Assets 98,673 95,715
Intangible Assets, net of accumulated amortization 5,743 6,843
Goodwill 37,572 37,572
Other Noncurrent Assets 66,416 51,377
Total Noncurrent Assets 3,022,041 2,672,398
Total Assets 3,652,082 3,242,568
Current Liabilities    
Short-Term Debt 69,615 81,422
Accounts Payable 113,574 94,428
Accrued Salaries and Wages 34,398 38,134
Accrued Taxes 17,314 26,590
Regulatory Liabilities 29,307 25,408
Other Current Liabilities 45,582 43,775
Total Current Liabilities 309,790 309,757
Noncurrent Liabilities and Deferred Credits    
Pension Benefit Liability 32,614 33,101
Other Postretirement Benefits Liability 27,385 27,676
Regulatory Liabilities 288,928 276,547
Deferred Income Taxes 267,745 237,273
Deferred Tax Credits 14,990 15,172
Other Noncurrent Liabilities 98,397 75,977
Total Noncurrent Liabilities and Deferred Credits 730,059 665,746
Commitments and Contingencies (Note 14)
Capitalization    
Long-Term Debt 943,734 824,059
Shareholders' Equity    
Common Stock: 50,000,000 shares authorized of $5 par value; 41,827,967 and 41,710,521 outstanding at December 31, 2024 and 2023 209,140 208,553
Additional Paid-In Capital 429,089 426,963
Retained Earnings 1,029,738 806,342
Accumulated Other Comprehensive Income 532 1,148
Total Shareholders' Equity 1,668,499 1,443,006
Total Capitalization 2,612,233 2,267,065
Total Liabilities and Shareholders' Equity $ 3,652,082 $ 3,242,568
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CONSOLIDATED BALANCE SHEETS (Parentheticals) - $ / shares
Dec. 31, 2024
Dec. 31, 2023
Statement of Financial Position [Abstract]    
Common stock, shares authorized (in shares) 50,000,000 50,000,000
Common stock, par value (in dollars per share) $ 5 $ 5
Common stock, shares outstanding (in shares) 41,827,967 41,710,521
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CONSOLIDATED STATEMENTS OF INCOME - USD ($)
shares in Thousands, $ in Thousands
 12 Months Ended
Dec. 31, 2024
Dec. 31, 2023
Dec. 31, 2022
Operating Revenues      
Total Operating Revenues $ 1,330,548 $ 1,349,166 $ 1,460,209
Operating Expenses      
Electric Production Fuel 60,945 60,339 65,110
Electric Purchased Power 61,561 78,292 100,281
Electric Operating and Maintenance Expenses 190,422 191,263 181,378
Cost of Products Sold (excluding depreciation) 434,522 454,122 542,944
Nonelectric Selling, General, and Administrative Expenses 80,065 72,663 69,718
Depreciation and Amortization 107,121 97,954 92,597
Electric Property Taxes 15,662 16,614 17,742
Total Operating Expenses 950,298 971,247 1,069,770
Operating Income 380,250 377,919 390,439
Other Income and (Expense)      
Interest Expense (41,815) (37,677) (36,016)
Nonservice Cost Components of Postretirement Benefits 9,609 10,597 1,075
Other Income (Expense), net 18,848 12,650 2,037
Income Before Income Taxes 366,892 363,489 357,535
Income Tax Expense 65,230 69,298 73,351
Net Income $ 301,662 $ 294,191 $ 284,184
Weighted-Average Common Shares Outstanding:      
Basic (in shares) 41,778 41,668 41,586
Diluted (in shares) 42,072 42,039 41,931
Earnings Per Share:      
Basic (in dollars per share) $ 7.22 $ 7.06 $ 6.83
Diluted (in dollars per share) $ 7.17 $ 7.00 $ 6.78
Electric      
Operating Revenues      
Total Operating Revenues $ 524,515 $ 528,359 $ 549,699
Product Sales      
Operating Revenues      
Total Operating Revenues $ 806,033 $ 820,807 $ 910,510
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CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2024
Dec. 31, 2023
Dec. 31, 2022
Statement of Comprehensive Income [Abstract]      
Net Income $ 301,662 $ 294,191 $ 284,184
Other Comprehensive Income (Loss):      
Unrealized Gain (Loss) on Available-for-Sale Securities, net of tax (expense) benefit of $(128), $(51) and $115 386 192 (432)
Unrealized Gain (Loss) on Pension and Other Postretirement Benefit Plans, net of tax (expense) benefit of $352, $(14) and $(2,769) (1,002) 41 7,871
Total Other Comprehensive Income (Loss) (616) 233 7,439
Total Comprehensive Income $ 301,046 $ 294,424 $ 291,623
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CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME (Parentheticals) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2024
Dec. 31, 2023
Dec. 31, 2022
Statement of Comprehensive Income [Abstract]      
Unrealized Gain (Loss) on Available-for-Sale Securities, tax (expense) benefit $ (128) $ (51) $ 115
Unrealized Gain (Loss) on Pension and Other Postretirement Benefit Plans, tax (expense) benefit $ 352 $ (14) $ (2,769)
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CONSOLIDATED STATEMENTS OF SHAREHOLDERS' EQUITY - USD ($)
$ in Thousands
Total
Common Shares
Additional Paid-In Capital
Retained Earnings
Accumulated Other Comprehensive Income (Loss)
Beginning balance (in shares) at Dec. 31, 2021   41,551,524      
Beginning balance at Dec. 31, 2021 $ 990,777 $ 207,758 $ 419,760 $ 369,783 $ (6,524)
Increase (Decrease) in Stockholders' Equity [Roll Forward]          
Employee Stock Purchase Plan Expenses (219)   (219)    
Stock Issued Under Share-Based Compensation Plans, Net of Shares Withheld for Employee Taxes (in shares)   79,589      
Stock Issued Under Share-Based Compensation Plans, Net of Shares Withheld for Employee Taxes (2,923) $ 398 (3,321)    
Stock Compensation Expense 6,814   6,814    
Net Income 284,184     284,184  
Other Comprehensive Income (Loss) 7,439       7,439
Common Dividends (68,755)     (68,755)  
Ending balance (in shares) at Dec. 31, 2022   41,631,113      
Ending balance at Dec. 31, 2022 1,217,317 $ 208,156 423,034 585,212 915
Increase (Decrease) in Stockholders' Equity [Roll Forward]          
Employee Stock Purchase Plan Expenses (339)   (339)    
Stock Issued Under Share-Based Compensation Plans, Net of Shares Withheld for Employee Taxes (in shares)   79,408      
Stock Issued Under Share-Based Compensation Plans, Net of Shares Withheld for Employee Taxes (3,088) $ 397 (3,485)    
Stock Compensation Expense 7,753   7,753    
Net Income 294,191     294,191  
Other Comprehensive Income (Loss) 233       233
Common Dividends $ (73,061)     (73,061)  
Ending balance (in shares) at Dec. 31, 2023 41,710,521 41,710,521      
Ending balance at Dec. 31, 2023 $ 1,443,006 $ 208,553 426,963 806,342 1,148
Increase (Decrease) in Stockholders' Equity [Roll Forward]          
Employee Stock Purchase Plan Expenses (359)   (359)    
Stock Issued Under Share-Based Compensation Plans, Net of Shares Withheld for Employee Taxes (in shares)   117,446      
Stock Issued Under Share-Based Compensation Plans, Net of Shares Withheld for Employee Taxes (6,457) $ 587 (7,044)    
Stock Compensation Expense 9,529   9,529    
Net Income 301,662     301,662  
Other Comprehensive Income (Loss) (616)       (616)
Common Dividends $ (78,266)     (78,266)  
Ending balance (in shares) at Dec. 31, 2024 41,827,967 41,827,967      
Ending balance at Dec. 31, 2024 $ 1,668,499 $ 209,140 $ 429,089 $ 1,029,738 $ 532
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CONSOLIDATED STATEMENTS OF SHAREHOLDERS' EQUITY (Parentheticals) - $ / shares
 12 Months Ended
Dec. 31, 2024
Dec. 31, 2023
Dec. 31, 2022
Statement of Stockholders' Equity [Abstract]      
Common Dividends (in dollars per share) $ 1.87 $ 1.75 $ 1.65
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CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2024
Dec. 31, 2023
Dec. 31, 2022
Operating Activities      
Net Income $ 301,662 $ 294,191 $ 284,184
Adjustments to Reconcile Net Income to Net Cash Provided by Operating Activities:      
Depreciation and Amortization 107,121 97,954 92,597
Deferred Tax Credits (182) (744) (745)
Deferred Income Taxes 23,057 13,508 32,424
Discretionary Contribution to Pension Plan 0 0 (20,000)
Investment (Gains) Losses (5,482) (7,222) 3,296
Stock Compensation Expense 9,529 7,753 6,814
Other, net (3,111) (423) (1,473)
Changes in Operating Assets and Liabilities:      
Receivables 11,179 (12,750) 30,560
Inventories 3,691 (2,450) 5,339
Regulatory Assets 5,194 12,479 (2,464)
Other Assets (11,640) 2,817 (368)
Accounts Payable 14,826 (9,988) (29,763)
Accrued and Other Liabilities (10,371) 6 (5,490)
Regulatory Liabilities 16,821 20,973 (6,846)
Pension and Other Postretirement Benefits (9,563) (11,605) 1,244
Net Cash Provided by Operating Activities 452,731 404,499 389,309
Investing Activities      
Capital Expenditures (358,650) (287,134) (171,134)
Proceeds from Disposal of Noncurrent Assets 8,849 6,225 4,346
Purchases of Investments and Other Assets (61,573) (8,378) (8,283)
Net Cash Used in Investing Activities (411,374) (289,287) (175,071)
Financing Activities      
Net (Repayments) Borrowings on Short-Term Debt (11,807) 73,218 (82,959)
Proceeds from Issuance of Long-Term Debt 120,000 0 90,000
Payments for Retirement of Long-Term Debt 0 0 (30,000)
Dividends Paid (78,266) (73,061) (68,755)
Payments for Shares Withheld for Employee Tax Obligations (6,457) (3,088) (2,942)
Other, net (549) (904) (2,123)
Net Cash Provided by (Used in) Financing Activities 22,921 (3,835) (96,779)
Net Change in Cash and Cash Equivalents 64,278 111,377 117,459
Cash and Cash Equivalents at Beginning of Period 230,373 118,996 1,537
Cash and Cash Equivalents at End of Period 294,651 230,373 118,996
Supplemental Disclosures of Cash Flow Information      
Interest, net of amount capitalized 39,484 36,956 35,699
Income Taxes 57,614 46,284 43,411
Supplemental Disclosure of Noncash Investing Activities      
Accrued Property, Plant and Equipment Additions $ 20,281 $ 13,001 $ 12,420
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Summary of Significant Accounting Policies
 12 Months Ended
Dec. 31, 2024
Accounting Policies [Abstract]  
Summary of Significant Accounting Policies Summary of Significant Accounting Policies
Overview
Otter Tail Corporation (OTC) and its subsidiaries (collectively, the "Company," "us," "our" or "we") form a diverse, multi-platform business consisting of a vertically integrated, regulated utility with generation, transmission and distribution facilities complemented by manufacturing businesses providing metal fabrication for custom machine parts and metal components, manufacturing of extruded and thermoformed plastic products, and manufacturing of PVC pipe products. We classify our business into three segments: Electric, Manufacturing and Plastics. Note 2 includes an additional description of the segments and financial information regarding each segment.
Principles of Consolidation
These consolidated financial statements are presented in accordance with U.S. generally accepted accounting principles and include the accounts of OTC and its wholly owned subsidiaries. All intercompany balances and transactions have been eliminated in consolidation except, as applicable, profits on sales to our regulated electric utility company from our nonregulated businesses, which is in accordance with the accounting requirements of regulated operations.
Use of Estimates
We use estimates based on the best information available in recording transactions and balances resulting from business operations. As better information becomes available or actual amounts are known, the recorded estimates are revised. Consequently, operating results can be affected by revisions to prior accounting estimates.
Regulatory Accounting
Our regulated electric utility company, OTP, is subject to regulation of rates and other matters by state utility commissions in Minnesota, North Dakota and South Dakota and by the FERC for certain interstate operations. OTP accounts for the financial effects of regulation in accordance with accounting guidance for regulated operations. This guidance allows for the recording of a regulatory asset for certain costs which otherwise would be recognized in the statements of income or comprehensive income based on an expectation that the cost will be recovered in future rates. This guidance also requires the recording of a regulatory liability for certain credits which would otherwise be recognized in the statements of income or comprehensive income based on an expectation that the amount will be returned to customers in future rates. Amounts recorded as regulatory assets and regulatory liabilities are generally recognized in the statements of income at the time they are reflected in customer rates. In the event OTP ceases to meet the criteria to apply the guidance for regulated operations, the regulatory assets and liabilities that no longer meet such criteria would be removed from the consolidated balance sheets and included in the consolidated statements of income as an expense or income item, or in the consolidated statements of comprehensive income as a loss or gain item, in the period in which the application of this guidance ceases.
Cash Equivalents
We consider all highly liquid investments purchased with maturity dates of 90 days or less to be cash equivalents.
Concentration of Deposits
We hold deposits with financial institutions which potentially subject us to a concentration risk. These deposits are guaranteed by the Federal Deposit Insurance Corporation up to an insurance limit of $250,000. Currently, our cash deposits exceed federally insured levels.
Revenue from Contracts with Customers
Due to our diverse business operations, the recognition of revenue from contracts with customers depends on the product produced and sold or service performed. We recognize revenue from contracts with customers at prices that are fixed or determinable as evidenced by an agreement with the customer, when we have met our performance obligation under the contract and it is probable that we will collect the amount to which we are entitled in exchange for the goods or services transferred or to be transferred to the customer. Depending on the product produced and sold or service performed and the terms of the agreement with the customer, we recognize revenue either over time, in the case of delivery or transmission of electricity or related services or the production and storage of certain custom-made products, or at a point in time for the delivery of standardized products and other products made to customer specifications where the terms of the contract require transfer of the completed product. Provisions for sales returns, early payment discounts and volume-based variable pricing incentives are recorded as reductions to revenue at the time revenue is recognized based on customer history, historical information and current trends. We include revenues received for shipping and handling in operating revenues. Expenses paid for shipping and handling are recorded as part of cost of products sold. Sales or other taxes collected from customers are excluded from operating revenues.
Electric Segment Revenues. Most Electric segment revenues are earned from the generation, transmission and sale of electricity to retail customers at rates approved by state regulatory commissions. OTP also earns revenue from the transmission of electricity for others over the transmission assets it owns separately or jointly with other transmission service providers, under rate tariffs established by the independent transmission system operator and approved by FERC. A third source of revenue for OTP comes from the generation and sale of electricity to wholesale customers at contract or market rates. Revenues from all these sources meet the criteria to be classified as revenue from contracts with customers and are recognized over time as energy is delivered or transmitted. Revenue is recognized based on the metered quantity of electricity delivered or transmitted at the applicable rates. For electricity delivered and consumed after a meter is read but not yet billed to a customer, OTP records revenue and an unbilled receivable based on estimates of the amount of energy delivered and a composite rate per kwh consumed.
Manufacturing Segment Revenues. Our Manufacturing segment businesses earn revenue predominantly from the production and delivery of custom-made or standardized parts and products to customers across several industries and from the production and sale of tools and dies to other manufacturers. For the production and delivery of standardized products and other products made to customer specifications where the terms of the contract require transfer of the completed product, we have met our performance obligation and recognize revenue at the point in time when the product is shipped. At this point we have no further obligation to provide services related to such products. The shipping terms used in these transactions are free on board (FOB) shipping point.
Plastics Segment Revenues. Our Plastics segment businesses earn revenue predominantly from the sale and delivery of standardized PVC pipe products produced at their manufacturing facilities. Revenue from the sale of these products is recognized at the point in time when the product is shipped as there is no further obligation to provide services related to such products and the shipping terms are FOB shipping point. We have one customer within our Plastics segment for which we produce and store a product made to the customer’s specifications and design under a build and hold agreement. For sales to this customer, we recognize revenue as the custom-made product is produced, adjusting the amount of revenue for volume rebate variable pricing considerations we expect the customer will earn and applicable early payment discounts we expect the customer will take. Ownership of the pipe transfers to the customer prior to delivery and we are paid a negotiated fee for storage of the pipe. Revenue for storage of the pipe is recognized over time as the pipe is stored.
Alternative Revenue
In addition to recognizing revenue from contracts with customers, our Electric segment business also records revenue under alternative revenue program (ARP) requirements. Certain rate rider mechanisms qualify as ARP revenues as they provide for adjustments to rates outside of a general rate case proceeding to encourage or incentivize investments in certain areas such as conservation, renewable energy, pollution reduction or control, improved infrastructure of the transmission grid or other programs that provide benefits to the general public under public policy, laws or regulations. ARP riders generally provide for the recovery of specified costs and investments and include an incentive component to provide the regulated utility with a return on amounts invested.
We accrue ARP revenue on the basis of costs incurred, investments made and returns on those investments that qualify for recovery through established riders. ARP revenue is disclosed separately from revenue from contracts with customers and we have elected to report ARP revenue on a net basis, whereby amounts initially recorded as ARP revenue in a period are presented net of the reversal of amounts previously recognized as ARP revenue that are reclassified and recorded as revenue from contracts with customers when such amounts are included in the price of electricity to customers.
Receivables and Allowance for Credit Losses
We grant credit to our customers in the normal course of business with repayment terms generally ranging from 30 to 90 days after the invoice date. Late fees are assessed on certain receivables once they are 30 days past due. Unbilled receivables represent estimates of energy delivered to customers but not yet billed.
Receivables are stated at the billed or estimated unbilled amount less an allowance for estimated credit losses. An allowance for credit losses is established based on losses expected to occur over the contractual life of the receivable. We estimate an allowance for credit losses on our trade and unbilled receivables by evaluating historical aging and write-off history, adjusted for current and forecasted economic conditions, for groups of receivables that share similar economic characteristics. Other receivables are evaluated by reviewing individual accounts, considering aging, financial condition of the debtor, recent payment history and other relevant factors. Account balances are written off in the period they are deemed to be uncollectible.
Inventories
Inventories are valued at the lower of cost or net realizable value. Costs for fuel, material and supply inventories of our Electric segment are determined on an average cost basis. Costs for raw material, work in process and finished goods inventories of our Manufacturing and Plastics segments are determined on a first-in first-out (FIFO) basis.
Inventories consist of the following as of December 31, 2024 and 2023:
(in thousands)20242023
Finished Goods$43,345 $47,614 
Work in Process22,637 26,354 
Raw Material, Fuel and Supplies82,903 75,733 
Total Inventories$148,885 $149,701 
Investments
We invest in and hold, through rabbi trusts, corporate-owned life insurance policies to provide future funding for obligations under our supplemental pension plan and a nonqualified deferred compensation plan. The polices are recorded at cash surrender value and there are no restrictions on our ability to surrender the policies.
We hold debt, mutual fund, and money market fund investments either as investments within our captive insurance entity, to provide future funding for obligations under nonqualified deferred compensation plans or provide a return on our available cash and liquidity. These investments are recorded at fair value. Debt securities are deemed to be available-for-sale securities, accordingly unrealized gains and losses are generally excluded from earnings and recognized in accumulated other comprehensive income. We evaluate whether declines in the fair value of debt securities below the cost basis are other-than-temporary. Declines in fair value deemed to be other-than-temporary result in the recognition of unrealized losses, or a portion thereof, in earnings. Unrealized gains and losses on mutual and money market funds are recognized in earnings immediately.
Property, Plant and Equipment
Electric plant is stated at original cost less accumulated depreciation. The cost of additions includes purchased assets, contracted work, direct labor and materials, allocable overheads and allowance for funds used during construction (AFUDC). The amount of interest capitalized to electric plant was $1.9 million in 2024, $1.9 million in 2023 and $0.9 million in 2022. Significant additions or improvements that extend an asset's useful life are capitalized, while repairs and maintenance costs are expensed as incurred.
Depreciation is recognized on a straight-line basis over the asset's estimated useful life. For certain asset classes, we employ a group or composite method of depreciation in which certain assets are combined and depreciated over the average life of the combined asset group. Actuarial studies are periodically performed to assess the remaining useful lives and salvage values of our assets, with any changes in these estimates incorporated into depreciation on a prospective basis. Gains or losses on group or composite asset dispositions are recorded to accumulated depreciation and impact current and future depreciation rates.
Amounts recovered in rates for future removal costs are recorded as regulatory liabilities. Removal costs, when incurred, are charged against the regulatory liability.
Property, plant and equipment of our nonelectric operations are carried at historical cost less accumulated depreciation. Depreciation is recognized on a straight-line basis over the asset's estimated useful life. The cost of additions includes purchased assets, contracted work, direct labor and materials, allocable overheads and capitalized interest, as applicable. No interest was capitalized in 2024, 2023 or 2022. Maintenance and repairs are expensed as incurred. Gains or losses on asset dispositions are included in the determination of operating income.
The estimated service lives for rate-regulated electric assets and nonelectric assets are included below:
 Service Life Range
(years)LowHigh
Electric Assets:  
Production Plant21114
Transmission Plant5180
Distribution Plant1072
General Plant556
Nonelectric Assets:
Equipment220
Buildings and Leasehold Improvements240
Jointly Owned Facilities
OTP is a joint owner in two coal-fired steam-powered electric generation plants: Big Stone Plant near Big Stone City, South Dakota and Coyote Station near Beulah, North Dakota. OTP is also a joint owner, with other regional utilities, in several major transmission lines. OTP's interest in each jointly owned facility is reflected in the consolidated balance sheets on a pro-rata basis and OTP's share
of direct revenue and expenses are included in operating revenues and expenses in the consolidated statements of income. Each participant in the jointly owned facilities finances their own investments.
Goodwill and Other Intangible Assets
Goodwill is recognized and initially measured as any excess of the acquisition-date consideration transferred in a business combination over amounts recognized for the net identifiable assets acquired. Goodwill is not amortized, but is tested for impairment annually, or more frequently if an event occurs or circumstances change that would more likely than not result in an impairment of goodwill. Impairment testing is performed at the reporting unit level, which is defined as an operating segment or one level below an operating segment. We perform our impairment testing in the fourth quarter of each year and have identified three reporting units that carry a goodwill balance. We historically tested goodwill for impairment as of December 31st each year; however, in 2024, we elected to change the date of our annual goodwill impairment test to October 1st. We believe this new testing date allows us to better align our annual goodwill impairment testing procedures with our year-end financial reporting, as well as our annual budgeting and forecasting process. This change did not delay, accelerate or avoid the recognition of an impairment charge.
We perform a quantitative impairment assessment, electing to forgo the optional qualitative assessment. The quantitative assessment is a single-step test that identifies both the existence of impairment and the amount of impairment loss by comparing the estimated fair value of a reporting unit to its carrying value, with any excess carrying value over the fair value being recognized as an impairment loss.
Intangible assets with finite lives, which primarily consist of customer relationships, are carried at estimated fair value at the time of acquisition less accumulated amortization. The costs of the intangible assets are amortized over their estimated useful lives, which generally range from 15 to 20 years.
Cloud Computing Costs
We capitalize implementation costs incurred in cloud computing arrangements that are service contracts consistent with capitalized implementation costs incurred to develop or obtain internal-use software. Costs are amortized on a straight-line basis over the life of the associated contract. Capitalized implementation costs are amortized over periods up to ten years. Capitalized costs and related accumulated amortization are included in other noncurrent assets on the consolidated balance sheets. Below are the amounts of capitalized cost and related accumulated amortization as of December 31, 2024 and 2023:
(in thousands)20242023
Cloud Computing Costs$15,741 $12,782 
Accumulated Amortization(3,796)(1,505)
Cloud Computing Costs, net$11,945 $11,277 
Amortization expense of capitalized implementation costs for each of the years ended December 31, 2024, 2023 and 2022 totaled $3.0 million, $1.3 million, and $1.4 million.
Leases
We recognize a right-of-use lease asset and a corresponding lease liability at the lease commencement date. The length of our lease agreements varies from less than one year to approximately ten years. We have elected to not record lease assets and liabilities for leases with a lease term at commencement of 12 months or less; such leases are expensed on a straight-line basis over the lease term. Certain of our leases contain options to renew or extend the lease term at our discretion if certain conditions are met. If a lease contains an option to extend the lease term and there is reasonable certainty the option will be exercised, the option is considered in the lease term at inception, or at such time when an event occurs which triggers the remeasurement of a lease, as applicable. In the determination of the lease term for one of our leased manufacturing facilities, we have incorporated the future lease renewals which we believe are reasonably certain to be exercised in the associated right-of-use asset and liability values.
We have elected to not separate non-lease components (e.g., common area maintenance) from lease components on real estate leases, accordingly the recognized lease asset and lease liability incorporate in their measurement payments for non-lease components. Certain leases include variable lease payments as the amounts are subject to change over the lease term; such amounts are not incorporated into the measurement of the right-of-use lease asset or lease liability. We are unable to determine the interest rate implicit in our leases, thus we apply our incremental borrowing rate to capitalize the right-of-use asset and lease liability. We estimate our incremental borrowing rate by reference to market interest rates on long-term debt, incorporating considerations of the credit quality of the lessee and the term of lease.
Recoverability of Long-Lived Assets
We review our long-lived assets including, among other assets, property, plant and equipment, amortizing intangible assets and right-of-use lease assets whenever events or changes in circumstances indicate the carrying amount of the assets may not be
recoverable. We determine potential impairment by comparing the carrying amount of the assets with the net cash flows expected to be provided by operating activities of the business or related assets. If the sum of the expected future net cash flows is less than the carrying amount of the assets, an impairment loss would be recognized. Such an impairment loss would be measured as the amount by which the carrying amount exceeds the fair value of the asset.
Pension Plans and Other Postretirement Benefits
We maintain pension and postretirement benefit plans for eligible employees. Recognizing the cost of providing benefits and measuring the projected benefit obligation of these plans requires management to make various assumptions and estimates. Certain unrecognized actuarial gains and losses and unrecognized prior service costs or credits are deferred as regulatory assets and liabilities, rather than recorded as other comprehensive income, based on regulatory recovery mechanisms.
We have elected to apply a minimum amortization method for determining the amount of amortization of net cumulative gains or losses to be included as a component of net periodic benefit cost for any annual period. Cumulative gains and losses recognized in accumulated other comprehensive income or as a deferred regulatory asset or liability that are in excess of 10% of the projected benefit obligation or the market value of pension plan assets are amortized over the expected remaining future service period of active plan participants. In periods in which the cumulative gains and losses do not exceed 10%, no amortization to net period benefit cost is recognized.
Asset Retirement Obligations
Legal obligations related to the future retirement of long-lived assets are recognized as asset retirement obligations (ARO). An ARO is recognized in the period in which the legal obligation is incurred and the amount of the obligation can be reasonably estimated, with an offsetting increase to the associated long-lived asset. AROs are initially recognized at fair value and increased with the passage of time (accretion). ARO estimates are revised periodically with any adjustments reflected in the ARO and associated long-lived asset.
Income Taxes
We use the asset and liability method to account for income taxes. Under this method, deferred tax assets and liabilities are recognized for the expected future tax consequences of all temporary differences between the carrying amounts of assets and liabilities and their respective tax bases. Deferred taxes are recorded using the tax rates scheduled by tax law to be in effect in the periods when the temporary differences reverse. Deferred tax assets are reduced by a valuation allowance when it is more likely than not that a portion or all of the deferred tax assets will not be realized. The realizability of deferred tax assets is determined by taking into consideration forecasts of future taxable income, the reversal of other existing temporary differences, available net operating loss carryforwards and available tax planning strategies. Changes in valuation allowances are included in the provision for income taxes in the period of the changes.
We recognize the tax effects of all tax positions that are more-likely-than-not to be sustained on audit based solely on the technical merits of those positions as of the balance sheet date. Changes in the recognition or measurement of such positions are recognized in the provision for income taxes in the period of the changes. We classify interest and penalties on tax uncertainties as components of the provision for income taxes within the consolidated statements of income.
We have elected to account for transferable tax credits as a component of our income tax provision. We recognize the benefit of PTCs as a reduction of income tax expense in the period the credit is generated, which corresponds to the period the energy production occurs. We apply the deferral method of accounting for ITCs and state wind energy credits. Under this method, ITCs and state wind energy credits are amortized as a reduction to income tax expense over the estimated useful lives of the underlying property that gave rise to the credit.
Deferred Compensation Plans
The Company sponsors two nonqualified deferred compensation plans for the benefit of executive officers and other select employees. Each plan allows participants to defer a specified amount or percentage of base wages or incentive compensation into the plan, subject to certain limitations. The Company, at its discretion, may make employer contributions to either plan during any annual period. Participant and employer deferred amounts are segregated into one or more accounts chosen by the participant. Participants earn a return on deferred amounts based on notional investments in the segregated accounts. Participants can elect lump sum distributions or annual installments of deferred balances during the participant's employment or upon retirement. As of December 31, 2024 and 2023, our liability to participants under these deferred compensation plans was $29.1 million and $24.6 million. Company contributions to these plans were $1.3 million, $1.2 million and $0.9 million for the years ended December 31, 2024, 2023 and 2022. Gains or (losses) recognized due to changes in our payment obligations in connection with these plans amounted to ($3.3 million), ($3.3 million) and $3.1 million for the years ended December 31, 2024, 2023 and 2022.
Stock-Based Compensation
Stock-based compensation awards are measured at the grant-date fair value of the award and compensation expense is recognized on a straight-line basis over the applicable service or performance period. The service period may be limited to the period until such
time that a recipient is retirement eligible as determined under the award agreement. Awards granted to employees eligible for retirement on the date of grant are expensed in the period of grant. We recognize the effects of award forfeitures as they occur.
Fair Value Measurements
Fair value is defined as the price that would be received for an asset or paid to transfer a liability (an exit price) in the principal or most advantageous market for the asset or liability in an orderly transaction between market participants. Three levels of inputs may be used to measure fair value:
Level 1 – Quoted prices are available in active markets for identical assets or liabilities as of the reported date. The types of assets and liabilities included in Level 1 are highly liquid and actively traded instruments with quoted prices, such as equities listed on the New York Stock Exchange and commodity derivative contracts listed on the New York Mercantile Exchange.
Level 2 – Pricing inputs are other than quoted prices in active markets but are either directly or indirectly observable as of the reported date. The types of assets and liabilities included in Level 2 are typically either comparable to actively traded securities or contracts, such as treasury securities with pricing interpolated from recent trades of similar securities, or priced with models using highly observable inputs, such as commodity options priced using observable forward prices and volatilities. 
Level 3 – Significant inputs to pricing have little or no observability as of the reporting date. The types of assets and liabilities included in Level 3 are those with inputs requiring significant management judgment or estimation and may include complex and subjective models and forecasts.
In instances where the determination of the fair value measurement is based on inputs from different levels within the hierarchy, the level in the hierarchy within which the entire fair value measurement falls is based on the lowest level input that is significant to the fair value measurement in its entirety.
Related Parties
The Otter Tail Corporation Foundation and Otter Tail Power Company Foundation are independent not-for-profit charitable entities affiliated with the Company and are not included in OTC's consolidated financial statements. Contribution obligations to the two foundations totaling $5.5 million and $5.5 million were recognized as of December 31, 2024 and 2023. Cash contributions paid to the two foundations during the years ended December 31, 2024, 2023 and 2022 were $5.5 million, $4.3 million, and $4.5 million.
Variable Interest Entity
In October 2012, the Coyote Station owners, including OTP, entered into an LSA with Coyote Creek Mining Company, LLC (CCMC), a subsidiary of The North American Coal Corporation, for the purchase of lignite coal to meet the coal supply requirements of Coyote Station for the period beginning in May 2016 and ending in December 2040. The price per ton paid by the Coyote Station owners under the LSA reflects the cost of production, along with an agreed upon profit and capital charge. CCMC was formed for the purpose of mining coal to meet the coal fuel supply requirements of Coyote Station from May 2016 through December 2040 and, based on the terms of the LSA, is considered a variable interest entity (VIE) due to the transfer of all operating and economic risk to the Coyote Station owners, as the agreement is structured so that the price of the coal would cover all costs of operations as well as future reclamation costs. The Coyote Station owners are required to buy certain assets of CCMC at book value should they terminate the contract prior to the end of the contract term and are providing a guarantee of the value of the equity of CCMC because the Coyote Station owners are required to buy the membership interests of CCMC at the end of the contract term at equity value. Under current accounting standards, the primary beneficiary of a VIE is required to include the assets, liabilities, results of operations and cash flows of the VIE in its consolidated financial statements. No single owner of Coyote Station owns a majority interest in Coyote Station and none, individually, has the power to direct the activities that most significantly impact CCMC. Therefore, none of the owners individually, including OTP, is considered the primary beneficiary of the VIE and the Company is not required to include CCMC in its consolidated financial statements.
If the LSA terminates prior to the expiration of its term or the production period terminates prior to December 31, 2040 and the Coyote Station owners purchase all of the outstanding membership interests of CCMC, the owners will satisfy or, if permitted by CCMC’s applicable lenders, assume all of CCMC’s obligations owed to CCMC’s lenders under its loans and leases. The Coyote Station owners have limited rights to assign their rights and obligations under the LSA without the consent of CCMC’s lenders during any period in which CCMC’s obligations to its lenders remain outstanding. In the event the contract is terminated prior to the end of the term due to certain events, OTP’s maximum loss exposure, as a result of its involvement with CCMC, could be as high as $40 million, or OTP’s 35% share of CCMC’s unrecovered costs as of December 31, 2024, if recovery of such a loss is denied by regulatory authorities.
Recently Adopted Accounting Pronouncements
Segment Reporting. In November 2023, the Financial Accounting Standards Board (FASB) issued amended authoritative guidance codified in Accounting Standards Codification (ASC) 280, Segment Reporting. The amended guidance expands annual and interim disclosure requirements for reportable segments, primarily through expanded disclosures about significant segment expenses. We adopted this updated standard in the 2024 annual period on a retrospective basis, as required by the updated
standard. The adoption of this updated standard resulted in additional disclosures related to our reportable segments and did not have an impact on our consolidated financial position or operating results.
Recent Accounting Pronouncements
Income Taxes. In December 2023, the FASB issued amended authoritative guidance codified in ASC 740, Income Taxes. The amended guidance requires additional disaggregated information in effective tax rate reconciliation disclosures and additional disaggregated information about income taxes paid. The updated standard is effective for our annual periods beginning in 2025. The amended guidance is to be applied on a prospective basis with the option to apply the standard retrospectively. We anticipate adopting the updated standard in our Form 10-K for the year ended December 31, 2025 and electing to apply the standard on a retrospective basis for all periods presented.
Disaggregated Income Statement Expenses. In November 2024, the FASB issued authoritative guidance codified in ASC 220, Income Statement—Reporting Comprehensive Income, which will require additional disclosure of certain costs and expenses within the notes to the financial statements. The updated standard is effective for our annual periods beginning in 2027 and interim periods beginning in the first quarter of fiscal 2028 and can be applied on either a prospective or retrospective basis. Early adoption is permitted. We are currently evaluating the impact that the updated standard will have on our financial statement disclosures.
v3.25.0.1
Segment Information
 12 Months Ended
Dec. 31, 2024
Segment Reporting [Abstract]  
Segment Information Segment Information
Our business is comprised of three reportable segments, Electric, Manufacturing and Plastics, consistent with our business strategy, organizational structure and our internal reporting and review processes. Our chief operating decision maker (CODM) is our Chief Executive Officer. Segment net income is the sole measure of segment profit or loss used by our CODM in assessing segment performance and allocating resources to our segments. Our CODM uses segment net income in assessing financial performance on a monthly basis, reviewing and approving annual operating budgets and periodic forecasts, allocating capital or financial resources to our segments, making strategic decisions and measuring returns on equity in comparison to internal thresholds or peer entities.
The operations of our three reportable segments are further described below. We have aggregated two operating segments within our Manufacturing reportable segment based on the similarity between these businesses and their economic characteristics.
Electric includes the production, transmission, distribution and sale of electric energy in Minnesota, North Dakota and South Dakota by OTP. In addition, OTP is a participant in the MISO markets. OTP’s operations have been our primary business since 1907.
Manufacturing consists of businesses in the following manufacturing activities: contract machining, metal parts stamping, fabrication and painting, and production of plastic thermoformed horticultural containers, life science and industrial packaging, and material handling components. These businesses have manufacturing facilities in Georgia, Illinois and Minnesota and sell products primarily in the United States.
Plastics consists of businesses producing PVC pipe at plants in North Dakota and Arizona. The PVC pipe is sold primarily in the western half of the United States and Canada.
Segment Profit or Loss
Information about each segment, including significant expenses and net income of each segment, for the years ended December 31, 2024, 2023 and 2022 are as follows:
Electric Segment
(in thousands)202420232022
Operating Revenue$524,515 $528,359 $549,699 
Production Fuel and Purchased Power122,506 138,631 165,391 
Operating and Maintenance Expenses190,422 191,263 181,378 
Depreciation and Amortization82,136 75,330 72,050 
Property Taxes15,662 16,614 17,742 
Interest Expense38,216 33,864 31,950 
Income Tax Expense (Benefit)
(1,544)1,648 5,065 
Other Segment Items(1)
(13,846)(13,415)(3,851)
Net Income$90,963 $84,424 $79,974 
(1) Other segment items includes nonservice components of postretirement benefits, allowance for funds used during construction, and other expenses (income).
Manufacturing Segment
(in thousands)202420232022
Operating Revenue$342,592 $402,781 $397,983 
Cost of Goods Sold283,390 324,245 327,228 
Selling, General, and Administrative Expenses40,110 49,396 41,690 
Interest Expense2,516 2,295 2,796 
Income Tax Expense2,895 5,390 5,321 
Other Segment Items— 1 (2)
Net Income$13,681 $21,454 $20,950 
Plastics Segment
(in thousands)202420232022
Operating Revenue$463,441 $418,026 $512,527 
Cost of Goods Sold166,628 143,521 227,571 
Selling, General, and Administrative Expenses24,908 20,103 20,378 
Interest Expense590 602 585 
Income Tax Expense70,644 66,066 68,688 
Other Segment Items(76)(14)(69)
Net Income$200,747 $187,748 $195,374 
Capital Expenditures and Identifiable Assets
The following provides capital expenditures for each reportable segment and our corporate cost center for the years ended December 31, 2024, 2023 and 2022:
(in thousands)202420232022
Capital Expenditures
Electric301,454 240,695 147,869 
Manufacturing32,159 23,284 17,954 
Plastics24,749 23,029 5,245 
Corporate288 126 66 
Total
$358,650 $287,134 $171,134 
The following provides the identifiable assets by segment and corporate assets as of December 31, 2024 and 2023:
(in thousands)20242023
Identifiable Assets
Electric$2,785,522 $2,533,831 
Manufacturing254,445 251,343 
Plastics186,043 164,179 
Corporate426,072 293,215 
Total
$3,652,082 $3,242,568 
Corporate assets consist primarily of cash and cash equivalents, prepaid expenses, investments and fixed assets.
Reconciliation to Consolidated Amounts
Certain costs are not allocated to our operating segments. Corporate operating costs include items such as corporate staff and overhead costs, the results of our captive insurance company and other items excluded from the measurement of operating segment performance. Corporate is not an operating segment, rather it is added to operating segment totals to reconcile to consolidated amounts.
Included below is a reconciliation of certain segment information and our unallocated corporate costs to consolidated amounts for the years ended December 31, 2024, 2023 and 2022:
(in thousands)202420232022
Depreciation and Amortization
Electric$82,136 $75,330 $72,050 
Manufacturing20,393 18,495 16,202 
Plastics4,494 4,027 4,205 
Corporate98 102 140 
Total
107,121 97,954 92,597 
Interest Expense
Total Interest Expense of Reportable Segments41,322 36,761 35,331 
Corporate Interest Expense493 916 685 
Total
41,815 37,677 36,016 
Income Tax Expense (Benefit)
Total Income Tax Expense of Reportable Segments71,995 73,104 79,074 
Corporate Income Tax Benefit(6,765)(3,806)(5,723)
Total
65,230 69,298 73,351 
Net Income (Loss)
Total Net Income of Reportable Segments305,391 293,626 296,298 
Corporate Net Income (Loss)(3,729)565 (12,114)
Total
301,662 294,191 284,184 
Concentrations
Our Plastics segment businesses use PVC resin as a critical component within their PVC pipe manufacturing process. There are a limited number of PVC resin suppliers in the U.S., and in 2024 we sourced all of our PVC resin needs from four vendors. Although there are a limited number of PVC resin suppliers, we believe that other suppliers could provide PVC resin on comparable terms. Additionally, most U.S. resin production plants are located in the Gulf Coast region. These plants are subject to the risk of damage and production shutdowns because of exposure to hurricanes or other extreme weather events that occur in this region. The loss of a key vendor, or any interruption or delay in the supply of PVC resin could cause production delays, a possible loss of sales or result in increased costs to secure resin, all of which would adversely affect our operating results.
For the year ended December 31, 2024, two customers combined accounted for 19% of Electric segment operating revenues, two customers combined to account for 36% of Manufacturing segment operating revenues and two customers combined to account for 52% of Plastics segment operating revenues, with one of those customers providing 11% of our consolidated operating revenues.
Entity-Wide Information
All of our long-lived assets are located within the United States and substantially all of our operating revenues are from customers located within the United States.
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Revenue
 12 Months Ended
Dec. 31, 2024
Revenue from Contract with Customer [Abstract]  
Revenue Revenue
We present our operating revenues from external customers, in total and by amounts arising from contracts with customers and ARP arrangements, disaggregated by revenue source and segment for the years ended December 31, 2024, 2023 and 2022:
(in thousands)202420232022
Operating Revenues
Electric Segment
Retail: Residential$133,408 $135,570 $143,888 
Retail: Commercial and Industrial311,968 312,551 318,494 
Retail: Other7,838 7,719 7,918 
  Total Retail453,214 455,840 470,300 
Transmission53,517 52,555 52,213 
Wholesale11,077 12,459 18,539 
Other6,707 7,505 8,647 
Total Electric Segment524,515 528,359 549,699 
Manufacturing Segment
Metal Parts and Tooling303,077 351,267 338,865 
Plastic Products and Tooling32,210 41,395 49,080 
Scrap Metal7,305 10,119 10,038 
Total Manufacturing Segment342,592 402,781 397,983 
Plastics Segment
PVC Pipe463,441 418,026 512,527 
Total Operating Revenue1,330,548 1,349,166 1,460,209 
Less: Noncontract Revenues Included Above
Electric Segment - ARP Revenues575 (4,310)(9,266)
Total Operating Revenues from Contracts with Customers$1,329,973 $1,353,476 $1,469,475 
v3.25.0.1
Receivables
 12 Months Ended
Dec. 31, 2024
Receivables [Abstract]  
Receivables Receivables
Receivables as of December 31, 2024 and 2023 are as follows:
(in thousands)20242023
Receivables
Trade$112,169 $129,257 
Other13,799 9,084 
Unbilled Receivables21,916 21,324 
Total Receivables147,884 159,665 
Less Allowance for Credit Losses1,920 2,522 
Receivables, net of allowance for credit losses$145,964 $157,143 
The following is a summary of activity in the allowance for credit losses for the years ended December 31, 2024 and 2023:
(in thousands)20242023
Beginning Balance$2,522 $1,648 
Additions Charged to Expense1,242 2,014 
Reductions for Amounts Written Off, Net of Recoveries(1,844)(1,140)
Ending Balance$1,920 $2,522 
v3.25.0.1
Investments
 12 Months Ended
Dec. 31, 2024
Investments, Debt and Equity Securities [Abstract]  
Investments Investments
The following is a summary of our investments as of December 31, 2024 and 2023:
(in thousands)20242023
Short-term Investments
Government Debt Securities
$753 $— 
Long-term Investments
Corporate-Owned Life Insurance Policies47,895 42,287 
Government Debt Securities
60,378 7,724 
Corporate Debt Securities
1,628 1,579 
Mutual Funds10,653 7,771 
Money Market Funds596 3,125 
Other Investments27 30 
Total Long-term Investments121,177 62,516 
Total Investments$121,930 $62,516 
In April 2024, we made a $50.1 million investment in U.S. treasuries which mature in September 2026. As of December 31, 2024, our government and corporate debt securities had maturity dates ranging from May 2025 to August 2029. During the years ended December 31, 2024 and 2023, our investment income, which consisted primarily of interest on our cash equivalent and debt security investments and gains on our corporate-owned life insurance policy investments, totaled $19.8 million and $15.2 million, which is included in other income in our consolidated statements of income.
Debt Securities
The following table summarizes the amortized cost and fair value of debt securities available for sale and the corresponding amounts of gross unrealized gains and losses as of December 31, 2024:
December 31, 2024
(in thousands)
Amortized Cost
Gross Unrealized Gains
Gross Unrealized (Losses)
Fair Value
Government Debt Securities$60,891 $424 $(184)$61,131 
Corporate Debt Securities1,629 9 (10)1,628 
Total$62,520 $433 $(194)$62,759 
Unrealized gains and losses on available-for-sale debt securities as of December 31, 2023 were not material. As of December 31, 2024 and December 31, 2023, no unrealized losses on debt securities were deemed to be other-than-temporary.
The following table summarizes the fair value of debt securities available for sale by contractual maturity date as of December 31, 2024:
(in thousands)December 31, 2024
Due in one year or less
$753 
Due in one to five years
62,006 
Total$62,759 
Equity Securities
The amount of net unrealized gains and losses during the years ended December 31, 2024 and 2023 on marketable equity securities still held as of December 31, 2024 and 2023, respectively, was not material.
v3.25.0.1
Regulatory Matters
 12 Months Ended
Dec. 31, 2024
Regulatory Assets and Liabilities Disclosure [Abstract]  
Regulatory Matters Regulatory Matters
Regulatory Assets and Liabilities
The following presents our current and long-term regulatory assets and liabilities as of December 31, 2024 and 2023 and the period we expect to recover or refund such amounts:
Period of20242023
(in thousands)Recovery/RefundCurrentLong-TermCurrentLong-Term
Regulatory Assets
Pension and Other Postretirement Benefit Plans1
See below$— $88,161 $154 $86,134 
Alternative Revenue Program Riders2
Up to 2 years
4,257 195 3,719 158 
Deferred Income TaxesAsset lives— 8,944 — 6,940 
Fuel Clause Adjustments1
Up to 1 year
2,218 — 7,294 — 
Derivative Instruments1
Up to 1 year
1,989 — 4,210 — 
Other1
Various1,498 1,373 750 2,483 
Total Regulatory Assets9,962 98,673 16,127 95,715 
Regulatory Liabilities
Deferred Income TaxesAsset lives— 130,387 — 136,022 
Plant Removal ObligationsAsset lives— 126,263 — 117,030 
Fuel Clause Adjustments
Up to 1 year
11,432 — 11,350 — 
Alternative Revenue Program Riders
Up to 1 year
14,255 — 6,885 — 
North Dakota PTC RefundsAsset lives— 20,099 — 12,011 
Pension and Other Postretirement Benefit PlansSee below2,547 10,758 6,138 11,307 
OtherVarious1,073 1,421 1,035 177 
Total Regulatory Liabilities$29,307 $288,928 $25,408 $276,547 
1Costs subject to recovery without a rate of return.
2Amount eligible for recovery includes an incentive or rate of return.
Pension and Other Postretirement Benefit Plans represent benefit costs and actuarial losses and gains subject to recovery or refund through rates as they are expensed or amortized. These unrecognized benefit costs and actuarial losses and gains are eligible for treatment as regulatory assets or liabilities based on their probable inclusion in future electric rates.
Alternative Revenue Program Riders regulatory assets and liabilities are revenues not yet collected from customers or amounts collected from customers that are subject to refund, respectively, primarily due to investments in qualifying transmission, conservation, renewable resource, environmental and other generation assets, and the impact of decoupling.
Deferred Income Taxes primarily represent the revaluation of accumulated deferred income taxes arising from the change in the federal income tax rate in 2017. This amount is being refunded to customers over the estimated lives of the property assets from which the deferred income taxes originated.
Fuel Clause Adjustments represent the under- or over-collection of fuel costs relative to the estimated cost of fuel included in customer rates, which will be collected from or returned to customers in future periods.
Derivative Instruments represent unrealized losses recognized on derivative instruments. On final settlement of such instruments, any realized losses are recovered from customers.
Plant Removal Obligations represent amounts collected from customers to be used to cover actual removal costs as incurred.
North Dakota PTC Refunds represent PTCs earned from our wind energy facilities. These amounts are being allocated to customers over the lives of the assets generating the credits.
Other regulatory assets and liabilities include other amounts that we expect to recover from, or return to, customers in future periods, such as the cost of abandoned projects, costs incurred in connection with recent rate cases and other items.
North Dakota Rate Case
On November 2, 2023, OTP filed a request with the NDPSC for an increase in revenue recoverable under general rates in North Dakota. In its filing, OTP requested a net increase in annual revenue of $17.4 million, or 8.4%, based on an allowed rate of return on rate base of 7.85% and an allowed rate of ROE of 10.6% on an equity ratio of 53.5% of total capital. The filing also included an interim rate request of a net increase in annual revenue of $12.4 million, or 6.0%, which was approved by the NDPSC on December 13, 2023. Interim rates went into effect on January 1, 2024. On July 3, 2024, OTP filed an update to the original request increasing the amount of the net annual revenue requirement from $17.4 million to $22.5 million, or a net increase of 10.9% in annual revenue, to account for certain items identified throughout the regulatory process.
On December 30, 2024, the NDPSC approved a settlement agreement between OTP and certain interested parties in the general rate case and issued its written order on final rates. The key provisions of the order include a revenue requirement of $225.6 million, based on a return on rate base of 7.53%, and an allowed ROE of 10.10% on an equity ratio of 53.5%. The net annual revenue requirement includes a net increase of $13.1 million, or 6.18%. OTP’s revenue requirement was reduced by approximately
$3.0 million primarily due to the inclusion of forecasted PTCs plus adjustments for new customer load additions, which were not included in OTP’s updated request filed on July 3, 2024. Through the settlement of the case, the parties also agreed to establish an earnings sharing mechanism, whereby 70% of actual earnings in excess of a 10.20% ROE would be returned to customers, with OTP retaining the remaining 30%.
v3.25.0.1
Property, Plant and Equipment
 12 Months Ended
Dec. 31, 2024
Property, Plant and Equipment [Abstract]  
Property, Plant and Equipment Property, Plant and Equipment
Major classes of property, plant and equipment as of December 31, 2024 and 2023 include:
(in thousands)20242023
Electric Plant in Service  
Production$1,469,008 $1,412,826 
Transmission820,415 777,613 
Distribution726,159 654,704 
General165,361 144,738 
Electric Plant in Service3,180,943 2,989,881 
Construction Work in Progress231,890 137,212 
Total Gross Electric Plant3,412,833 3,127,093 
Less Accumulated Depreciation
899,049 851,148 
Net Electric Plant2,513,784 2,275,945 
Nonelectric Property, Plant and Equipment
Equipment260,307 233,571 
Buildings and Leasehold Improvements88,680 64,753 
Land13,578 13,600 
Nonelectric Property, Plant and Equipment362,565 311,924 
Construction Work in Progress40,536 38,062 
Total Gross Nonelectric Property, Plant and Equipment403,101 349,986 
Less Accumulated Depreciation
224,425 207,556 
Net Nonelectric Property, Plant and Equipment178,676 142,430 
Net Property, Plant and Equipment$2,692,460 $2,418,375 
Depreciation expense for the years ended December 31, 2024, 2023 and 2022 totaled $99.4 million, $90.8 million and $84.4 million.
The following table provides OTP’s ownership percentages and amounts included in the December 31, 2024 and 2023 consolidated balance sheets for OTP’s share of each of these jointly owned facilities:
 (dollars in thousands)Ownership
Percentage		Electric Plant
in Service		Construction
Work in
Progress		Accumulated
Depreciation		Net Plant
December 31, 2024     
Big Stone Plant53.9 %$345,990 $459 $(135,065)$211,384 
Coyote Station35.0 %188,066 813 (118,268)70,611 
Big Stone South–Ellendale 345 kV line50.0 %106,185 — (8,445)97,740 
Fargo–Monticello 345 kV line14.2 %78,184 — (12,247)65,937 
Big Stone South–Brookings 345 kV line50.0 %53,167 — (5,822)47,345 
Brookings–Southeast Twin Cities 345 kV line4.8 %28,013 1,131 (3,941)25,203 
Bemidji–Grand Rapids 230 kV line14.8 %16,331 — (3,693)12,638 
Jamestown– Ellendale 345 kV line50.0 %— 5,509 — 5,509 
Big Stone South–Alexandria 345 kV line40.0 %— 2,418 — 2,418 
Alexandria–Big Oaks 345 kV line14.2 %— 417 — 417 
Oslo - Lake Ardoch 115 kV line72.0 %— 2,646 — 2,646 
December 31, 2023
Big Stone Plant53.9 %$341,683 $820 $(126,904)$215,599 
Coyote Station35.0 %188,656 104 (115,306)73,454 
Big Stone South–Ellendale 345 kV line50.0 %106,185 — (7,181)99,004 
Fargo–Monticello 345 kV line14.2 %78,184 — (11,238)66,946 
Big Stone South–Brookings 345 kV line50.0 %53,170 — (5,207)47,963 
Brookings–Southeast Twin Cities 345 kV line4.8 %26,409 83 (3,617)22,875 
Bemidji–Grand Rapids 230 kV line14.8 %16,331 — (3,568)12,763 
Jamestown–Ellendale 345 kV line
50.0 %— 1,121 — 1,121 
Big Stone South–Alexandria 345 kV line40.0 %— 555 — 555 
Alexandria–Big Oaks 345 kV line14.2 %— 343 — 343 
v3.25.0.1
Intangible Assets
 12 Months Ended
Dec. 31, 2024
Goodwill and Intangible Assets Disclosure [Abstract]  
Intangible Assets Intangible Assets
The following table summarizes our goodwill by segment as of December 31, 2024 and 2023: 
(in thousands)20242023
Manufacturing$18,270 $18,270 
Plastics19,302 19,302 
Total Goodwill$37,572 $37,572 
Our annual goodwill impairment testing, performed in the fourth quarters of 2024 and 2023, indicated no impairment existed as of the test date.
The following table summarizes the components of our intangible assets as of December 31, 2024 and 2023:
(in thousands)Gross
Amount		Accumulated
Amortization		Net Carrying
Amount
December 31, 2024
Customer Relationships$22,491 $16,766 $5,725 
Other26 8 18 
Total$22,517 $16,774 $5,743 
December 31, 2023
Customer Relationships$22,491 $15,667 $6,824 
Other26 7 19 
Total$22,517 $15,674 $6,843 
Amortization expense for these intangible assets for each of the years ended December 31, 2024, 2023 and 2022 totaled $1.1 million.
Annual amortization expense for these intangible assets for the next five years is: 
(in thousands)20252026202720282029
Amortization Expense$1,100 $1,092 $1,090 $554 $285 
v3.25.0.1
Leases
 12 Months Ended
Dec. 31, 2024
Leases [Abstract]  
Leases Leases 
We lease rail cars, warehouse and office space, land, and certain office, manufacturing, material handling and other equipment under varying terms and conditions. All leases are classified as operating leases.
The components of lease cost and lease cash flows for the years ended December 31, 2024, 2023, and 2022 are as follows:
(in thousands)202420232022
Lease Cost
Operating Lease Cost$6,688 $6,309 $5,606 
Variable Lease Cost1,460 1,433 1,386 
Short-Term Lease Cost2,746 2,525 1,517 
Total Lease Cost$10,894 $10,267 $8,509 
Lease Cash Flows
Operating Cash Flows from Operating Leases$6,762 $6,424 $5,592 
A summary of operating lease right-of-use lease assets and lease liabilities as of December 31, 2024 and 2023 is as follows: 
(in thousands)20242023
Right of Use Lease Assets1
$28,179 $16,788 
Lease Liabilities
Current2
4,776 5,756 
Long-Term3
23,567 11,258 
Total Lease Liabilities$28,343 $17,014 
1Included in Other Noncurrent Assets in the consolidated balance sheets.
2Included in Other Current Liabilities in the consolidated balance sheets.
3Included in Other Noncurrent Liabilities in the consolidated balance sheets.
Operating lease assets obtained in exchange for new operating lease liabilities amounted to $17.6 million and $3.6 million for the years ended December 31, 2024 and 2023.
Maturities of lease liabilities as of December 31, 2024 for each of the next five years and in the aggregate thereafter are as follows:
(in thousands)Operating Leases
2025$6,350 
20265,638 
20274,785 
20283,604 
20292,750 
Thereafter12,921 
Total Lease Payments36,048 
Less: Interest7,705 
Present Value of Lease Liabilities$28,343 
The weighted-average remaining lease term and the weighted-average discount rate as of December 31, 2024 and 2023 are as follows:
20242023
Weighted-Average Remaining Lease Term (in years)7.93.4
Weighted-Average Discount Rate6.37 %5.40 %
v3.25.0.1
Short-Term and Long-Term Borrowings
 12 Months Ended
Dec. 31, 2024
Debt Disclosure [Abstract]  
Short-Term and Long-Term Borrowings Short-Term and Long-Term Borrowings
The following is a summary of our outstanding short- and long-term borrowings by borrower, OTC or OTP, as of December 31, 2024 and 2023:
20242023
(in thousands)OTCOTPTotalOTCOTPTotal
Short-Term Debt$— $69,615 $69,615 $— $81,422 $81,422 
Long-Term Debt79,900 863,834 943,734 79,849 744,210 824,059 
Total$79,900 $933,449 $1,013,349 $79,849 $825,632 $905,481 
Short-Term Debt
The following is a summary of our lines of credit as of December 31, 2024 and 2023:
20242023
(in thousands)Line LimitAmount OutstandingLetters
of Credit		Amount AvailableAmount Available
OTC Credit Agreement$170,000 $— $— $170,000 $170,000 
OTP Credit Agreement220,000 69,615 8,772 141,613 79,446 
Total$390,000 $69,615 $8,772 $311,613 $249,446 
On December 11, 2024, OTC entered into a Sixth Amended and Restated Credit Agreement (the OTC Credit Agreement) and OTP entered into a Fifth Amended and Restated Credit Agreement (the OTP Credit Agreement), in each case amending and restating the previously existing credit agreements, to extend the maturity date of each credit facility and adjust the maximum debt to total capitalization covenant. The OTP Credit Agreement was also amended to increase the maximum borrowing capacity. The OTC agreement provides for a $170.0 million unsecured revolving line of credit and the OTP agreement provides for a $220.0 million unsecured revolving line of credit to support operations, fund capital expenditures, refinance certain indebtedness and provide for the issuance of letters of credit in an aggregate amount not to exceed $40.0 million under the OTC Credit Agreement and $50.0 million under the OTP Credit Agreement. Each credit facility includes an accordion provision allowing the borrower to increase the borrowing capacity under the facility, subject to certain conditions, up to $290.0 million and $300.0 million under the OTC Credit Agreement and OTP Credit Agreement, respectively.
Borrowings under each credit facility are subject to a variable rate of interest on outstanding balances and a commitment fee is charged based on the average unused amount available to be drawn under the respective facility. The variable rate of interest to be charged is based on a benchmark interest rate, either SOFR or a Base Rate, as defined in the credit agreements, selected by the borrower at the time of an advance, subject to the conditions of each agreement, plus an applicable credit spread. The credit spread ranges from zero to 2.00%, depending on the benchmark interest rate selected, and is subject to adjustment based on the credit ratings of the relevant borrower. The weighted-average interest rate on all outstanding borrowings as of December 31, 2024 and 2023 was 5.61% and 6.70%.
Each credit facility contains a number of restrictions on the borrower, including restrictions on the ability to merge, sell assets, make investments, create or incur liens on assets, guarantee the obligations of any other party and engage in transactions with related parties. The agreements also require the borrower to maintain various financial covenants, as further described below. Each credit facility includes a cross-default provision whereby an event of default of other outstanding indebtedness will trigger an event of default under the agreement. Each credit facility expires on December 11, 2029.
Long-Term Debt
The following is a summary of outstanding long-term debt by borrower as of December 31, 2024 and 2023: 
(in thousands)
EntityDebt InstrumentRateMaturity20242023
OTCGuaranteed Senior Notes3.55%12/15/26$80,000 $80,000 
OTPSeries 2007C Senior Unsecured Notes6.37%08/02/2742,000 42,000 
OTPSeries 2013A Senior Unsecured Notes4.68%02/27/2960,000 60,000 
OTPSeries 2019A Senior Unsecured Notes 3.07%10/10/2910,000 10,000 
OTPSeries 2020A Senior Unsecured Notes3.22%02/25/3010,000 10,000 
OTPSeries 2020B Senior Unsecured Notes3.22%08/20/3040,000 40,000 
OTPSeries 2021A Senior Unsecured Notes2.74%11/29/3140,000 40,000 
OTPSeries 2024A Senior Unsecured Notes5.48%04/01/3460,000 — 
OTPSeries 2007D Senior Unsecured Notes6.47%08/20/3750,000 50,000 
OTPSeries 2019B Senior Unsecured Notes3.52%10/10/3926,000 26,000 
OTPSeries 2020C Senior Unsecured Notes3.62%02/25/4010,000 10,000 
OTPSeries 2013B Senior Unsecured Notes5.47%02/27/4490,000 90,000 
OTPSeries 2018A Senior Unsecured Notes4.07%02/07/48100,000 100,000 
OTPSeries 2019C Senior Unsecured Notes3.82%10/10/4964,000 64,000 
OTPSeries 2020D Senior Unsecured Notes3.92%02/25/5015,000 15,000 
OTPSeries 2021B Senior Unsecured Notes3.69%11/29/51100,000 100,000 
OTPSeries 2022A Senior Unsecured Notes3.77%05/20/5290,000 90,000 
OTPSeries 2024B Senior Unsecured Notes5.77%04/01/5460,000 — 
Total947,000 827,000 
Less:Unamortized Long-Term Debt Issuance Costs3,266 2,941 
Total Long-Term Debt Net of Unamortized Debt Issuance Costs$943,734 $824,059 
On March 28, 2024, OTP entered into a Note Purchase Agreement pursuant to which OTP issued, in a private placement transaction, $120.0 million of senior unsecured notes consisting of (a) $60.0 million of 5.48% Series 2024A Senior Unsecured Notes due April 1, 2034, and (b) $60.0 million of 5.77% Series 2024B Senior Unsecured Notes due April 1, 2054.
Per the terms of the agreement, OTP may prepay all or any part of the notes (in an amount not less than 10% of the aggregate principal amount of the notes then outstanding in the case of a partial prepayment) at 100% of the principal amount so prepaid, together with unpaid accrued interest and a make-whole amount, as defined in the agreement; provided that no default or event of default exists under the agreement. Any prepayment of the Series 2024A Notes then outstanding on or after January 1, 2034, or the Series 2024B Notes then outstanding on or after October 1, 2053, will be made without any make-whole amount. Consistent with other of our borrowings, the agreement contains a number of restrictions on the business of OTP, including restrictions on OTP’s ability to merge, sell substantially all assets, create or incur liens on assets, guarantee the obligations of any other party, and engage in certain transactions with affiliates.
Our guaranteed and unsecured notes require the borrower to maintain various financial covenants, as further described below. These notes provide for prepayment options allowing for a full or partial prepayment at 100% of the principal amount so prepaid, together with unpaid accrued interest and a make-whole amount, as defined. These notes also include restrictions on the borrower, including its ability to merge, sell assets, create or incur liens on assets, guarantee the obligations of any other party and engage in transactions with related parties. The notes include a cross-default provision whereby an event of default of other outstanding indebtedness will trigger an event of default under the note.
Aggregate maturities of long-term debt obligations on December 31, 2024 for each of the next five years are as follows:
(in thousands)20252026202720282029
Debt Maturities$— $80,000 $42,000 $— $70,000 
Financial Covenants
Certain of OTC's and OTP's short-term and long-term debt agreements require the borrower, whether OTC or OTP, to maintain certain financial covenants, including a maximum debt to total capitalization of either 0.60 to 1.00 or 0.65 to 1.00, depending on the debt agreement, a minimum interest and dividend coverage ratio of 1.50 to 1.00, and a maximum level of priority indebtedness. As of December 31, 2024, OTC and OTP were in compliance with these financial covenants.
Guaranties
OTC's obligations under the terms of its Guaranteed Senior Notes are unconditionally and irrevocably guaranteed by its subsidiaries, Varistar Corporation, BTD Manufacturing, Inc., Northern Pipe Products, Inc. and Vinyltech Corporation.
v3.25.0.1
Employee Postretirement Benefits
 12 Months Ended
Dec. 31, 2024
Retirement Benefits [Abstract]  
Employee Postretirement Benefits Employee Postretirement Benefits
Pension Plan and Other Postretirement Benefits
The Company sponsors a noncontributory funded pension plan (the Pension Plan), an unfunded, nonqualified Executive Survivor and Supplemental Retirement Plan (ESSRP), both accounted for as defined benefit pension plans, and a postretirement healthcare plan accounted for as an other postretirement benefit plan.
The Pension Plan, which previously covered substantially all corporate and OTP employees, was closed to new employees in 2013. The plan provides retirement compensation to all covered employees at age 65, with reduced compensation in cases of retirement prior to age 62. Participants are fully vested after completing five years of vesting service. The plan assets consist of equity funds, fixed income funds, cash and cash equivalents and alternative investments. None of the plan assets are invested in common stock or debt securities of the Company.
The ESSRP, an unfunded plan, provides for defined benefit payments to executive officers and certain key management employees on their retirement for life, or to their beneficiaries on their death. The ESSRP was amended and restated in 2019 to i) freeze the participation in the restoration retirement benefit component of the plan and ii) freeze benefit accruals under the restoration retirement benefit component of the plan for all participants of the plan except any participants deemed to be grandfathered participants.
The postretirement healthcare plan, closed to new participants in 2010, provides a portion of health insurance benefits for retired and covered corporate and OTP employees. To be eligible for retiree health insurance benefits, the employee must be 55 years of age with a minimum of 10 years of service. The plan is an unfunded plan and accordingly holds no plan assets.
Pension Plan Assets. We have established a Retirement Plans Administration Committee to develop and monitor our investment strategy for our Pension Plan assets. Our investment strategy includes the following objectives:
•The assets of the plan will be invested in accordance with all applicable laws in a manner consistent with fiduciary standards including Employee Retirement Income Security Act standards of 1974 (ERISA) (if applicable). Specifically:
◦The safeguards and diversity that a prudent investor would adhere to must be present in the investment program.
◦All transactions undertaken on behalf of the Pension Plan must be in the best interest of plan participants and their beneficiaries.
•The primary objective is to provide a source of retirement income for its participants and beneficiaries.
•The near-term primary financial objective is to improve and protect the funded status of the plan.
•A secondary financial objective is to minimize pension funding and expense volatility where possible.
We have developed an asset allocation target, measured at investment market value, to provide guideline percentages of investment mix. This investment mix is intended to achieve the financial objectives of the plan. The permitted range is a guide and will at times not reflect the actual asset allocation due to market conditions, actions of our investment managers and required cash flows to and from the Pension Plan.
The following table presents our target asset allocation permitted range along with the actual asset allocation as of December 31, 2024 and 2023:
 PermittedActual Allocation
Asset ClassRange20242023
Return Enhancement35 –60%41 %48 %
Risk Management40 –80%59 51 
Alternatives0 –20%— 1 
Total100 %100 %
Return Enhancement investments are those that seek to provide equity-like, long-term capital appreciation. Examples include equity securities, including dynamic asset allocation funds, and higher yielding fixed income securities, such as high yield bonds and emerging market debt.
Risk Management investments seek to decrease downside risk or act as a hedge against plan liabilities. Examples are cash and fixed income instruments.
Alternative investments seek to either provide return enhancement through long-term appreciation or risk management through decreased downside risk. The defining characteristic of these asset types is uncorrelated source of returns, less liquidity and private market access. Examples include investments in the SEI Energy Debt Collective Fund.
The following presents the fair value inputs classified within the fair value hierarchy used to measure Pension Plan assets at December 31, 2024 and 2023 and assets measured using the net asset value (NAV) practical expedient:
(in thousands)Level 1Level 2Level 3NAVTotal
December 31, 2024
Equity Funds$116,889 $— $— $— $116,889 
Fixed Income Funds175,310 — — — 175,310 
Hybrid Funds10,106 — — — 10,106 
U.S. Treasury Securities23,909 — — — 23,909 
SEI Energy Debt Collective Fund— — — 1,061 1,061 
Total$326,214 $— $— $1,061 $327,275 
December 31, 2023
Equity Funds$127,159 $— $— $— $127,159 
Fixed Income Funds167,604 — — — 167,604 
Hybrid Funds10,980 — — — 10,980 
U.S. Treasury Securities23,218 — — — 23,218 
SEI Energy Debt Collective Fund— — — 1,518 1,518 
Total$328,961 $— $— $1,518 $330,479 
The investments held by the SEI Energy Debt Collective Fund on December 31, 2024 and 2023 consist mainly of below investment grade high yield bonds and loans of U.S. energy companies which trade at a discount to fair value. Redemptions are allowed semi-annually with a 95-day notice period, subject to fund director consent and certain gate, holdback and suspension restrictions. Subscriptions are allowed monthly with a three-year lock up on subscriptions. The fund’s assets are valued in accordance with valuations reported by the fund’s sub-advisor or the fund’s underlying investments or other independent third-party sources, although SEI, in its discretion, may use other valuation methods, subject to compliance with ERISA, as applicable. On an annual basis, as determined by the investment manager in its sole discretion, an independent valuation agent is retained to provide a valuation of the illiquid assets of the fund and of any other asset of the fund.
Funded Status. The following table provides a reconciliation of the changes in the fair value of plan assets and the actuarially computed benefit obligation for the years ended December 31, 2024 and 2023 and the funded status of the plans as of December 31, 2024 and 2023:
Pension Benefits (Pension Plan) Pension Benefits (ESSRP)Postretirement Benefits
(in thousands)202420232024202320242023
Change in Fair Value of Plan Assets:
Fair Value of Plan Assets at January 1$330,479 $313,797 $— $— $— $— 
Actual Return on Plan Assets14,976 34,196 — — — — 
Company Contributions— — 2,694 2,197 1,568 3,167 
Benefit Payments(18,180)(17,514)(2,694)(2,197)(3,734)(8,900)
Participant Premium Payments— — — — 2,166 5,733 
Fair Value of Plan Assets at December 31$327,275 $330,479 $— $— $— $— 
Change in Benefit Obligation:
Benefit Obligation at January 1$318,801 $308,055 $35,780 $35,624 $30,145 $49,947 
Service Cost3,886 3,698 — 72 490 565 
Interest Cost17,189 16,436 1,897 1,889 1,600 2,416 
Benefit Payments(18,180)(17,514)(2,694)(2,197)(3,734)(8,900)
Participant Premium Payments— — — — 2,166 5,733 
Plan Amendments— — — — — (17,493)
Actuarial (Gain) Loss(7,686)8,126 331 392 (664)(2,123)
Benefit Obligation at December 31314,010 318,801 35,314 35,780 30,003 30,145 
Funded Status$13,265 $11,678 $(35,314)$(35,780)$(30,003)$(30,145)
Amounts Recognized in Consolidated Balance Sheets at December 31:
Noncurrent Assets$13,265 $11,678 $— $— $— $— 
Current Liabilities— — (2,700)(2,679)(2,618)(2,469)
Noncurrent Liabilities
— — (32,614)(33,101)(27,385)(27,676)
Net Asset (Liability)$13,265 $11,678 $(35,314)$(35,780)$(30,003)$(30,145)
The accumulated benefit obligation of our Pension Plan was $288.5 million and $288.8 million as of December 31, 2024 and 2023. The accumulated benefit obligation of our ESSRP was $35.3 million and $35.8 million as of December 31, 2024 and 2023.
The following assumptions were used to determine benefit obligations as of December 31, 2024 and 2023:
Pension Benefits (Pension Plan)Pension Benefits (ESSRP)Postretirement Benefits
 202420232024202320242023
Discount Rate5.70 %5.57 %5.60 %5.53 %5.61 %5.53 %
Long-Term Rate of Compensation Increasen/an/a3.00 %3.00 %n/an/a
Participants up to Age 39(1)
4.50 %4.50 %n/an/an/an/a
Participants Ages 40 to 49(2)
4.50 %4.50 %n/an/an/an/a
Participants Age 50 and Older(3)
3.75 %3.75 %n/an/an/an/a
Healthcare Cost Immediate Trend Raten/an/an/an/a6.44 %6.97 %
Healthcare Cost Ultimate Trend Raten/an/an/an/a4.00 %4.00 %
Year the Rate Reaches the Ultimate Trend Raten/an/an/an/a20482048
(1) Amount reflects rate of compensation increases for both union and non-union employees.
(2) Amount reflects rate of compensation increases for union employees. The rate of compensation increases for non-union employees is 3.50%.
(3) Amount reflects rate of compensation increases for union employees. The rate of compensation increases for non-union employees is 3.00%.
The measurement of the plan asset or benefit obligation recognized for our Pension Plan, ESSRP and postretirement healthcare benefit plan included the following significant actuarial adjustments:
•For the Pension Plan, an increase in the discount rate in 2024 and 2023 reduced our obligation by $4.7 million and $2.2 million. Changes in plan participant census data decreased our benefit obligation by $3.0 million in 2024. Actual
returns on Pension Plan assets in 2024 were $15.0 million, compared to an expected return of $25.5 million, impacting our net obligation by $10.5 million.
•For the ESSRP, an increase in the discount rate in 2024 and 2023 reduced our obligation by $0.2 million and $0.1 million.
•For the postretirement healthcare plan, an increase in the discount rate in 2024 and 2023 reduced our obligation by $0.2 million and $1.3 million. Revised estimates of healthcare cost trends and participant contribution assumptions increased the benefit obligation by $0.4 million in 2024. Changes in plan participant census data decreased our benefit obligation by $0.9 million in 2024.
Net Periodic Benefit Cost. A portion of service cost may be capitalized as a cost of self-constructed property, plant and equipment. When recognized in the consolidated statements of income, service cost is recognized within one of the components of operating expenses. Nonservice cost components of net periodic benefit cost may be deferred and recognized as a regulatory asset under the accounting guidance for regulated operations. When recognized in the consolidated statements of income, nonservice cost components are recognized as nonservice cost components of postretirement benefits.
The following table lists the components of net periodic benefit cost of our defined benefit pension plans and other postretirement benefits for the years ended December 31, 2024, 2023 and 2022:
Pension Benefits (Pension Plan)Pension Benefits (ESSRP)Postretirement Benefits
(in thousands)202420232022202420232022202420232022
Service Cost$3,886 $3,698 $6,576 $— $72 $195 $490 $565 $1,338 
Interest Cost17,189 16,436 12,344 1,897 1,889 1,341 1,600 2,416 2,041 
Expected Return on Assets(25,518)(25,914)(23,684)— — — — — — 
Amortization of Prior Service Cost— — — — — — (6,302)(6,649)(5,733)
Amortization of Net Actuarial Loss158 — 7,865 — — 567 — — 3,063 
Net Periodic Benefit Cost$(4,285)$(5,780)$3,101 $1,897 $1,961 $2,103 $(4,212)$(3,668)$709 
The following table includes the impact of regulation on the recognition of periodic benefit cost arising from pension and other postretirement benefits for the years ended December 31, 2024, 2023 and 2022:
(in thousands)202420232022
Net Periodic Benefit Cost$(6,600)$(7,487)$5,913 
Net Amount Amortized Due to the Effect of Regulation1,367 1,225 1,121 
Net Periodic Benefit Cost Recognized$(5,233)$(6,262)$7,034 
The following assumptions were used to determine net periodic benefit cost for the years ended December 31, 2024, 2023 and 2022:
Pension Benefits (Pension Plan)Pension Benefits (ESSRP)Postretirement Benefits
 202420232022202420232022202420232022
Discount Rate5.57 %5.51 %3.03 %5.53 %5.51 %2.93 %5.53 %5.52 %3.01 %
Long-Term Rate of Return on Plan Assets7.00 %7.00 %6.30 %n/an/an/an/an/an/a
Long-Term Rate of Compensation Increasen/an/an/a3.00 %3.00 %3.00 %n/an/an/a
Participants to Age 394.50 %4.50 %4.50 %n/an/an/an/an/an/a
Participants Ages 40 to 494.00 %3.50 %3.50 %n/an/an/an/an/an/a
Participants Age 50 and Older3.38 %2.75 %2.75 %n/an/an/an/an/an/a
We develop our estimated discount rate through the use of a hypothetical bond portfolio method. This method derives the discount rate from the average yield of a collection of high credit quality bonds which produce cash flows similar to our anticipated future benefit payments. We estimate the assumed long-term rate of return on plan assets based primarily on asset category studies using historical market return and volatility data with forward-looking estimates based on existing financial market conditions and forecasts of capital markets. Modest excess return expectations versus some market indices are incorporated into the return projections based on the actively managed structure of the investment programs and their records of achieving such returns historically.
The following table presents the amounts not yet recognized as components of net periodic benefit cost as of December 31, 2024 and 2023:
Pension Benefits (Pension Plan)Pension Benefits (ESSRP)Postretirement Benefits
(in thousands)202420232024202320242023
Regulatory Assets (Liabilities):
Unrecognized Prior Service Cost$— $— $— $— $(12,703)$(18,845)
Unrecognized Actuarial Loss87,868 85,227 292 1,061 1,121 1,759 
Net Regulatory Assets (Liabilities)$87,868 $85,227 $292 $1,061 $(11,582)$(17,086)
Accumulated Other Comprehensive Income (Loss):
Unrecognized Prior Service Cost$— $— $— $— $339 $498 
Unrecognized Actuarial Gain (Loss)1,937 1,994 (2,502)(1,403)732 707 
Total Accumulated Other Comprehensive Income (Loss)$1,937 $1,994 $(2,502)$(1,403)$1,071 $1,205 
Cash Flows. We did not make any contributions to our Pension Plan in 2024 or 2023. We made a discretionary contribution of $20.0 million in in 2022. As of December 31, 2024, we had no minimum funding requirements for our Pension Plan. Contributions to our ESSRP and postretirement healthcare plan are equal to the benefits paid to plan participants.
The following reflects anticipated benefit payments to be paid in each of the next five years and in the aggregate for the five-year period thereafter under our pension plans and postretirement healthcare plan:
(in thousands)202520262027202820292030-2034
Projected Pension Plan Benefit Payments$19,274 $19,804 $20,284 $20,859 $21,420 $111,275 
Projected ESSRP Benefit Payments2,769 2,899 3,059 2,999 3,070 14,275 
Projected Postretirement Benefit Payments2,618 2,614 2,576 2,488 2,517 12,019 
Total$24,661 $25,317 $25,919 $26,346 $27,007 $137,569 
401K Plan
We sponsor a 401K plan for the benefit of all corporate and subsidiary company employees. Contributions made to these plans totaled $9.3 million for 2024, $7.8 million for 2023 and $6.7 million for 2022.
v3.25.0.1
Asset Retirement Obligations
 12 Months Ended
Dec. 31, 2024
Asset Retirement Obligation Disclosure [Abstract]  
Asset Retirement Obligations Asset Retirement Obligations
We have recognized asset retirement obligations (AROs) related to our coal-fired generation plants, natural gas combustion turbines, solar facility and wind turbines. The cost of AROs include items such as site restoration, closure or removal of ash pits and removal of certain structures, generators, asbestos and storage tanks. We have other legal obligations associated with the retirement of a variety of other long-lived tangible assets used in electric operations where the estimated settlement costs are individually and collectively immaterial. We have no assets legally restricted for the settlement of any AROs. As of December 31, 2024 and 2023, $0.1 million and $0.1 million, respectively, was included in other current liabilities and $42.1 million and $36.4 million, respectively, was included in other noncurrent liabilities in the consolidated balance sheets related to AROs.
A reconciliation of the carrying amounts of AROs for the years ended December 31, 2024 and 2023 is as follows: 
(in thousands)20242023
Beginning Balance$36,477 $25,182 
New Obligations Recognized2,991 4,506 
Adjustments Due to Revisions in Cash Flow Estimates1,098 8,394 
Accrued Accretion1,676 1,191 
Settlements(79)(2,796)
Ending Balance$42,163 $36,477 
Coal Combustion Residual Regulations
In May 2024, the Environmental Protection Agency (EPA) published a final rule amending coal combustion residual (CCR) regulations. The final rule introduces new requirements for the management of coal ash at active coal-fired power plants and inactive coal-fired
power plants with a legacy surface impoundment. The regulations impose new requirements including groundwater monitoring, closure standards, post-closure care obligations and potential remediation activities. At this time, we do not believe there are any significant new requirements which are applicable to our coal-fired power plants, except for Big Stone Plant. During the fourth quarter of 2024, a site evaluation was performed at Big Stone Plant to assess the presence and estimated volumes of coal ash stored at the facility. Based on this and our assessment of the regulations, we believe the plant will be impacted by the new requirements. As of December 31, 2024, we recognized $3.0 million of additional liabilities for new obligations resulting from the EPA's final CCR rule for costs associated with coal ash removal and groundwater monitoring we expect to incur in the future. The final rule requires facility evaluations to be performed in the future. Revisions to our estimated compliance costs or further obligations could be identified through the process of performing the additional evaluations. Should such revisions be necessary or if additional cost obligations are identified, we will update our cash flow estimates and resulting retirement obligation at that time.
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Income Taxes
 12 Months Ended
Dec. 31, 2024
Income Tax Disclosure [Abstract]  
Income Taxes Income Taxes
Income before income taxes for the years ended December 31, 2024, 2023 and 2022 consists entirely of domestic earnings.
The provision for income taxes charged to income for the years ended December 31, 2024, 2023 and 2022 consisted of the following:
(in thousands)202420232022
Current
Federal Income Taxes$36,238 $41,253 $31,949 
State Income Taxes6,533 15,126 9,568 
Deferred
Federal Income Taxes13,078 9,832 22,480 
State Income Taxes9,979 3,676 9,943 
Tax Credits
North Dakota Wind Tax Credit Amortization, Net of Federal Tax(586)(586)(586)
Investment Tax Credit Amortization(12)(3)(3)
Total$65,230 $69,298 $73,351 
The reconciliation of the statutory federal income tax rate to our effective tax rate for each of the years ended December 31, 2024, 2023 and 2022 is as follows:
202420232022
Income Taxes at Federal Statutory Rate$77,047 21.0 %$76,332 21.0 %$75,082 21.0 %
Increases (Decreases) in Tax from:
State Taxes on Income, Net of Federal Tax14,360 3.9 14,429 4.0 15,049 4.2 
Production Tax Credits (PTCs)(20,106)(5.5)(17,394)(4.8)(14,985)(4.2)
Amortization of Excess Deferred Income Taxes(2,788)(0.8)(2,205)(0.6)(1,625)(0.5)
North Dakota Wind Tax Credit Amortization, Net of Federal Tax(586)(0.2)(586)(0.2)(586)(0.2)
Other, Net(2,697)(0.6)(1,278)(0.3)416 0.2 
Income Taxes at Effective Tax Rate$65,230 17.8 %$69,298 19.1 %$73,351 20.5 %
PTCs, North Dakota wind tax credits and excess deferred income taxes arising from the federal tax rate reduction in the 2017 Tax Cuts and Jobs Act are returned to customers as a reduction of the rates they are charged and result in a reduction of operating revenues.
Deferred tax assets and liabilities were composed of the following on December 31, 2024 and 2023:
(in thousands)20242023
Deferred Tax Assets  
Employee Benefits$37,456 $39,959 
Regulatory Liabilities52,664 56,479 
Tax Credit Carryforwards18,268 21,836 
Cost of Removal35,374 32,993 
Asset Retirement Obligations10,948 9,494 
Net Operating Loss Carryforward2,289 2,336 
Other19,449 11,310 
Total Deferred Tax Assets$176,448 $174,407 
Deferred Tax Liabilities
Differences Related to Property$(375,120)$(347,885)
Retirement Benefits Regulatory Asset(22,892)(22,458)
Pension Expense(26,034)(24,875)
Other(20,147)(16,462)
Total Deferred Tax Liabilities(444,193)(411,680)
Deferred Income Taxes$(267,745)$(237,273)
As of December 31, 2024, we had net operating loss carryforwards for state tax purposes totaling $2.3 million that expire between 2029 and 2037, and state tax credits totaling $18.3 million which expire between 2040 and 2043.
The following table summarizes the activity for unrecognized tax benefits for the years ended December 31, 2024, 2023 and 2022:
(in thousands)202420232022
Balance on January 1$1,489 $923 $827 
Increases (Decreases) for tax positions taken during a prior period
(189)596 44 
Increases for tax positions taken during the current period188 163 260 
Decreases due to settlements with taxing authorities— — — 
Decreases as a result of a lapse of applicable statutes of limitations(363)(193)(208)
Balance on December 31$1,125 $1,489 $923 
The balance of unrecognized tax benefits as of December 31, 2024 would reduce our effective tax rate if recognized. The total amount of unrecognized tax benefits as of December 31, 2024 is not expected to change significantly within the next 12 months.
The Company and its subsidiaries file a consolidated U.S. federal income tax return and various state income tax returns. As of December 31, 2024, with limited exceptions, we are no longer subject to examinations by taxing authorities for tax years prior to 2021 for federal and North Dakota income taxes and prior to 2020 for Minnesota state income taxes.
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Commitments and Contingencies
 12 Months Ended
Dec. 31, 2024
Commitments and Contingencies Disclosure [Abstract]  
Commitments and Contingencies Commitments and Contingencies
Commitments
Electric Utility Capacity and Energy Requirements. OTP has commitments for the purchase of capacity and energy requirements under contractual agreements, including wind power purchase agreements extending into 2048. Generally, the terms of OTP's wind power purchase agreements require OTP to purchase all of the electricity generated by a particular wind farm, but do not include fixed or minimum payments. The required payments are variable and the amounts due are determined based upon the amount of capacity available or electricity generated. Capacity and energy requirement costs under these agreements totaled $6.0 million, $5.6 million and $13.1 million for the years ended December 31, 2024, 2023 and 2022.
Coal Purchase Commitments. OTP has contracts providing for the purchase and delivery of its coal requirements. OTP’s current coal purchase agreement with CCMC for Coyote Station expires on December 31, 2040. All of Coyote Station’s coal requirements for the period covered must be purchased under this agreement. The agreement is structured so that the price of the coal covers all of CCMC's operating, financing and future mine reclamation costs. In the table below, we have estimated the future payments to be made under the terms of the agreement until its maturity. OTP has an agreement for the purchase of Big Stone Plant’s coal requirements through December 31, 2026. There is no fixed minimum purchase requirement, and no amounts for this agreement have been included in the table below; however, under this agreement all of Big Stone Plant’s coal requirements for the period
covered must be purchased under this agreement. Coal purchase costs under these two agreements totaled $44.7 million, $43.7 million and $45.1 million for the years ended December 31, 2024, 2023 and 2022.
Land Easement Payments. OTP has commitments to make payments for land easements not classified as leases. The contractual terms of these easements are generally 99 years or do not have a stated maturity date; however, per the terms of the agreements, our requirement to make payment ends once we cease use of the land. As such, in the table below, we have included payments under these easements through the estimated useful lives of the facilities associated with the easement. The commitments under these arrangements extend into 2055 and total approximately $62.0 million. Land easement costs under these agreements totaled $1.8 million, $1.8 million and $1.4 million for the years ended December 31, 2024, 2023 and 2022.
Other Commitments. As of December 31, 2024, we had commitments under contracts for plant maintenance, software subscriptions and other services extending into 2046 which totaled approximately $10.6 million.
Our future commitments as of December 31, 2024 were as follows:
(in thousands)Coal Purchase
Commitments		Land
 Easement
Payments		Other Commitments
2025$24,192 $1,897 $1,817 
202624,416 1,902 1,518 
202725,127 1,941 661 
202825,859 1,981 544 
202927,102 2,021 272 
Beyond 2029314,713 52,261 5,758 
Total$441,409 $62,003 $10,570 
Solar Development. On October 30, 2024, OTP entered into an agreement to acquire the assets of a solar facility currently under development. The assets to be acquired include real property rights and interests, interconnection agreements, state and local permits, and other development assets. Per the agreement, the purchase price is equal to $23.6 million, plus the reimbursement of certain interconnection costs and costs to purchase and store the main power transformer. Closing of the transaction is expected to occur in late 2025 or early 2026, and remains subject to certain conditions to close, including regulatory and other approvals. OTP would be subject to a termination fee of up to $5.0 million if the seller has satisfied all required conditions to close but the transaction is not consummated.
Contingencies
FERC ROE. In November 2013 and February 2015, customers filed complaints with FERC seeking to reduce the ROE component of the transmission rates that MISO transmission owners, including OTP, may collect under the MISO tariff rate. FERC issued an order on November 19, 2020, which adopted a revised ROE methodology and set the base ROE at 10.02% (10.52% with an adder) effective for the fifteen-month period from November 2013 to February 2015 and on a prospective basis beginning in September 2016. The order also dismissed any complaints covering the period from February 2015 to May 2016. On August 9, 2022, the U.S. Court of Appeals for the District of Columbia Circuit vacated the FERC order citing a lack of reasoned explanation by FERC in its adoption of its revised ROE methodology as outlined in its November 2020 order and remanded the matter to FERC to reopen the proceedings.
On October 17, 2024, FERC issued an Order on Remand modifying its ROE methodology and establishing a base ROE of 9.98% (10.48% with an adder) effective for the fifteen-month period from November 2013 to February 2015 and on a prospective basis beginning in September 2016, and required MISO transmission owners to provide refunds to customers for collections in excess of the base ROE of 9.98% for the applicable period, plus interest. In addition, FERC concluded the evidentiary record continues to support the ROE established for the period from February 2015 to May 2016.
Prior to FERC's Order on Remand, we had deferred recognition of certain revenues and recognized a refund liability which reflected the amount previously collected under the MISO tariff rate that we anticipated would be refunded to customers. Our previous estimated refund amount was larger than the actual amount ordered by FERC in the Order on Remand and was therefore reduced, which resulted in a pre-tax benefit of $2.5 million recognized in our consolidated statements of income for the year ended December 31, 2024. The balance of the recorded refund liability as of December 31, 2024 was $0.5 million.
Self-Funding of Transmission Upgrades for Generator Interconnections. FERC has granted transmission owners within MISO and other regional transmission organizations (RTOs) the unilateral authority to determine the funding mechanism for interconnection transmission upgrades that are necessary to accommodate new generation facilities connecting to the electrical grid. Under existing FERC orders, transmission owners can unilaterally determine whether the generator pays the transmission owner in advance for the transmission upgrade or, alternatively, the transmission owner can elect to fund the upgrade and recover over time from the generator the cost of and a return on the upgrade investment (a self-funding). FERC’s orders granting transmission owners this unilateral funding authority have been judicially contested on the basis that transmission owners may be motivated to discriminate among generators in making funding determinations. In the most recent judicial proceedings, the petitioners argued to the U.S. Court of Appeals for the District of Columbia that FERC did not comply with a previous judicial order to fully develop a record regarding the risk of discrimination and the financial risk absorbed by transmission owners for generator-funded upgrades. In December 2022, the Court of Appeals ruled in favor of the petitioners remanding the matter to FERC, instructing the agency to adequately explain the basis of its orders. The Court of Appeals decision did not vacate transmission owners’ unilateral funding authority.
In June 2024, FERC issued an Order to Show Cause proceeding against four RTOs, including MISO. Within its order, FERC indicates that the transmission tariffs of the RTOs appear to be unjust, unreasonable, and unduly discriminatory or preferential because they allow transmission owners to unilaterally elect transmission owner self-funding, which may increase costs, impose barriers to transmission interconnection and result in undue discrimination among interconnection customers.
The order required each RTO to submit filings to either 1) show cause as to why the transmission tariff remains just and reasonable and not duly discriminatory or preferential, or 2) to explain what changes to the tariff it believes would remedy the identified concerns. FERC has received a number of responses to its Order to Show Cause. In September 2024, in separate filings, MISO and transmission owners within MISO, including OTP, filed responses outlining the reasons why the self-funding option remains just and reasonable and not unduly discriminatory or preferential. Other responses have been provided by other RTOs, individual transmission owners, developers of renewable generation facilities and other interested parties.
OTP, as a transmission owner in MISO, has exercised its authority and elected to self-fund previous transmission upgrades necessary to accommodate new system generation. Under such an election, OTP is recovering the cost of the transmission upgrade and a return on that investment from the generator over a contractual period of time. Should the resolution of this matter eliminate transmission owners’ unilateral funding authority on either a prospective or retrospective basis, our financial results would be impacted. We cannot at this time reasonably predict the outcome of this matter given the uncertainty as to how FERC may ultimately decide on the matter after RTOs' filings in response to the Order to Show Cause.
Class Action Lawsuits. Several class action complaints against certain PVC pipe manufacturers, including OTC, have been filed in the U.S. District Court for the Northern District of Illinois alleging violations of antitrust laws. The first of the complaints was filed on August 23, 2024. The various complaints have been consolidated under the caption In re: PVC Pipe Antitrust Litigation (Case No. 1:24-cv-07639). Specifically, the complaints allege, among other things, that beginning in at least January 2021, the defendants conspired and combined to fix, raise, maintain and stabilize the price of PVC municipal water and electrical conduit pipe in violation of U.S. antitrust laws. The plaintiffs are seeking treble damages, injunctive relief, pre- and post-judgment interest, costs and attorneys’ fees.
In addition, on August 27, 2024, the Company received a grand jury subpoena issued by the U.S. District Court for the Northern District of California, from the U.S. Department of Justice (DOJ) Antitrust Division. The subpoena calls for production of documents regarding the manufacturing, selling and pricing of PVC pipe. The Company is responding to the subpoena and intends to comply with its obligations under the subpoena.
At this time, we are unable to determine the likelihood of an outcome or estimate a range of reasonably possible losses, if any, arising from the class action complaints or the DOJ investigation. However, if an antitrust violation by the Company is found, it could have a material impact on the Company’s financial condition, operating results and liquidity. The Company believes that there are factual and legal defenses to the allegations in the complaints and intends to defend itself accordingly.
Other Contingencies. We are party to litigation and regulatory matters arising in the normal course of business. We regularly analyze relevant information and, as necessary, estimate and record accrued liabilities for legal, regulatory enforcement and other matters in which a loss is probable of occurring and can be reasonably estimated. We believe the effect on our consolidated operating results, financial position and cash flows, if any, for the disposition of all matters pending as of December 31, 2024, other than those discussed above, will not be material.
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Stockholders' Equity
 12 Months Ended
Dec. 31, 2024
Equity [Abstract]  
Stockholders' Equity Stockholders' Equity
Capital Structure
In addition to authorized and outstanding common stock, the Company has 1,500,000 authorized no par value cumulative preferred shares and 1,000,000 authorized no par value cumulative preference shares. No cumulative preferred or cumulative preference shares were outstanding at December 31, 2024 or 2023.
Registration Statements
On May 3, 2024, we filed a shelf registration statement with the SEC under which we may offer for sale, from time to time, either separately or together in any combination, equity, debt or other securities described in the shelf registration statement. The registration statement expires in May 2027. No shares were issued pursuant to the shelf registration statement in 2024.
On May 3, 2024, we filed a second registration statement with the SEC for the issuance of up to 1,500,000 common shares under an Automatic Dividend Reinvestment and Share Purchase Plan, which provides shareholders, retail customers of OTP and other interested investors methods of purchasing our common shares by reinvesting their dividends or making optional cash investments. Shares purchased under the plan may be new issue common shares or common shares purchased on the open market. In 2024, we issued 70,469 common shares under this program and no proceeds were received, as all shares issued were purchased on the open market. As of December 31, 2024, 1,429,531 shares remained available for purchase or issuance under the plan. The registration statement expires in May 2027.
Dividend Restrictions
OTC is a holding company with no significant operations of its own. The primary source of funds for payments of dividends to our shareholders is from intercompany distributions made by OTC's subsidiaries to OTC.
As a result of certain statutory limitations or regulatory or financing agreements, restrictions could occur on the amount of distributions allowed to be made by OTC's subsidiaries, as further described below:
Both the OTC Credit Agreement and OTP Credit Agreement contain restrictions on the payment of cash dividends upon a default or event of default, including failure to maintain certain financial covenants. As of December 31, 2024, we were in compliance with these financial covenants.
Under the Federal Power Act, a public utility may not pay dividends from any funds properly included in a capital account. What constitutes “funds properly included in a capital account” is undefined in the Federal Power Act and the related regulations; however, the FERC has consistently interpreted the provision to allow dividends to be paid as long as i) the source of the dividends is clearly disclosed, ii) the dividend is not excessive and iii) there is no self-dealing on the part of corporate officials.
The MPUC indirectly limits the amount of dividends OTP can pay to OTC by requiring an equity-to-total-capitalization ratio between 47.2% and 57.7%, with total capitalization not to exceed $2.2 billion based on OTP’s capital structure requirements as of December 31, 2024. As of December 31, 2024, OTP’s equity-to-total-capitalization ratio including short-term debt was 53.1% and its net assets restricted from distribution totaled approximately $834.5 million.
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Accumulated Other Comprehensive Income (Loss)
 12 Months Ended
Dec. 31, 2024
Equity [Abstract]  
Accumulated Other Comprehensive Income (Loss) Accumulated Other Comprehensive Income (Loss)
The Company's other comprehensive income (loss) consists of unamortized actuarial losses and prior service costs related to pension and other postretirement benefits and unrealized gains and losses on marketable securities classified as available-for-sale. The income tax expense or benefit associated with amounts reclassified from accumulated other comprehensive income (loss) and reflected in the consolidated statements of income are recognized in the same period as the amounts are reclassified.
The following table shows the changes in accumulated other comprehensive Income (loss) for the years ended December 31, 2024, 2023 and 2022:
(in thousands)Pension and Other Postretirement BenefitsNet Unrealized Gain (Losses) on Available-for-Sale SecuritiesTotal
Balance, December 31, 2021
$(6,537)$13 $(6,524)
Other Comprehensive Income (Loss) Before Reclassifications, net of tax7,331 (433)6,898 
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss)540 
(1)
1 
(2)
541 
Total Other Comprehensive Income (Loss)7,871 (432)7,439 
Balance, December 31, 2022
1,334 (419)915 
Other Comprehensive Income Before Reclassifications, net of tax
59 180 239 
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss)(18)
(1)
12 
(2)
(6)
Total Other Comprehensive Income
41 192 233 
Balance, December 31, 2023
1,375 (227)1,148 
Other Comprehensive Income Before Reclassifications, net of tax501 407 908 
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss)(1,503)
(1)
(21)
(2)
(1,524)
Total Other Comprehensive Income (Loss)
(1,002)386 (616)
Balance, December 31, 2024
$373 $159 $532 
(1) Included in the computation of net periodic pension and other postretirement benefit costs. See Note 11 for further information.
(2) Included in other income (expense), net on the accompanying consolidated statements of income.
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Share-Based Payments
 12 Months Ended
Dec. 31, 2024
Share-Based Payment Arrangement [Abstract]  
Share-Based Payments Share-Based Payments
Employee Stock Purchase Plan
The 1999 Employee Stock Purchase Plan authorizes the issuance of 1,400,000 common shares, allowing eligible employees to purchase our common shares through payroll withholding at a discount of up to 15% off the market price at the end of each six-month purchase period. Employee withholding amounts may not be less than $10 or more than $2,000 per month, subject to certain limitations, as described in the plan. A plan participant may cease making payroll deductions at any time. A participant may not purchase more than 2,000 shares in a given six-month purchase period under the plan and may not purchase more than $25,000 (fair market value) of common shares under the plan and all other purchase plans (if any) in a calendar year. A participant may withdraw from the plan at any time and elect to receive the balance of their contributions to the plan that have not yet been used to purchase shares. Shares purchased under the plan are automatically enrolled in the Company's dividend reinvestment plan. Shares purchased under the plan may not be assigned, transferred, pledged, or otherwise disposed, except for certain situations allowed by the plan, such as upon death, for a period of 18 months after purchase. At our discretion, shares purchased under the plan can be either new issue shares or shares purchased in the open market. The plan shall automatically terminate when all of the shares authorized under the plan have been issued.
We recognize the 15% discount to the fair market value of the purchased shares as stock-based compensation expense, which amounted to $0.4 million, $0.3 million and $0.3 million for the years ended December 31, 2024, 2023 and 2022. For the years ended December 31, 2024, 2023 and 2022, the amount of shares issued under the plan amounted to 31,252, 26,348 and 26,420 shares. As of December 31, 2024, there were 206,115 shares available for purchase under the plan.
Share-Based Compensation Plan
The 2023 Stock Incentive Plan, which was approved by our shareholders in April 2023, authorizes the issuance of 979,891 common shares, including 500,000 newly requested common shares, for the granting of stock options, stock appreciation rights, restricted
stock, restricted stock units, dividend equivalents, performance awards and other stock-based awards. In addition, common shares subject to any outstanding awards under our prior stock incentive plans that are forfeited, canceled or reacquired by the Company will become available for re-issuance under the 2023 Stock Incentive Plan. As of December 31, 2024, 828,761 shares were available for issuance under the plan. The plan terminates on April 17, 2033.
We grant restricted stock awards to our employees and members of our Board of Directors and stock performance awards to our executive officers and certain other key employees as part of our long-term compensation and retention program. Stock-based compensation cost, recognized within operating expenses in the consolidated statements of income, amounted to $9.1 million, $7.4 million and $6.6 million for the years ended December 31, 2024, 2023 and 2022. The related income tax benefit recognized for these periods amounted to $2.7 million, $1.6 million and $1.7 million.
Restricted Stock Awards. Restricted stock awards are granted to executive officers and other key employees and members of the Company's Board of Directors. The awards vest, depending on award recipient, either ratably over a period of three to four years or cliff vest after four years. Vesting is accelerated in certain circumstances, including upon retirement. Awards granted to members of the Board of Directors are issued and outstanding upon grant and carry the same voting and dividend rights of unrestricted outstanding common stock. Awards granted to executive officers are eligible to receive dividend equivalent payments during the vesting period, subject to forfeiture under the terms of the agreement, but such awards are not issued or outstanding upon grant and do not provide for voting rights.
The grant-date fair value of each restricted stock award is determined based on the market price of the Company's common stock on the date of grant adjusted to exclude the value of dividends for those awards that do not receive dividend or dividend equivalent payments during the vesting period.
The following is a summary of restricted stock award activity for the year ended December 31, 2024:
SharesWeighted-Average
Grant-Date
Fair Value
Nonvested, Beginning of Year148,913 $56.48 
Granted52,425 85.25 
Vested(57,771)52.78 
Forfeited(150)66.39 
Nonvested, End of Year143,417 $68.47 
The weighted-average grant-date fair value of granted awards was $85.25, $68.03 and $59.95 during the years ended December 31, 2024, 2023 and 2022. The fair value of vested awards was $5.1 million, $3.1 million and $3.0 million during the years ended December 31, 2024, 2023 and 2022. As of December 31, 2024, there was $3.7 million of unrecognized compensation cost for unvested restricted stock awards to be recognized over a weighted-average period of 1.5 years.
Stock Performance Awards. Stock performance awards are granted to executive officers and certain other key employees. The awards vest at the end of a three-year performance period. The number of common shares awarded, if any, at the end of the performance period ranges from zero to 150% of the target amount based on two performance measures i) total shareholder return relative to a peer group (TSR component) and ii) return on equity (ROE component). The awards have no voting or dividend rights during the vesting period. Vesting of the awards is accelerated in certain circumstances, including upon retirement. The number of common shares awarded on an accelerated vesting is based on actual performance at the end of the performance period.
The grant-date fair value of the ROE component of the stock performance awards granted during the years ended December 31, 2024, 2023 and 2022 was determined using the grant date stock price and a discounted cash flow analysis to adjust for expected unearned dividends during the vesting period. The grant-date fair value of the TSR component of the stock performance awards granted during the years ended December 31, 2024, 2023 and 2022 was determined using a Monte Carlo fair value simulation model incorporating the following assumptions:
202420232022
Risk-free interest rate4.16 %4.15 %1.52 %
Expected term (in years)3.003.003.00
Expected volatility35.10 %34.00 %32.00 %
Dividend yield2.40 %2.50 %2.90 %
The risk-free interest rate was derived from yields on U.S. government bonds of a similar term. The expected term of the award is equal to the three-year performance period. Expected volatility was estimated based on actual historical volatility of our common stock over a five-year period. Dividend yield was estimated based on historic and future yield estimates.
The following is a summary of stock performance award activity for the year ended December 31, 2024 (share amounts reflect awards at target):
 SharesWeighted-Average
Grant-Date
Fair Value
Nonvested, Beginning of Year194,200 $50.33 
Granted43,400 94.45 
Vested(92,800)42.06 
Forfeited— — 
Nonvested, End of Year144,800 $68.85 
The weighted-average grant-date fair value of granted awards was $94.45, $61.97 and $54.91 during the years ended December 31, 2024, 2023 and 2022. The fair value of vested awards was $12.3 million, $5.3 million and $5.1 million during the years ended December 31, 2024, 2023 and 2022. As of December 31, 2024, there was $0.5 million of unrecognized compensation cost of unvested stock performance awards to be recognized over a weighted-average period of 0.65 years.
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Earnings Per Share
 12 Months Ended
Dec. 31, 2024
Earnings Per Share [Abstract]  
Earnings Per Share Earnings Per Share
The numerator used in the calculation of both basic and diluted earnings per share is net income. The denominator used in the calculation of basic earnings per share is the weighted-average number of shares outstanding during the period. The denominator used in the calculation of diluted earnings per share is derived by adjusting basic shares outstanding for the dilutive effect of potential shares outstanding, which consist of shares associated with time- and performance-based stock awards and our employee stock purchase plan.
The following includes the computation of the denominator for basic and diluted weighted-average shares outstanding for the years ended December 31, 2024, 2023 and 2022:
(in thousands)202420232022
Weighted Average Common Shares Outstanding – Basic41,778 41,668 41,586 
Effect of Dilutive Securities:
Stock Performance Awards196 269 248 
Restricted Stock Awards96 100 95 
Employee Stock Purchase Plan Shares and Other2 2 2 
Dilutive Effect of Potential Common Shares294 371 345 
Weighted Average Common Shares Outstanding – Diluted42,072 42,039 41,931 
The number of shares excluded from diluted weighted-average common shares outstanding because such shares were anti-dilutive was not material for the years ended December 31, 2024, 2023 and 2022.
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Derivative Instruments
 12 Months Ended
Dec. 31, 2024
Derivative Instruments and Hedging Activities Disclosure [Abstract]  
Derivative Instruments Derivative Instruments
OTP enters into derivative instruments to manage its exposure to future commodity price variability, specifically future wholesale energy and natural gas prices, and reduce volatility in prices for our retail electric customers. These derivative instruments are not designated as qualifying hedging transactions but provide for an economic hedge against future price variability. The instruments are recorded at fair value on the consolidated balance sheets on a gross basis with assets and liabilities presented separately. In
accordance with rate-making and cost recovery processes, we recognize a regulatory asset or liability to defer losses or gains from derivative activity until settlement of the associated derivative instrument.
As of December 31, 2024 and 2023, OTP had multiple outstanding pay-fixed, receive-variable swap agreements. The contracts outstanding as of December 31, 2024 had various settlement dates throughout 2025. The following presents the notional amounts and fair value of our derivative instruments as of December 31, 2024 and 2023:
(in thousands)20242023
Megawatt hours of electricity167187
Derivative Liabilities:
Other Current Liabilities$1,989 $4,210 
Other Noncurrent Liabilities— — 
Total Derivative Liabilities$1,989 $4,210 
During the years ended December 31, 2024 and 2023, contracts matured and were settled in an aggregate amount of a $3.5 million loss and a $16.5 million loss, respectively. Gains and losses recognized on the settlement of derivative instruments are returned to, or recovered from, our electric customers through fuel recovery mechanisms in each state. When recognized in the consolidated statements of income, these gains or losses are included in electric purchased power. Gains or losses related to the settlement of derivative instruments are included in cash flows from operations in the consolidated statements of cash flows.
v3.25.0.1
Fair Value Measurements
 12 Months Ended
Dec. 31, 2024
Fair Value Disclosures [Abstract]  
Fair Value Measurements Fair Value Measurements
The following tables present our assets and liabilities measured at fair value on a recurring basis as of December 31, 2024 and 2023 classified by the input method used to measure fair value:
Level 1Level 2Level 3
December 31, 2024
Assets
Investments:
Money Market Funds$596 $— $— 
Mutual Funds10,653 — — 
Corporate Debt Securities— 1,628 — 
Government Debt Securities— 61,131 — 
Total Assets11,249 62,759 — 
Liabilities
Derivative Instruments— 1,989 — 
Total Liabilities$— $1,989 $— 
(in thousands)Level 1Level 2Level 3
December 31, 2023
Assets
Investments:
Money Market Funds$3,125 $— $— 
Mutual Funds7,771 — — 
Corporate Debt Securities— 1,579 — 
Government Debt Securities— 7,724 — 
Total Assets$10,896 $9,303 $— 
Liabilities
Derivative Instruments$— $4,210 $— 
Total Liabilities$— $4,210 $— 
Level 1 fair value measurements are based on quoted prices (unadjusted) in active markets for identical assets or liabilities that we have the ability to access at the measurement date.
The level 2 fair value measurements for government and corporate debt securities are determined based on valuations provided by third parties which utilize industry accepted valuation models and observable market inputs to determine valuation. Some valuations or model inputs used by the pricing services may be based on broker quotes.
The level 2 fair value measurements for derivative instruments are determined by using inputs such as forward electric commodity prices, adjusted for location differences. These inputs are observable in the marketplace throughout the full term of the instrument, can be derived from observable data or are supported by observable levels at which transactions are executed in the marketplace.
In addition to assets recorded at fair value on a recurring basis, we also hold financial instruments that are not recorded at fair value in the consolidated balance sheets but for which disclosure of the fair value of these financial instruments is provided. The following reflects the carrying value and estimated fair value of these assets and liabilities as of December 31, 2024 and 2023:
 December 31, 2024December 31, 2023
(in thousands)Carrying
Amount		Fair ValueCarrying
Amount		Fair Value
Assets:
Cash and Cash Equivalents$294,651 $294,651 $230,373 $230,373 
Total294,651 294,651 230,373 230,373 
Liabilities:
Short-Term Debt69,615 69,615 81,422 81,422 
Long-Term Debt943,734 806,826 824,059 710,839 
Total$1,013,349 $876,441 $905,481 $792,261 
The following methods and assumptions were used to estimate the fair value of each class of financial instruments for which it is practicable to estimate that fair value:
Cash Equivalents: The carrying amount approximates fair value because of the short-term maturity of these instruments. Fair value is determined based on quoted prices in active markets, a Level 1 fair value input.
Short-Term Debt: The carrying amount approximates fair value because the debt obligations are short-term in nature and balances outstanding are subject to variable rates of interest which reset frequently, a Level 2 fair value input.
Long-Term Debt: The fair value of long-term debt is estimated based on current market indications for borrowings of similar maturities with similar terms, a Level 2 fair value input.
v3.25.0.1
Schedule I - Condensed Financial Information of Registrant
 12 Months Ended
Dec. 31, 2024
Condensed Financial Information Disclosure [Abstract]  
Schedule I - Condensed Financial Information of Registrant
SCHEDULE I - CONDENSED FINANCIAL INFORMATION OF REGISTRANT
OTTER TAIL CORPORATION (PARENT COMPANY)
CONDENSED BALANCE SHEETS
December 31,
(in thousands)20242023
Assets
Current Assets
Cash and Cash Equivalents$291,575 $228,137 
Accounts Receivable from Subsidiaries5,642 2,555 
Interest Receivable from Subsidiaries117 117 
Notes Receivable from Subsidiaries4,706 — 
Other3,538 977 
Total Current Assets305,578 231,786 
Investments in Subsidiaries2,006,239 1,725,584 
Notes Receivable from Subsidiaries78,900 78,900 
Deferred Income Taxes69,781 65,244 
Other Assets109,057 50,795 
Total Assets$2,569,555 $2,152,309 
Liabilities and Stockholders' Equity
Current Liabilities
Accounts Payable to Subsidiaries$7 $7 
Notes Payable to Subsidiaries752,625 568,672 
Other19,100 15,320 
Total Current Liabilities771,732 583,999 
Other Noncurrent Liabilities49,424 45,455 
Commitments and Contingencies
Capitalization
Long-Term Debt79,900 79,849 
Common Stockholders' Equity1,668,499 1,443,006 
Total Capitalization1,748,399 1,522,855 
Total Liabilities and Stockholders' Equity$2,569,555 $2,152,309 
See accompanying notes to condensed financial statements.
OTTER TAIL CORPORATION (PARENT COMPANY)
CONDENSED STATEMENTS OF INCOME
Years Ended December 31,
(in thousands)202420232022
Income
Equity Income in Earnings of Subsidiaries$304,525 $294,467 $296,833 
Interest Income from Subsidiaries3,107 2,898 3,382 
Other Income15,085 10,496 466 
Total Income322,717 307,861 300,681 
Expense
Nonelectric Selling, General, and Administrative Expenses23,016 12,816 17,269 
Interest Expense3,599 3,813 4,066 
Interest Expense from Subsidiaries5 6 5 
Nonservice Cost Components of Postretirement Benefits970 1,063 1,023 
Total Expense27,590 17,698 22,363 
Income Before Income Taxes295,127 290,163 278,318 
Income Tax Benefit6,535 4,028 5,866 
Net Income$301,662 $294,191 $284,184 
See accompanying notes to condensed financial statements.
OTTER TAIL CORPORATION (PARENT COMPANY)
CONDENSED STATEMENTS OF CASH FLOWS
Years Ended December 31,
(in thousands)202420232022
Cash Flows from Operating Activities
Net Cash Provided by Operating Activities$76,333 $77,139 $28,807 
Cash Flows from Investing Activities
Investment in Subsidiaries(55,000)(40,000)(50,000)
Purchases of Investments and Other Assets
(53,085)(1,754)(3,175)
Other, net1,394 1,686 1,480 
Net Cash Used in Investing Activities(106,691)(40,068)(51,695)
Cash Flows from Financing Activities
Net (Repayments) Borrowings on Short-Term Debt— — (22,637)
Borrowings from Subsidiaries179,247 148,308 236,926 
Payments for Shares Withheld for Employee Tax Obligations(6,457)(3,088)(2,942)
Dividends Paid(78,265)(73,061)(68,755)
Other, net(729)(339)(461)
Net Cash Provided by Financing Activities
93,796 71,820 142,131 
Net Change in Cash and Cash Equivalents63,438 108,891 119,243 
Cash and Cash Equivalents at Beginning of Period228,137 119,246 3 
Cash and Cash Equivalents at End of Period$291,575 $228,137 $119,246 
 
See accompanying notes to condensed financial statements.
OTTER TAIL CORPORATION (PARENT COMPANY)
NOTES TO CONDENSED FINANCIAL STATEMENTS
Incorporated by Reference
OTC’s consolidated statements of comprehensive income and common shareholders’ equity in Part II, Item 8 are incorporated by reference.
Basis of Presentation
The condensed financial information of OTC is presented to comply with Rule 12-04 of Regulation S-X. The unconsolidated condensed financial statements do not reflect all of the information and notes normally included with financial statements prepared in accordance with generally accepted accounting principles. Therefore, these condensed financial statements should be read with the consolidated financial statements and related notes included in this report on Form 10-K.
OTC’s investments in subsidiaries are presented under the equity method of accounting. Under this method, the assets and liabilities of the subsidiaries are not consolidated. The investments in net assets of the subsidiaries are recorded in the balance sheets. The income from operations of the subsidiaries is reported on a net basis as equity income in earnings of subsidiaries.
Related Party Transactions
Outstanding receivables from and payables to OTC's subsidiaries as of December 31, 2024 and 2023 are as follows:
(in thousands)Accounts
Receivable		Interest
Receivable		Current
Notes
Receivable		Long-Term
Notes
Receivable		Accounts
Payable
Current
Notes
Payable
December 31, 2024
Otter Tail Power Company$5,223 $— $— $— $7 $— 
Northern Pipe Products, Inc.36 7 — 5,000 — 66,170 
Vinyltech Corporation— 17 — 11,500 — 90,764 
BTD Manufacturing, Inc.— 78 — 52,000 — 5,662 
T.O. Plastics, Inc.42 15 4,706 10,400 — — 
Varistar Corporation— — — — — 590,029 
Otter Tail Assurance Limited341 — — — — — 
Total
$5,642 $117 $4,706 $78,900 $7 $752,625 
December 31, 2023
Otter Tail Power Company$2,415 $— $— $— $7 $— 
Northern Pipe Products, Inc.— 7 — 5,000 — 56,917 
Vinyltech Corporation14 17 — 11,500 — 98,016 
BTD Manufacturing, Inc.— 78 — 52,000 — 6,291 
T.O. Plastics, Inc.36 15 — 10,400 — 980 
Varistar Corporation— — — — — 406,468 
Otter Tail Assurance Limited90 — — — — — 
Total
$2,555 $117 $— $78,900 $7 $568,672 
Dividends
Dividends paid to OTC (the Parent) from its subsidiaries were as follows:
(in thousands)202420232022
Cash Dividends Paid to Parent by Subsidiaries$78,191 $72,982 $68,680 
See OTC’s notes to consolidated financial statements in Part II, Item 8 for other disclosures.
v3.25.0.1
Schedule II - Valuation and Qualifying Accounts and Reserves
 12 Months Ended
Dec. 31, 2024
SEC Schedule, 12-09, Valuation and Qualifying Accounts [Abstract]  
Schedule II - Valuation and Qualifying Accounts and Reserves
SCHEDULE II - VALUATION AND QUALIFYING ACCOUNTS AND RESERVES
OTTER TAIL CORPORATION
Below is a summary of activity within valuation and qualifying accounts for the years ended December 31, 2024, 2023 and 2022:
(in thousands)Balance, January 1Charged to Cost and Expenses
Deductions(1)
Balance, December 31
Allowance for Credit Losses
2024$2,522 $1,242 $(1,844)$1,920 
20231,648 2,014 (1,140)2,522 
20221,836 909 (1,097)1,648 
(1)Amounts reflect deductions to the allowance for amounts written-off, net of recoveries.
v3.25.0.1
Pay vs Performance Disclosure - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2024
Dec. 31, 2023
Dec. 31, 2022
Pay vs Performance Disclosure      
Net Income $ 301,662 $ 294,191 $ 284,184
v3.25.0.1
Insider Trading Arrangements
 3 Months Ended
Dec. 31, 2024
Trading Arrangements, by Individual  
Rule 10b5-1 Arrangement Adopted false
Non-Rule 10b5-1 Arrangement Adopted false
Rule 10b5-1 Arrangement Terminated false
Non-Rule 10b5-1 Arrangement Terminated false
v3.25.0.1
Insider Trading Policies and Procedures
 12 Months Ended
Dec. 31, 2024
Insider Trading Policies and Procedures [Line Items]  
Insider Trading Policies and Procedures Adopted true
v3.25.0.1
Cybersecurity Risk Management and Strategy Disclosure
 12 Months Ended
Dec. 31, 2024
Cybersecurity Risk Management, Strategy, and Governance [Line Items]  
Cybersecurity Risk Management Processes for Assessing, Identifying, and Managing Threats [Text Block]
Our cybersecurity policies and practices, which are based on the Center for Information Security (CIS) Critical Security Controls, are governed by our information and cybersecurity governance program. The CIS Critical Security Controls are a set of 18 cybersecurity-related controls which aid companies in designing an effective control environment and are viewed as best practices by organizations worldwide. A significant number of our cybersecurity policies and practices associated with our electric utility operations are also subject to regulation by multiple governmental and other agencies.
Our information and cybersecurity governance program is the foundation of our cybersecurity risk management strategy. The program includes policies which authorize and guide the development of procedures, standards and guidelines for personnel activities, incident prevention and reporting, and compliance monitoring. Cybersecurity policies, procedures and controls are reviewed and approved by our Information and Cybersecurity Program (ICSP) group annually, with amendments made as deemed necessary for any updates for regulatory compliance and best practices, legal privacy protection and information protection, or to reflect current technology or new methods for ensuring secure business procedures.
We perform a corporate risk assessment annually, which includes specific consideration and assessment of cybersecurity risk. As part of our risk assessment process, we incorporate results from procedures performed by third-party consultants. We utilize third-party consultants to perform penetration and vulnerability testing and monitoring, as well as overall cybersecurity control testing. Potential risks associated with the use of third-party service providers are monitored and managed through an established service provider management policy. Service providers must meet certain security requirements such as security incident or data breach notification and response protocols, data encryption requirements and data disposal commitments.
In managing cybersecurity risk, we employ a defense-in-depth strategy and regularly monitor our cyber environment for potential new threats. Our strategy includes employee training and awareness on cybersecurity risks and related best practices, required
password complexity, the use of multi-factor authentication, information security protocols, anti-virus and anti-ransomware software, a patch management program, the execution of tabletop exercises on a periodic basis, established policies and protocols for cyber incident response planning and reporting, and ongoing internal cybersecurity testing.
Cybersecurity Risk Management Processes Integrated [Flag] true
Cybersecurity Risk Management Processes Integrated [Text Block] Our cybersecurity risk management is integrated into our overall risk management system through our internal business risk management process.
Cybersecurity Risk Management Third Party Engaged [Flag] true
Cybersecurity Risk Third Party Oversight and Identification Processes [Flag] true
Cybersecurity Risk Materially Affected or Reasonably Likely to Materially Affect Registrant [Flag] false
Cybersecurity Risk Board of Directors Oversight [Text Block] Our Board of Directors provides oversight of our cybersecurity program through quarterly and annual risk reviews and cybersecurity reporting. On a quarterly basis, cybersecurity risk and mitigation strategies are reviewed as part of our business risk management group's reporting to the Board of Directors, which includes the reporting of significant business risks, including cybersecurity mitigation strategies employed to manage these risks and a review of any emerging risks. At least annually, our Vice President of IT provides an overview of our cybersecurity program to the Board of Directors, including a review of key strategies, emerging risks and a summary of key performance indicators. In addition, annually the Board of Directors reviews the results of our penetration and vulnerability testing.
Cybersecurity Risk Board Committee or Subcommittee Responsible for Oversight [Text Block] At the management level, our cyber program is managed by our ICSP group. The ICSP group consists of Information Technology (IT) managers, IT security subject matter experts, and internal audit personnel and is led by our Vice President of IT who has more than 25 years of experience in IT, enterprise security and cyber risk management, a Bachelor's degree of Science, CIS, Information Technology and Master's of Business, Information Systems, and holds Certified Information Systems Security Professional, Certified Information Security Manager and Certified Data Privacy Solution Engineer designations.
Cybersecurity Risk Process for Informing Board Committee or Subcommittee Responsible for Oversight [Text Block]
Our cybersecurity risk management is integrated into our overall risk management system through our internal business risk management process. Our business risk management group works closely with our ICSP group to regularly assess and identify possible material risks from cybersecurity threats, including but not limited to, financial, operations, reputational and regulatory impact to the Company, as well as impacts on our employees and customers. Their risk assessment results are reported to our Executive Risk Committee on a quarterly basis. The Executive Risk Committee, which is comprised of our executive officers, meets quarterly to identify and assess short-, medium- and long-term risks, and to ensure adequate mitigation strategies are implemented. During these meetings, the Executive Risk Committee reviews significant and emerging risks, including cybersecurity risks, and assesses the Company’s plans to mitigate or otherwise manage and monitor those risks.
Our Board of Directors provides oversight of our cybersecurity program through quarterly and annual risk reviews and cybersecurity reporting. On a quarterly basis, cybersecurity risk and mitigation strategies are reviewed as part of our business risk management group's reporting to the Board of Directors, which includes the reporting of significant business risks, including cybersecurity mitigation strategies employed to manage these risks and a review of any emerging risks. At least annually, our Vice President of IT provides an overview of our cybersecurity program to the Board of Directors, including a review of key strategies, emerging risks and a summary of key performance indicators. In addition, annually the Board of Directors reviews the results of our penetration and vulnerability testing.
Cybersecurity Risk Role of Management [Text Block]
At the management level, our cyber program is managed by our ICSP group. The ICSP group consists of Information Technology (IT) managers, IT security subject matter experts, and internal audit personnel and is led by our Vice President of IT who has more than 25 years of experience in IT, enterprise security and cyber risk management, a Bachelor's degree of Science, CIS, Information Technology and Master's of Business, Information Systems, and holds Certified Information Systems Security Professional, Certified Information Security Manager and Certified Data Privacy Solution Engineer designations. The ICSP group is in charge of developing, maintaining and measuring compliance with the information and cybersecurity governance program, as well as monitoring cyber incidents and implementing mitigation measures as part of an evolving, dynamic external environment. Our approach to cybersecurity incident reporting and response planning is governed by our incident response plans established for each of our business units. The plans outline the processes related to detecting, assessing, investigating, mitigating and remediating cyber incidents, as well the communication and reporting plan and the required personnel to be included in the process and communications.
Our cybersecurity risk management is integrated into our overall risk management system through our internal business risk management process. Our business risk management group works closely with our ICSP group to regularly assess and identify possible material risks from cybersecurity threats, including but not limited to, financial, operations, reputational and regulatory impact to the Company, as well as impacts on our employees and customers. Their risk assessment results are reported to our Executive Risk Committee on a quarterly basis. The Executive Risk Committee, which is comprised of our executive officers, meets quarterly to identify and assess short-, medium- and long-term risks, and to ensure adequate mitigation strategies are implemented. During these meetings, the Executive Risk Committee reviews significant and emerging risks, including cybersecurity risks, and assesses the Company’s plans to mitigate or otherwise manage and monitor those risks.
Cybersecurity Risk Management Positions or Committees Responsible [Flag] true
Cybersecurity Risk Management Positions or Committees Responsible [Text Block] At the management level, our cyber program is managed by our ICSP group. The ICSP group consists of Information Technology (IT) managers, IT security subject matter experts, and internal audit personnel and is led by our Vice President of IT who has more than 25 years of experience in IT, enterprise security and cyber risk management, a Bachelor's degree of Science, CIS, Information Technology and Master's of Business, Information Systems, and holds Certified Information Systems Security Professional, Certified Information Security Manager and Certified Data Privacy Solution Engineer designations. The ICSP group is in charge of developing, maintaining and measuring compliance with the information and cybersecurity governance program, as well as monitoring cyber incidents and implementing mitigation measures as part of an evolving, dynamic external environment. Our approach to cybersecurity incident reporting and response planning is governed by our incident response plans established for each of our business units. The plans outline the processes related to detecting, assessing, investigating, mitigating and remediating cyber incidents, as well the communication and reporting plan and the required personnel to be included in the process and communications.
Cybersecurity Risk Management Expertise of Management Responsible [Text Block] The ICSP group consists of Information Technology (IT) managers, IT security subject matter experts, and internal audit personnel and is led by our Vice President of IT who has more than 25 years of experience in IT, enterprise security and cyber risk management, a Bachelor's degree of Science, CIS, Information Technology and Master's of Business, Information Systems, and holds Certified Information Systems Security Professional, Certified Information Security Manager and Certified Data Privacy Solution Engineer designations.
Cybersecurity Risk Process for Informing Management or Committees Responsible [Text Block]
At the management level, our cyber program is managed by our ICSP group. The ICSP group consists of Information Technology (IT) managers, IT security subject matter experts, and internal audit personnel and is led by our Vice President of IT who has more than 25 years of experience in IT, enterprise security and cyber risk management, a Bachelor's degree of Science, CIS, Information Technology and Master's of Business, Information Systems, and holds Certified Information Systems Security Professional, Certified Information Security Manager and Certified Data Privacy Solution Engineer designations. The ICSP group is in charge of developing, maintaining and measuring compliance with the information and cybersecurity governance program, as well as monitoring cyber incidents and implementing mitigation measures as part of an evolving, dynamic external environment. Our approach to cybersecurity incident reporting and response planning is governed by our incident response plans established for each of our business units. The plans outline the processes related to detecting, assessing, investigating, mitigating and remediating cyber incidents, as well the communication and reporting plan and the required personnel to be included in the process and communications.
Our cybersecurity risk management is integrated into our overall risk management system through our internal business risk management process. Our business risk management group works closely with our ICSP group to regularly assess and identify possible material risks from cybersecurity threats, including but not limited to, financial, operations, reputational and regulatory impact to the Company, as well as impacts on our employees and customers. Their risk assessment results are reported to our Executive Risk Committee on a quarterly basis. The Executive Risk Committee, which is comprised of our executive officers, meets quarterly to identify and assess short-, medium- and long-term risks, and to ensure adequate mitigation strategies are implemented. During these meetings, the Executive Risk Committee reviews significant and emerging risks, including cybersecurity risks, and assesses the Company’s plans to mitigate or otherwise manage and monitor those risks.
Cybersecurity Risk Management Positions or Committees Responsible Report to Board [Flag] true
v3.25.0.1
Summary of Significant Accounting Policies (Policies)
 12 Months Ended
Dec. 31, 2024
Accounting Policies [Abstract]  
Basis of Accounting These consolidated financial statements are presented in accordance with U.S. generally accepted accounting principles and include the accounts of OTC and its wholly owned subsidiaries.
Principles of Consolidation All intercompany balances and transactions have been eliminated in consolidation except, as applicable, profits on sales to our regulated electric utility company from our nonregulated businesses, which is in accordance with the accounting requirements of regulated operations.
Use of Estimates
Use of Estimates
We use estimates based on the best information available in recording transactions and balances resulting from business operations. As better information becomes available or actual amounts are known, the recorded estimates are revised. Consequently, operating results can be affected by revisions to prior accounting estimates.
Regulatory Accounting
Regulatory Accounting
Our regulated electric utility company, OTP, is subject to regulation of rates and other matters by state utility commissions in Minnesota, North Dakota and South Dakota and by the FERC for certain interstate operations. OTP accounts for the financial effects of regulation in accordance with accounting guidance for regulated operations. This guidance allows for the recording of a regulatory asset for certain costs which otherwise would be recognized in the statements of income or comprehensive income based on an expectation that the cost will be recovered in future rates. This guidance also requires the recording of a regulatory liability for certain credits which would otherwise be recognized in the statements of income or comprehensive income based on an expectation that the amount will be returned to customers in future rates. Amounts recorded as regulatory assets and regulatory liabilities are generally recognized in the statements of income at the time they are reflected in customer rates. In the event OTP ceases to meet the criteria to apply the guidance for regulated operations, the regulatory assets and liabilities that no longer meet such criteria would be removed from the consolidated balance sheets and included in the consolidated statements of income as an expense or income item, or in the consolidated statements of comprehensive income as a loss or gain item, in the period in which the application of this guidance ceases.
Cash Equivalents
Cash Equivalents
We consider all highly liquid investments purchased with maturity dates of 90 days or less to be cash equivalents.
Concentration of Deposits
Concentration of Deposits
We hold deposits with financial institutions which potentially subject us to a concentration risk. These deposits are guaranteed by the Federal Deposit Insurance Corporation up to an insurance limit of $250,000. Currently, our cash deposits exceed federally insured levels.
Revenue from Contracts with Customers
Revenue from Contracts with Customers
Due to our diverse business operations, the recognition of revenue from contracts with customers depends on the product produced and sold or service performed. We recognize revenue from contracts with customers at prices that are fixed or determinable as evidenced by an agreement with the customer, when we have met our performance obligation under the contract and it is probable that we will collect the amount to which we are entitled in exchange for the goods or services transferred or to be transferred to the customer. Depending on the product produced and sold or service performed and the terms of the agreement with the customer, we recognize revenue either over time, in the case of delivery or transmission of electricity or related services or the production and storage of certain custom-made products, or at a point in time for the delivery of standardized products and other products made to customer specifications where the terms of the contract require transfer of the completed product. Provisions for sales returns, early payment discounts and volume-based variable pricing incentives are recorded as reductions to revenue at the time revenue is recognized based on customer history, historical information and current trends. We include revenues received for shipping and handling in operating revenues. Expenses paid for shipping and handling are recorded as part of cost of products sold. Sales or other taxes collected from customers are excluded from operating revenues.
Electric Segment Revenues. Most Electric segment revenues are earned from the generation, transmission and sale of electricity to retail customers at rates approved by state regulatory commissions. OTP also earns revenue from the transmission of electricity for others over the transmission assets it owns separately or jointly with other transmission service providers, under rate tariffs established by the independent transmission system operator and approved by FERC. A third source of revenue for OTP comes from the generation and sale of electricity to wholesale customers at contract or market rates. Revenues from all these sources meet the criteria to be classified as revenue from contracts with customers and are recognized over time as energy is delivered or transmitted. Revenue is recognized based on the metered quantity of electricity delivered or transmitted at the applicable rates. For electricity delivered and consumed after a meter is read but not yet billed to a customer, OTP records revenue and an unbilled receivable based on estimates of the amount of energy delivered and a composite rate per kwh consumed.
Manufacturing Segment Revenues. Our Manufacturing segment businesses earn revenue predominantly from the production and delivery of custom-made or standardized parts and products to customers across several industries and from the production and sale of tools and dies to other manufacturers. For the production and delivery of standardized products and other products made to customer specifications where the terms of the contract require transfer of the completed product, we have met our performance obligation and recognize revenue at the point in time when the product is shipped. At this point we have no further obligation to provide services related to such products. The shipping terms used in these transactions are free on board (FOB) shipping point.
Plastics Segment Revenues. Our Plastics segment businesses earn revenue predominantly from the sale and delivery of standardized PVC pipe products produced at their manufacturing facilities. Revenue from the sale of these products is recognized at the point in time when the product is shipped as there is no further obligation to provide services related to such products and the shipping terms are FOB shipping point. We have one customer within our Plastics segment for which we produce and store a product made to the customer’s specifications and design under a build and hold agreement. For sales to this customer, we recognize revenue as the custom-made product is produced, adjusting the amount of revenue for volume rebate variable pricing considerations we expect the customer will earn and applicable early payment discounts we expect the customer will take. Ownership of the pipe transfers to the customer prior to delivery and we are paid a negotiated fee for storage of the pipe. Revenue for storage of the pipe is recognized over time as the pipe is stored.
Alternative Revenue
Alternative Revenue
In addition to recognizing revenue from contracts with customers, our Electric segment business also records revenue under alternative revenue program (ARP) requirements. Certain rate rider mechanisms qualify as ARP revenues as they provide for adjustments to rates outside of a general rate case proceeding to encourage or incentivize investments in certain areas such as conservation, renewable energy, pollution reduction or control, improved infrastructure of the transmission grid or other programs that provide benefits to the general public under public policy, laws or regulations. ARP riders generally provide for the recovery of specified costs and investments and include an incentive component to provide the regulated utility with a return on amounts invested.
We accrue ARP revenue on the basis of costs incurred, investments made and returns on those investments that qualify for recovery through established riders. ARP revenue is disclosed separately from revenue from contracts with customers and we have elected to report ARP revenue on a net basis, whereby amounts initially recorded as ARP revenue in a period are presented net of the reversal of amounts previously recognized as ARP revenue that are reclassified and recorded as revenue from contracts with customers when such amounts are included in the price of electricity to customers.
Receivables and Allowance for Credit Losses
Receivables and Allowance for Credit Losses
We grant credit to our customers in the normal course of business with repayment terms generally ranging from 30 to 90 days after the invoice date. Late fees are assessed on certain receivables once they are 30 days past due. Unbilled receivables represent estimates of energy delivered to customers but not yet billed.
Receivables are stated at the billed or estimated unbilled amount less an allowance for estimated credit losses. An allowance for credit losses is established based on losses expected to occur over the contractual life of the receivable. We estimate an allowance for credit losses on our trade and unbilled receivables by evaluating historical aging and write-off history, adjusted for current and forecasted economic conditions, for groups of receivables that share similar economic characteristics. Other receivables are evaluated by reviewing individual accounts, considering aging, financial condition of the debtor, recent payment history and other relevant factors. Account balances are written off in the period they are deemed to be uncollectible.
Inventories
Inventories
Inventories are valued at the lower of cost or net realizable value. Costs for fuel, material and supply inventories of our Electric segment are determined on an average cost basis. Costs for raw material, work in process and finished goods inventories of our Manufacturing and Plastics segments are determined on a first-in first-out (FIFO) basis.
Investments
Investments
We invest in and hold, through rabbi trusts, corporate-owned life insurance policies to provide future funding for obligations under our supplemental pension plan and a nonqualified deferred compensation plan. The polices are recorded at cash surrender value and there are no restrictions on our ability to surrender the policies.
We hold debt, mutual fund, and money market fund investments either as investments within our captive insurance entity, to provide future funding for obligations under nonqualified deferred compensation plans or provide a return on our available cash and liquidity. These investments are recorded at fair value. Debt securities are deemed to be available-for-sale securities, accordingly unrealized gains and losses are generally excluded from earnings and recognized in accumulated other comprehensive income. We evaluate whether declines in the fair value of debt securities below the cost basis are other-than-temporary. Declines in fair value deemed to be other-than-temporary result in the recognition of unrealized losses, or a portion thereof, in earnings. Unrealized gains and losses on mutual and money market funds are recognized in earnings immediately.
Property, Plant and Equipment
Property, Plant and Equipment
Electric plant is stated at original cost less accumulated depreciation. The cost of additions includes purchased assets, contracted work, direct labor and materials, allocable overheads and allowance for funds used during construction (AFUDC). The amount of interest capitalized to electric plant was $1.9 million in 2024, $1.9 million in 2023 and $0.9 million in 2022. Significant additions or improvements that extend an asset's useful life are capitalized, while repairs and maintenance costs are expensed as incurred.
Depreciation is recognized on a straight-line basis over the asset's estimated useful life. For certain asset classes, we employ a group or composite method of depreciation in which certain assets are combined and depreciated over the average life of the combined asset group. Actuarial studies are periodically performed to assess the remaining useful lives and salvage values of our assets, with any changes in these estimates incorporated into depreciation on a prospective basis. Gains or losses on group or composite asset dispositions are recorded to accumulated depreciation and impact current and future depreciation rates.
Amounts recovered in rates for future removal costs are recorded as regulatory liabilities. Removal costs, when incurred, are charged against the regulatory liability.
Property, plant and equipment of our nonelectric operations are carried at historical cost less accumulated depreciation. Depreciation is recognized on a straight-line basis over the asset's estimated useful life. The cost of additions includes purchased assets, contracted work, direct labor and materials, allocable overheads and capitalized interest, as applicable. No interest was capitalized in 2024, 2023 or 2022. Maintenance and repairs are expensed as incurred. Gains or losses on asset dispositions are included in the determination of operating income.
Jointly Owned Facilities
Jointly Owned Facilities
OTP is a joint owner in two coal-fired steam-powered electric generation plants: Big Stone Plant near Big Stone City, South Dakota and Coyote Station near Beulah, North Dakota. OTP is also a joint owner, with other regional utilities, in several major transmission lines. OTP's interest in each jointly owned facility is reflected in the consolidated balance sheets on a pro-rata basis and OTP's share
of direct revenue and expenses are included in operating revenues and expenses in the consolidated statements of income. Each participant in the jointly owned facilities finances their own investments.
Goodwill and Other Intangible Assets
Goodwill and Other Intangible Assets
Goodwill is recognized and initially measured as any excess of the acquisition-date consideration transferred in a business combination over amounts recognized for the net identifiable assets acquired. Goodwill is not amortized, but is tested for impairment annually, or more frequently if an event occurs or circumstances change that would more likely than not result in an impairment of goodwill. Impairment testing is performed at the reporting unit level, which is defined as an operating segment or one level below an operating segment. We perform our impairment testing in the fourth quarter of each year and have identified three reporting units that carry a goodwill balance. We historically tested goodwill for impairment as of December 31st each year; however, in 2024, we elected to change the date of our annual goodwill impairment test to October 1st. We believe this new testing date allows us to better align our annual goodwill impairment testing procedures with our year-end financial reporting, as well as our annual budgeting and forecasting process. This change did not delay, accelerate or avoid the recognition of an impairment charge.
We perform a quantitative impairment assessment, electing to forgo the optional qualitative assessment. The quantitative assessment is a single-step test that identifies both the existence of impairment and the amount of impairment loss by comparing the estimated fair value of a reporting unit to its carrying value, with any excess carrying value over the fair value being recognized as an impairment loss.
Intangible assets with finite lives, which primarily consist of customer relationships, are carried at estimated fair value at the time of acquisition less accumulated amortization. The costs of the intangible assets are amortized over their estimated useful lives, which generally range from 15 to 20 years.
Cloud Computing Costs
Cloud Computing Costs
We capitalize implementation costs incurred in cloud computing arrangements that are service contracts consistent with capitalized implementation costs incurred to develop or obtain internal-use software. Costs are amortized on a straight-line basis over the life of the associated contract. Capitalized implementation costs are amortized over periods up to ten years. Capitalized costs and related accumulated amortization are included in other noncurrent assets on the consolidated balance sheets.
Leases
Leases
We recognize a right-of-use lease asset and a corresponding lease liability at the lease commencement date. The length of our lease agreements varies from less than one year to approximately ten years. We have elected to not record lease assets and liabilities for leases with a lease term at commencement of 12 months or less; such leases are expensed on a straight-line basis over the lease term. Certain of our leases contain options to renew or extend the lease term at our discretion if certain conditions are met. If a lease contains an option to extend the lease term and there is reasonable certainty the option will be exercised, the option is considered in the lease term at inception, or at such time when an event occurs which triggers the remeasurement of a lease, as applicable. In the determination of the lease term for one of our leased manufacturing facilities, we have incorporated the future lease renewals which we believe are reasonably certain to be exercised in the associated right-of-use asset and liability values.
We have elected to not separate non-lease components (e.g., common area maintenance) from lease components on real estate leases, accordingly the recognized lease asset and lease liability incorporate in their measurement payments for non-lease components. Certain leases include variable lease payments as the amounts are subject to change over the lease term; such amounts are not incorporated into the measurement of the right-of-use lease asset or lease liability. We are unable to determine the interest rate implicit in our leases, thus we apply our incremental borrowing rate to capitalize the right-of-use asset and lease liability. We estimate our incremental borrowing rate by reference to market interest rates on long-term debt, incorporating considerations of the credit quality of the lessee and the term of lease.
Recoverability of Long-Lived Assets
Recoverability of Long-Lived Assets
We review our long-lived assets including, among other assets, property, plant and equipment, amortizing intangible assets and right-of-use lease assets whenever events or changes in circumstances indicate the carrying amount of the assets may not be
recoverable. We determine potential impairment by comparing the carrying amount of the assets with the net cash flows expected to be provided by operating activities of the business or related assets. If the sum of the expected future net cash flows is less than the carrying amount of the assets, an impairment loss would be recognized. Such an impairment loss would be measured as the amount by which the carrying amount exceeds the fair value of the asset.
Pension Plans and Other Postretirement Benefits
Pension Plans and Other Postretirement Benefits
We maintain pension and postretirement benefit plans for eligible employees. Recognizing the cost of providing benefits and measuring the projected benefit obligation of these plans requires management to make various assumptions and estimates. Certain unrecognized actuarial gains and losses and unrecognized prior service costs or credits are deferred as regulatory assets and liabilities, rather than recorded as other comprehensive income, based on regulatory recovery mechanisms.
We have elected to apply a minimum amortization method for determining the amount of amortization of net cumulative gains or losses to be included as a component of net periodic benefit cost for any annual period. Cumulative gains and losses recognized in accumulated other comprehensive income or as a deferred regulatory asset or liability that are in excess of 10% of the projected benefit obligation or the market value of pension plan assets are amortized over the expected remaining future service period of active plan participants. In periods in which the cumulative gains and losses do not exceed 10%, no amortization to net period benefit cost is recognized.
Asset Retirement Obligations
Asset Retirement Obligations
Legal obligations related to the future retirement of long-lived assets are recognized as asset retirement obligations (ARO). An ARO is recognized in the period in which the legal obligation is incurred and the amount of the obligation can be reasonably estimated, with an offsetting increase to the associated long-lived asset. AROs are initially recognized at fair value and increased with the passage of time (accretion). ARO estimates are revised periodically with any adjustments reflected in the ARO and associated long-lived asset.
Income Taxes
Income Taxes
We use the asset and liability method to account for income taxes. Under this method, deferred tax assets and liabilities are recognized for the expected future tax consequences of all temporary differences between the carrying amounts of assets and liabilities and their respective tax bases. Deferred taxes are recorded using the tax rates scheduled by tax law to be in effect in the periods when the temporary differences reverse. Deferred tax assets are reduced by a valuation allowance when it is more likely than not that a portion or all of the deferred tax assets will not be realized. The realizability of deferred tax assets is determined by taking into consideration forecasts of future taxable income, the reversal of other existing temporary differences, available net operating loss carryforwards and available tax planning strategies. Changes in valuation allowances are included in the provision for income taxes in the period of the changes.
We recognize the tax effects of all tax positions that are more-likely-than-not to be sustained on audit based solely on the technical merits of those positions as of the balance sheet date. Changes in the recognition or measurement of such positions are recognized in the provision for income taxes in the period of the changes. We classify interest and penalties on tax uncertainties as components of the provision for income taxes within the consolidated statements of income.
We have elected to account for transferable tax credits as a component of our income tax provision. We recognize the benefit of PTCs as a reduction of income tax expense in the period the credit is generated, which corresponds to the period the energy production occurs. We apply the deferral method of accounting for ITCs and state wind energy credits. Under this method, ITCs and state wind energy credits are amortized as a reduction to income tax expense over the estimated useful lives of the underlying property that gave rise to the credit.
Deferred Compensation Plans
Deferred Compensation Plans
The Company sponsors two nonqualified deferred compensation plans for the benefit of executive officers and other select employees. Each plan allows participants to defer a specified amount or percentage of base wages or incentive compensation into the plan, subject to certain limitations. The Company, at its discretion, may make employer contributions to either plan during any annual period. Participant and employer deferred amounts are segregated into one or more accounts chosen by the participant. Participants earn a return on deferred amounts based on notional investments in the segregated accounts. Participants can elect lump sum distributions or annual installments of deferred balances during the participant's employment or upon retirement.
Stock-Based Compensation
Stock-Based Compensation
Stock-based compensation awards are measured at the grant-date fair value of the award and compensation expense is recognized on a straight-line basis over the applicable service or performance period. The service period may be limited to the period until such
time that a recipient is retirement eligible as determined under the award agreement. Awards granted to employees eligible for retirement on the date of grant are expensed in the period of grant. We recognize the effects of award forfeitures as they occur.
Fair Value Measurements
Fair Value Measurements
Fair value is defined as the price that would be received for an asset or paid to transfer a liability (an exit price) in the principal or most advantageous market for the asset or liability in an orderly transaction between market participants. Three levels of inputs may be used to measure fair value:
Level 1 – Quoted prices are available in active markets for identical assets or liabilities as of the reported date. The types of assets and liabilities included in Level 1 are highly liquid and actively traded instruments with quoted prices, such as equities listed on the New York Stock Exchange and commodity derivative contracts listed on the New York Mercantile Exchange.
Level 2 – Pricing inputs are other than quoted prices in active markets but are either directly or indirectly observable as of the reported date. The types of assets and liabilities included in Level 2 are typically either comparable to actively traded securities or contracts, such as treasury securities with pricing interpolated from recent trades of similar securities, or priced with models using highly observable inputs, such as commodity options priced using observable forward prices and volatilities. 
Level 3 – Significant inputs to pricing have little or no observability as of the reporting date. The types of assets and liabilities included in Level 3 are those with inputs requiring significant management judgment or estimation and may include complex and subjective models and forecasts.
In instances where the determination of the fair value measurement is based on inputs from different levels within the hierarchy, the level in the hierarchy within which the entire fair value measurement falls is based on the lowest level input that is significant to the fair value measurement in its entirety.
Related Parties
Related Parties
The Otter Tail Corporation Foundation and Otter Tail Power Company Foundation are independent not-for-profit charitable entities affiliated with the Company and are not included in OTC's consolidated financial statements.
Variable Interest Entity
Variable Interest Entity
In October 2012, the Coyote Station owners, including OTP, entered into an LSA with Coyote Creek Mining Company, LLC (CCMC), a subsidiary of The North American Coal Corporation, for the purchase of lignite coal to meet the coal supply requirements of Coyote Station for the period beginning in May 2016 and ending in December 2040. The price per ton paid by the Coyote Station owners under the LSA reflects the cost of production, along with an agreed upon profit and capital charge. CCMC was formed for the purpose of mining coal to meet the coal fuel supply requirements of Coyote Station from May 2016 through December 2040 and, based on the terms of the LSA, is considered a variable interest entity (VIE) due to the transfer of all operating and economic risk to the Coyote Station owners, as the agreement is structured so that the price of the coal would cover all costs of operations as well as future reclamation costs. The Coyote Station owners are required to buy certain assets of CCMC at book value should they terminate the contract prior to the end of the contract term and are providing a guarantee of the value of the equity of CCMC because the Coyote Station owners are required to buy the membership interests of CCMC at the end of the contract term at equity value. Under current accounting standards, the primary beneficiary of a VIE is required to include the assets, liabilities, results of operations and cash flows of the VIE in its consolidated financial statements. No single owner of Coyote Station owns a majority interest in Coyote Station and none, individually, has the power to direct the activities that most significantly impact CCMC. Therefore, none of the owners individually, including OTP, is considered the primary beneficiary of the VIE and the Company is not required to include CCMC in its consolidated financial statements.
If the LSA terminates prior to the expiration of its term or the production period terminates prior to December 31, 2040 and the Coyote Station owners purchase all of the outstanding membership interests of CCMC, the owners will satisfy or, if permitted by CCMC’s applicable lenders, assume all of CCMC’s obligations owed to CCMC’s lenders under its loans and leases. The Coyote Station owners have limited rights to assign their rights and obligations under the LSA without the consent of CCMC’s lenders during any period in which CCMC’s obligations to its lenders remain outstanding. In the event the contract is terminated prior to the end of the term due to certain events, OTP’s maximum loss exposure, as a result of its involvement with CCMC, could be as high as $40 million, or OTP’s 35% share of CCMC’s unrecovered costs as of December 31, 2024, if recovery of such a loss is denied by regulatory authorities.
Recently Adopted Accounting Pronouncements and Recent Accounting Pronouncements
Recently Adopted Accounting Pronouncements
Segment Reporting. In November 2023, the Financial Accounting Standards Board (FASB) issued amended authoritative guidance codified in Accounting Standards Codification (ASC) 280, Segment Reporting. The amended guidance expands annual and interim disclosure requirements for reportable segments, primarily through expanded disclosures about significant segment expenses. We adopted this updated standard in the 2024 annual period on a retrospective basis, as required by the updated
standard. The adoption of this updated standard resulted in additional disclosures related to our reportable segments and did not have an impact on our consolidated financial position or operating results.
Recent Accounting Pronouncements
Income Taxes. In December 2023, the FASB issued amended authoritative guidance codified in ASC 740, Income Taxes. The amended guidance requires additional disaggregated information in effective tax rate reconciliation disclosures and additional disaggregated information about income taxes paid. The updated standard is effective for our annual periods beginning in 2025. The amended guidance is to be applied on a prospective basis with the option to apply the standard retrospectively. We anticipate adopting the updated standard in our Form 10-K for the year ended December 31, 2025 and electing to apply the standard on a retrospective basis for all periods presented.
Disaggregated Income Statement Expenses. In November 2024, the FASB issued authoritative guidance codified in ASC 220, Income Statement—Reporting Comprehensive Income, which will require additional disclosure of certain costs and expenses within the notes to the financial statements. The updated standard is effective for our annual periods beginning in 2027 and interim periods beginning in the first quarter of fiscal 2028 and can be applied on either a prospective or retrospective basis. Early adoption is permitted. We are currently evaluating the impact that the updated standard will have on our financial statement disclosures.
v3.25.0.1
Summary of Significant Accounting Policies (Tables)
 12 Months Ended
Dec. 31, 2024
Accounting Policies [Abstract]  
Schedule of Inventories
Inventories consist of the following as of December 31, 2024 and 2023:
(in thousands)20242023
Finished Goods$43,345 $47,614 
Work in Process22,637 26,354 
Raw Material, Fuel and Supplies82,903 75,733 
Total Inventories$148,885 $149,701 
Schedule of Estimated Service Lives for Rate-Regulated Electric and Nonelectric Assets
The estimated service lives for rate-regulated electric assets and nonelectric assets are included below:
 Service Life Range
(years)LowHigh
Electric Assets:  
Production Plant21114
Transmission Plant5180
Distribution Plant1072
General Plant556
Nonelectric Assets:
Equipment220
Buildings and Leasehold Improvements240
Schedule of Capitalized Cost and Related Accumulated Amortization Below are the amounts of capitalized cost and related accumulated amortization as of December 31, 2024 and 2023:
(in thousands)20242023
Cloud Computing Costs$15,741 $12,782 
Accumulated Amortization(3,796)(1,505)
Cloud Computing Costs, net$11,945 $11,277 
v3.25.0.1
Segment Information (Tables)
 12 Months Ended
Dec. 31, 2024
Segment Reporting [Abstract]  
Schedule of Segment Reporting Information, by Segment
Information about each segment, including significant expenses and net income of each segment, for the years ended December 31, 2024, 2023 and 2022 are as follows:
Electric Segment
(in thousands)202420232022
Operating Revenue$524,515 $528,359 $549,699 
Production Fuel and Purchased Power122,506 138,631 165,391 
Operating and Maintenance Expenses190,422 191,263 181,378 
Depreciation and Amortization82,136 75,330 72,050 
Property Taxes15,662 16,614 17,742 
Interest Expense38,216 33,864 31,950 
Income Tax Expense (Benefit)
(1,544)1,648 5,065 
Other Segment Items(1)
(13,846)(13,415)(3,851)
Net Income$90,963 $84,424 $79,974 
(1) Other segment items includes nonservice components of postretirement benefits, allowance for funds used during construction, and other expenses (income).
Manufacturing Segment
(in thousands)202420232022
Operating Revenue$342,592 $402,781 $397,983 
Cost of Goods Sold283,390 324,245 327,228 
Selling, General, and Administrative Expenses40,110 49,396 41,690 
Interest Expense2,516 2,295 2,796 
Income Tax Expense2,895 5,390 5,321 
Other Segment Items— 1 (2)
Net Income$13,681 $21,454 $20,950 
Plastics Segment
(in thousands)202420232022
Operating Revenue$463,441 $418,026 $512,527 
Cost of Goods Sold166,628 143,521 227,571 
Selling, General, and Administrative Expenses24,908 20,103 20,378 
Interest Expense590 602 585 
Income Tax Expense70,644 66,066 68,688 
Other Segment Items(76)(14)(69)
Net Income$200,747 $187,748 $195,374 
Capital Expenditures and Identifiable Assets
The following provides capital expenditures for each reportable segment and our corporate cost center for the years ended December 31, 2024, 2023 and 2022:
(in thousands)202420232022
Capital Expenditures
Electric301,454 240,695 147,869 
Manufacturing32,159 23,284 17,954 
Plastics24,749 23,029 5,245 
Corporate288 126 66 
Total
$358,650 $287,134 $171,134 
The following provides the identifiable assets by segment and corporate assets as of December 31, 2024 and 2023:
(in thousands)20242023
Identifiable Assets
Electric$2,785,522 $2,533,831 
Manufacturing254,445 251,343 
Plastics186,043 164,179 
Corporate426,072 293,215 
Total
$3,652,082 $3,242,568 
Included below is a reconciliation of certain segment information and our unallocated corporate costs to consolidated amounts for the years ended December 31, 2024, 2023 and 2022:
(in thousands)202420232022
Depreciation and Amortization
Electric$82,136 $75,330 $72,050 
Manufacturing20,393 18,495 16,202 
Plastics4,494 4,027 4,205 
Corporate98 102 140 
Total
107,121 97,954 92,597 
Interest Expense
Total Interest Expense of Reportable Segments41,322 36,761 35,331 
Corporate Interest Expense493 916 685 
Total
41,815 37,677 36,016 
Income Tax Expense (Benefit)
Total Income Tax Expense of Reportable Segments71,995 73,104 79,074 
Corporate Income Tax Benefit(6,765)(3,806)(5,723)
Total
65,230 69,298 73,351 
Net Income (Loss)
Total Net Income of Reportable Segments305,391 293,626 296,298 
Corporate Net Income (Loss)(3,729)565 (12,114)
Total
301,662 294,191 284,184 
v3.25.0.1
Revenue (Tables)
 12 Months Ended
Dec. 31, 2024
Revenue from Contract with Customer [Abstract]  
Schedule of Disaggregation of Revenue
We present our operating revenues from external customers, in total and by amounts arising from contracts with customers and ARP arrangements, disaggregated by revenue source and segment for the years ended December 31, 2024, 2023 and 2022:
(in thousands)202420232022
Operating Revenues
Electric Segment
Retail: Residential$133,408 $135,570 $143,888 
Retail: Commercial and Industrial311,968 312,551 318,494 
Retail: Other7,838 7,719 7,918 
  Total Retail453,214 455,840 470,300 
Transmission53,517 52,555 52,213 
Wholesale11,077 12,459 18,539 
Other6,707 7,505 8,647 
Total Electric Segment524,515 528,359 549,699 
Manufacturing Segment
Metal Parts and Tooling303,077 351,267 338,865 
Plastic Products and Tooling32,210 41,395 49,080 
Scrap Metal7,305 10,119 10,038 
Total Manufacturing Segment342,592 402,781 397,983 
Plastics Segment
PVC Pipe463,441 418,026 512,527 
Total Operating Revenue1,330,548 1,349,166 1,460,209 
Less: Noncontract Revenues Included Above
Electric Segment - ARP Revenues575 (4,310)(9,266)
Total Operating Revenues from Contracts with Customers$1,329,973 $1,353,476 $1,469,475 
v3.25.0.1
Receivables (Tables)
 12 Months Ended
Dec. 31, 2024
Receivables [Abstract]  
Schedule of Accounts Receivable
Receivables as of December 31, 2024 and 2023 are as follows:
(in thousands)20242023
Receivables
Trade$112,169 $129,257 
Other13,799 9,084 
Unbilled Receivables21,916 21,324 
Total Receivables147,884 159,665 
Less Allowance for Credit Losses1,920 2,522 
Receivables, net of allowance for credit losses$145,964 $157,143 
Schedule of Activity in the Allowance for Credit Losses
The following is a summary of activity in the allowance for credit losses for the years ended December 31, 2024 and 2023:
(in thousands)20242023
Beginning Balance$2,522 $1,648 
Additions Charged to Expense1,242 2,014 
Reductions for Amounts Written Off, Net of Recoveries(1,844)(1,140)
Ending Balance$1,920 $2,522 
v3.25.0.1
Investments (Tables)
 12 Months Ended
Dec. 31, 2024
Investments, Debt and Equity Securities [Abstract]  
Schedule of Investments
The following is a summary of our investments as of December 31, 2024 and 2023:
(in thousands)20242023
Short-term Investments
Government Debt Securities
$753 $— 
Long-term Investments
Corporate-Owned Life Insurance Policies47,895 42,287 
Government Debt Securities
60,378 7,724 
Corporate Debt Securities
1,628 1,579 
Mutual Funds10,653 7,771 
Money Market Funds596 3,125 
Other Investments27 30 
Total Long-term Investments121,177 62,516 
Total Investments$121,930 $62,516 
Schedule of Amortized Costs and Fair Value of Debt Securities, Available-for-Sale
The following table summarizes the amortized cost and fair value of debt securities available for sale and the corresponding amounts of gross unrealized gains and losses as of December 31, 2024:
December 31, 2024
(in thousands)
Amortized Cost
Gross Unrealized Gains
Gross Unrealized (Losses)
Fair Value
Government Debt Securities$60,891 $424 $(184)$61,131 
Corporate Debt Securities1,629 9 (10)1,628 
Total$62,520 $433 $(194)$62,759 
Schedule of Investments Classified by Contractual Maturity Date
The following table summarizes the fair value of debt securities available for sale by contractual maturity date as of December 31, 2024:
(in thousands)December 31, 2024
Due in one year or less
$753 
Due in one to five years
62,006 
Total$62,759 
v3.25.0.1
Regulatory Matters (Tables)
 12 Months Ended
Dec. 31, 2024
Regulatory Assets and Liabilities Disclosure [Abstract]  
Schedule of Regulatory Assets and Liabilities
The following presents our current and long-term regulatory assets and liabilities as of December 31, 2024 and 2023 and the period we expect to recover or refund such amounts:
Period of20242023
(in thousands)Recovery/RefundCurrentLong-TermCurrentLong-Term
Regulatory Assets
Pension and Other Postretirement Benefit Plans1
See below$— $88,161 $154 $86,134 
Alternative Revenue Program Riders2
Up to 2 years
4,257 195 3,719 158 
Deferred Income TaxesAsset lives— 8,944 — 6,940 
Fuel Clause Adjustments1
Up to 1 year
2,218 — 7,294 — 
Derivative Instruments1
Up to 1 year
1,989 — 4,210 — 
Other1
Various1,498 1,373 750 2,483 
Total Regulatory Assets9,962 98,673 16,127 95,715 
Regulatory Liabilities
Deferred Income TaxesAsset lives— 130,387 — 136,022 
Plant Removal ObligationsAsset lives— 126,263 — 117,030 
Fuel Clause Adjustments
Up to 1 year
11,432 — 11,350 — 
Alternative Revenue Program Riders
Up to 1 year
14,255 — 6,885 — 
North Dakota PTC RefundsAsset lives— 20,099 — 12,011 
Pension and Other Postretirement Benefit PlansSee below2,547 10,758 6,138 11,307 
OtherVarious1,073 1,421 1,035 177 
Total Regulatory Liabilities$29,307 $288,928 $25,408 $276,547 
1Costs subject to recovery without a rate of return.
2Amount eligible for recovery includes an incentive or rate of return.
v3.25.0.1
Property, Plant and Equipment (Tables)
 12 Months Ended
Dec. 31, 2024
Property, Plant and Equipment [Abstract]  
Schedule of Property, Plant and Equipment
Major classes of property, plant and equipment as of December 31, 2024 and 2023 include:
(in thousands)20242023
Electric Plant in Service  
Production$1,469,008 $1,412,826 
Transmission820,415 777,613 
Distribution726,159 654,704 
General165,361 144,738 
Electric Plant in Service3,180,943 2,989,881 
Construction Work in Progress231,890 137,212 
Total Gross Electric Plant3,412,833 3,127,093 
Less Accumulated Depreciation
899,049 851,148 
Net Electric Plant2,513,784 2,275,945 
Nonelectric Property, Plant and Equipment
Equipment260,307 233,571 
Buildings and Leasehold Improvements88,680 64,753 
Land13,578 13,600 
Nonelectric Property, Plant and Equipment362,565 311,924 
Construction Work in Progress40,536 38,062 
Total Gross Nonelectric Property, Plant and Equipment403,101 349,986 
Less Accumulated Depreciation
224,425 207,556 
Net Nonelectric Property, Plant and Equipment178,676 142,430 
Net Property, Plant and Equipment$2,692,460 $2,418,375 
Schedule of Jointly Owned Utility Plants
The following table provides OTP’s ownership percentages and amounts included in the December 31, 2024 and 2023 consolidated balance sheets for OTP’s share of each of these jointly owned facilities:
 (dollars in thousands)Ownership
Percentage		Electric Plant
in Service		Construction
Work in
Progress		Accumulated
Depreciation		Net Plant
December 31, 2024     
Big Stone Plant53.9 %$345,990 $459 $(135,065)$211,384 
Coyote Station35.0 %188,066 813 (118,268)70,611 
Big Stone South–Ellendale 345 kV line50.0 %106,185 — (8,445)97,740 
Fargo–Monticello 345 kV line14.2 %78,184 — (12,247)65,937 
Big Stone South–Brookings 345 kV line50.0 %53,167 — (5,822)47,345 
Brookings–Southeast Twin Cities 345 kV line4.8 %28,013 1,131 (3,941)25,203 
Bemidji–Grand Rapids 230 kV line14.8 %16,331 — (3,693)12,638 
Jamestown– Ellendale 345 kV line50.0 %— 5,509 — 5,509 
Big Stone South–Alexandria 345 kV line40.0 %— 2,418 — 2,418 
Alexandria–Big Oaks 345 kV line14.2 %— 417 — 417 
Oslo - Lake Ardoch 115 kV line72.0 %— 2,646 — 2,646 
December 31, 2023
Big Stone Plant53.9 %$341,683 $820 $(126,904)$215,599 
Coyote Station35.0 %188,656 104 (115,306)73,454 
Big Stone South–Ellendale 345 kV line50.0 %106,185 — (7,181)99,004 
Fargo–Monticello 345 kV line14.2 %78,184 — (11,238)66,946 
Big Stone South–Brookings 345 kV line50.0 %53,170 — (5,207)47,963 
Brookings–Southeast Twin Cities 345 kV line4.8 %26,409 83 (3,617)22,875 
Bemidji–Grand Rapids 230 kV line14.8 %16,331 — (3,568)12,763 
Jamestown–Ellendale 345 kV line
50.0 %— 1,121 — 1,121 
Big Stone South–Alexandria 345 kV line40.0 %— 555 — 555 
Alexandria–Big Oaks 345 kV line14.2 %— 343 — 343 
v3.25.0.1
Intangible Assets (Tables)
 12 Months Ended
Dec. 31, 2024
Goodwill and Intangible Assets Disclosure [Abstract]  
Schedule of Goodwill by Segment
The following table summarizes our goodwill by segment as of December 31, 2024 and 2023: 
(in thousands)20242023
Manufacturing$18,270 $18,270 
Plastics19,302 19,302 
Total Goodwill$37,572 $37,572 
Schedule of Components of Intangible Assets
The following table summarizes the components of our intangible assets as of December 31, 2024 and 2023:
(in thousands)Gross
Amount		Accumulated
Amortization		Net Carrying
Amount
December 31, 2024
Customer Relationships$22,491 $16,766 $5,725 
Other26 8 18 
Total$22,517 $16,774 $5,743 
December 31, 2023
Customer Relationships$22,491 $15,667 $6,824 
Other26 7 19 
Total$22,517 $15,674 $6,843 
Schedule of Intangible Assets Future Amortization Expenses
Annual amortization expense for these intangible assets for the next five years is: 
(in thousands)20252026202720282029
Amortization Expense$1,100 $1,092 $1,090 $554 $285 
v3.25.0.1
Leases (Tables)
 12 Months Ended
Dec. 31, 2024
Leases [Abstract]  
Schedule of Lease Costs
The components of lease cost and lease cash flows for the years ended December 31, 2024, 2023, and 2022 are as follows:
(in thousands)202420232022
Lease Cost
Operating Lease Cost$6,688 $6,309 $5,606 
Variable Lease Cost1,460 1,433 1,386 
Short-Term Lease Cost2,746 2,525 1,517 
Total Lease Cost$10,894 $10,267 $8,509 
Lease Cash Flows
Operating Cash Flows from Operating Leases$6,762 $6,424 $5,592 
Schedule of Assets And Liabilities, Lessee
A summary of operating lease right-of-use lease assets and lease liabilities as of December 31, 2024 and 2023 is as follows: 
(in thousands)20242023
Right of Use Lease Assets1
$28,179 $16,788 
Lease Liabilities
Current2
4,776 5,756 
Long-Term3
23,567 11,258 
Total Lease Liabilities$28,343 $17,014 
1Included in Other Noncurrent Assets in the consolidated balance sheets.
2Included in Other Current Liabilities in the consolidated balance sheets.
3Included in Other Noncurrent Liabilities in the consolidated balance sheets.
The weighted-average remaining lease term and the weighted-average discount rate as of December 31, 2024 and 2023 are as follows:
20242023
Weighted-Average Remaining Lease Term (in years)7.93.4
Weighted-Average Discount Rate6.37 %5.40 %
Schedule of Lessee, Operating Lease, Liability, Maturity
Maturities of lease liabilities as of December 31, 2024 for each of the next five years and in the aggregate thereafter are as follows:
(in thousands)Operating Leases
2025$6,350 
20265,638 
20274,785 
20283,604 
20292,750 
Thereafter12,921 
Total Lease Payments36,048 
Less: Interest7,705 
Present Value of Lease Liabilities$28,343 
v3.25.0.1
Short-Term and Long-Term Borrowings (Tables)
 12 Months Ended
Dec. 31, 2024
Debt Disclosure [Abstract]  
Schedule of Debt
The following is a summary of our outstanding short- and long-term borrowings by borrower, OTC or OTP, as of December 31, 2024 and 2023:
20242023
(in thousands)OTCOTPTotalOTCOTPTotal
Short-Term Debt$— $69,615 $69,615 $— $81,422 $81,422 
Long-Term Debt79,900 863,834 943,734 79,849 744,210 824,059 
Total$79,900 $933,449 $1,013,349 $79,849 $825,632 $905,481 
The following is a summary of outstanding long-term debt by borrower as of December 31, 2024 and 2023: 
(in thousands)
EntityDebt InstrumentRateMaturity20242023
OTCGuaranteed Senior Notes3.55%12/15/26$80,000 $80,000 
OTPSeries 2007C Senior Unsecured Notes6.37%08/02/2742,000 42,000 
OTPSeries 2013A Senior Unsecured Notes4.68%02/27/2960,000 60,000 
OTPSeries 2019A Senior Unsecured Notes 3.07%10/10/2910,000 10,000 
OTPSeries 2020A Senior Unsecured Notes3.22%02/25/3010,000 10,000 
OTPSeries 2020B Senior Unsecured Notes3.22%08/20/3040,000 40,000 
OTPSeries 2021A Senior Unsecured Notes2.74%11/29/3140,000 40,000 
OTPSeries 2024A Senior Unsecured Notes5.48%04/01/3460,000 — 
OTPSeries 2007D Senior Unsecured Notes6.47%08/20/3750,000 50,000 
OTPSeries 2019B Senior Unsecured Notes3.52%10/10/3926,000 26,000 
OTPSeries 2020C Senior Unsecured Notes3.62%02/25/4010,000 10,000 
OTPSeries 2013B Senior Unsecured Notes5.47%02/27/4490,000 90,000 
OTPSeries 2018A Senior Unsecured Notes4.07%02/07/48100,000 100,000 
OTPSeries 2019C Senior Unsecured Notes3.82%10/10/4964,000 64,000 
OTPSeries 2020D Senior Unsecured Notes3.92%02/25/5015,000 15,000 
OTPSeries 2021B Senior Unsecured Notes3.69%11/29/51100,000 100,000 
OTPSeries 2022A Senior Unsecured Notes3.77%05/20/5290,000 90,000 
OTPSeries 2024B Senior Unsecured Notes5.77%04/01/5460,000 — 
Total947,000 827,000 
Less:Unamortized Long-Term Debt Issuance Costs3,266 2,941 
Total Long-Term Debt Net of Unamortized Debt Issuance Costs$943,734 $824,059 
Schedule of Line of Credit Facilities
The following is a summary of our lines of credit as of December 31, 2024 and 2023:
20242023
(in thousands)Line LimitAmount OutstandingLetters
of Credit		Amount AvailableAmount Available
OTC Credit Agreement$170,000 $— $— $170,000 $170,000 
OTP Credit Agreement220,000 69,615 8,772 141,613 79,446 
Total$390,000 $69,615 $8,772 $311,613 $249,446 
Schedule of Maturities of Long-Term Debt
Aggregate maturities of long-term debt obligations on December 31, 2024 for each of the next five years are as follows:
(in thousands)20252026202720282029
Debt Maturities$— $80,000 $42,000 $— $70,000 
v3.25.0.1
Employee Postretirement Benefits (Tables)
 12 Months Ended
Dec. 31, 2024
Retirement Benefits [Abstract]  
Schedule of Target Asset Allocation Ranges By Asset Class
The following table presents our target asset allocation permitted range along with the actual asset allocation as of December 31, 2024 and 2023:
 PermittedActual Allocation
Asset ClassRange20242023
Return Enhancement35 –60%41 %48 %
Risk Management40 –80%59 51 
Alternatives0 –20%— 1 
Total100 %100 %
Schedule of Allocation of Plan Assets
The following presents the fair value inputs classified within the fair value hierarchy used to measure Pension Plan assets at December 31, 2024 and 2023 and assets measured using the net asset value (NAV) practical expedient:
(in thousands)Level 1Level 2Level 3NAVTotal
December 31, 2024
Equity Funds$116,889 $— $— $— $116,889 
Fixed Income Funds175,310 — — — 175,310 
Hybrid Funds10,106 — — — 10,106 
U.S. Treasury Securities23,909 — — — 23,909 
SEI Energy Debt Collective Fund— — — 1,061 1,061 
Total$326,214 $— $— $1,061 $327,275 
December 31, 2023
Equity Funds$127,159 $— $— $— $127,159 
Fixed Income Funds167,604 — — — 167,604 
Hybrid Funds10,980 — — — 10,980 
U.S. Treasury Securities23,218 — — — 23,218 
SEI Energy Debt Collective Fund— — — 1,518 1,518 
Total$328,961 $— $— $1,518 $330,479 
Schedule of Changes in Projected Benefit Obligations and Changes in Plan Assets The following table provides a reconciliation of the changes in the fair value of plan assets and the actuarially computed benefit obligation for the years ended December 31, 2024 and 2023 and the funded status of the plans as of December 31, 2024 and 2023:
Pension Benefits (Pension Plan) Pension Benefits (ESSRP)Postretirement Benefits
(in thousands)202420232024202320242023
Change in Fair Value of Plan Assets:
Fair Value of Plan Assets at January 1$330,479 $313,797 $— $— $— $— 
Actual Return on Plan Assets14,976 34,196 — — — — 
Company Contributions— — 2,694 2,197 1,568 3,167 
Benefit Payments(18,180)(17,514)(2,694)(2,197)(3,734)(8,900)
Participant Premium Payments— — — — 2,166 5,733 
Fair Value of Plan Assets at December 31$327,275 $330,479 $— $— $— $— 
Change in Benefit Obligation:
Benefit Obligation at January 1$318,801 $308,055 $35,780 $35,624 $30,145 $49,947 
Service Cost3,886 3,698 — 72 490 565 
Interest Cost17,189 16,436 1,897 1,889 1,600 2,416 
Benefit Payments(18,180)(17,514)(2,694)(2,197)(3,734)(8,900)
Participant Premium Payments— — — — 2,166 5,733 
Plan Amendments— — — — — (17,493)
Actuarial (Gain) Loss(7,686)8,126 331 392 (664)(2,123)
Benefit Obligation at December 31314,010 318,801 35,314 35,780 30,003 30,145 
Funded Status$13,265 $11,678 $(35,314)$(35,780)$(30,003)$(30,145)
Amounts Recognized in Consolidated Balance Sheets at December 31:
Noncurrent Assets$13,265 $11,678 $— $— $— $— 
Current Liabilities— — (2,700)(2,679)(2,618)(2,469)
Noncurrent Liabilities
— — (32,614)(33,101)(27,385)(27,676)
Net Asset (Liability)$13,265 $11,678 $(35,314)$(35,780)$(30,003)$(30,145)
Schedule of Assumptions Used in Calculating Net Periodic Benefit Cost
The following assumptions were used to determine benefit obligations as of December 31, 2024 and 2023:
Pension Benefits (Pension Plan)Pension Benefits (ESSRP)Postretirement Benefits
 202420232024202320242023
Discount Rate5.70 %5.57 %5.60 %5.53 %5.61 %5.53 %
Long-Term Rate of Compensation Increasen/an/a3.00 %3.00 %n/an/a
Participants up to Age 39(1)
4.50 %4.50 %n/an/an/an/a
Participants Ages 40 to 49(2)
4.50 %4.50 %n/an/an/an/a
Participants Age 50 and Older(3)
3.75 %3.75 %n/an/an/an/a
Healthcare Cost Immediate Trend Raten/an/an/an/a6.44 %6.97 %
Healthcare Cost Ultimate Trend Raten/an/an/an/a4.00 %4.00 %
Year the Rate Reaches the Ultimate Trend Raten/an/an/an/a20482048
(1) Amount reflects rate of compensation increases for both union and non-union employees.
(2) Amount reflects rate of compensation increases for union employees. The rate of compensation increases for non-union employees is 3.50%.
(3) Amount reflects rate of compensation increases for union employees. The rate of compensation increases for non-union employees is 3.00%.
The following assumptions were used to determine net periodic benefit cost for the years ended December 31, 2024, 2023 and 2022:
Pension Benefits (Pension Plan)Pension Benefits (ESSRP)Postretirement Benefits
 202420232022202420232022202420232022
Discount Rate5.57 %5.51 %3.03 %5.53 %5.51 %2.93 %5.53 %5.52 %3.01 %
Long-Term Rate of Return on Plan Assets7.00 %7.00 %6.30 %n/an/an/an/an/an/a
Long-Term Rate of Compensation Increasen/an/an/a3.00 %3.00 %3.00 %n/an/an/a
Participants to Age 394.50 %4.50 %4.50 %n/an/an/an/an/an/a
Participants Ages 40 to 494.00 %3.50 %3.50 %n/an/an/an/an/an/a
Participants Age 50 and Older3.38 %2.75 %2.75 %n/an/an/an/an/an/a
Schedule of Net Benefit Costs
The following table lists the components of net periodic benefit cost of our defined benefit pension plans and other postretirement benefits for the years ended December 31, 2024, 2023 and 2022:
Pension Benefits (Pension Plan)Pension Benefits (ESSRP)Postretirement Benefits
(in thousands)202420232022202420232022202420232022
Service Cost$3,886 $3,698 $6,576 $— $72 $195 $490 $565 $1,338 
Interest Cost17,189 16,436 12,344 1,897 1,889 1,341 1,600 2,416 2,041 
Expected Return on Assets(25,518)(25,914)(23,684)— — — — — — 
Amortization of Prior Service Cost— — — — — — (6,302)(6,649)(5,733)
Amortization of Net Actuarial Loss158 — 7,865 — — 567 — — 3,063 
Net Periodic Benefit Cost$(4,285)$(5,780)$3,101 $1,897 $1,961 $2,103 $(4,212)$(3,668)$709 
The following table includes the impact of regulation on the recognition of periodic benefit cost arising from pension and other postretirement benefits for the years ended December 31, 2024, 2023 and 2022:
(in thousands)202420232022
Net Periodic Benefit Cost$(6,600)$(7,487)$5,913 
Net Amount Amortized Due to the Effect of Regulation1,367 1,225 1,121 
Net Periodic Benefit Cost Recognized$(5,233)$(6,262)$7,034 
Schedule of Amounts Not Yet Recognized as Components of Net Periodic Benefit Cost
The following table presents the amounts not yet recognized as components of net periodic benefit cost as of December 31, 2024 and 2023:
Pension Benefits (Pension Plan)Pension Benefits (ESSRP)Postretirement Benefits
(in thousands)202420232024202320242023
Regulatory Assets (Liabilities):
Unrecognized Prior Service Cost$— $— $— $— $(12,703)$(18,845)
Unrecognized Actuarial Loss87,868 85,227 292 1,061 1,121 1,759 
Net Regulatory Assets (Liabilities)$87,868 $85,227 $292 $1,061 $(11,582)$(17,086)
Accumulated Other Comprehensive Income (Loss):
Unrecognized Prior Service Cost$— $— $— $— $339 $498 
Unrecognized Actuarial Gain (Loss)1,937 1,994 (2,502)(1,403)732 707 
Total Accumulated Other Comprehensive Income (Loss)$1,937 $1,994 $(2,502)$(1,403)$1,071 $1,205 
Schedule of Expected Benefit Payments
The following reflects anticipated benefit payments to be paid in each of the next five years and in the aggregate for the five-year period thereafter under our pension plans and postretirement healthcare plan:
(in thousands)202520262027202820292030-2034
Projected Pension Plan Benefit Payments$19,274 $19,804 $20,284 $20,859 $21,420 $111,275 
Projected ESSRP Benefit Payments2,769 2,899 3,059 2,999 3,070 14,275 
Projected Postretirement Benefit Payments2,618 2,614 2,576 2,488 2,517 12,019 
Total$24,661 $25,317 $25,919 $26,346 $27,007 $137,569 
v3.25.0.1
Asset Retirement Obligations (Tables)
 12 Months Ended
Dec. 31, 2024
Asset Retirement Obligation Disclosure [Abstract]  
Schedule of Change in Asset Retirement Obligations
A reconciliation of the carrying amounts of AROs for the years ended December 31, 2024 and 2023 is as follows: 
(in thousands)20242023
Beginning Balance$36,477 $25,182 
New Obligations Recognized2,991 4,506 
Adjustments Due to Revisions in Cash Flow Estimates1,098 8,394 
Accrued Accretion1,676 1,191 
Settlements(79)(2,796)
Ending Balance$42,163 $36,477 
v3.25.0.1
Income Taxes (Tables)
 12 Months Ended
Dec. 31, 2024
Income Tax Disclosure [Abstract]  
Schedule of Income Tax Expense (Benefit)
The provision for income taxes charged to income for the years ended December 31, 2024, 2023 and 2022 consisted of the following:
(in thousands)202420232022
Current
Federal Income Taxes$36,238 $41,253 $31,949 
State Income Taxes6,533 15,126 9,568 
Deferred
Federal Income Taxes13,078 9,832 22,480 
State Income Taxes9,979 3,676 9,943 
Tax Credits
North Dakota Wind Tax Credit Amortization, Net of Federal Tax(586)(586)(586)
Investment Tax Credit Amortization(12)(3)(3)
Total$65,230 $69,298 $73,351 
Schedule of Effective Income Tax Rate Reconciliation
The reconciliation of the statutory federal income tax rate to our effective tax rate for each of the years ended December 31, 2024, 2023 and 2022 is as follows:
202420232022
Income Taxes at Federal Statutory Rate$77,047 21.0 %$76,332 21.0 %$75,082 21.0 %
Increases (Decreases) in Tax from:
State Taxes on Income, Net of Federal Tax14,360 3.9 14,429 4.0 15,049 4.2 
Production Tax Credits (PTCs)(20,106)(5.5)(17,394)(4.8)(14,985)(4.2)
Amortization of Excess Deferred Income Taxes(2,788)(0.8)(2,205)(0.6)(1,625)(0.5)
North Dakota Wind Tax Credit Amortization, Net of Federal Tax(586)(0.2)(586)(0.2)(586)(0.2)
Other, Net(2,697)(0.6)(1,278)(0.3)416 0.2 
Income Taxes at Effective Tax Rate$65,230 17.8 %$69,298 19.1 %$73,351 20.5 %
Schedule of Deferred Tax Assets and Liabilities
Deferred tax assets and liabilities were composed of the following on December 31, 2024 and 2023:
(in thousands)20242023
Deferred Tax Assets  
Employee Benefits$37,456 $39,959 
Regulatory Liabilities52,664 56,479 
Tax Credit Carryforwards18,268 21,836 
Cost of Removal35,374 32,993 
Asset Retirement Obligations10,948 9,494 
Net Operating Loss Carryforward2,289 2,336 
Other19,449 11,310 
Total Deferred Tax Assets$176,448 $174,407 
Deferred Tax Liabilities
Differences Related to Property$(375,120)$(347,885)
Retirement Benefits Regulatory Asset(22,892)(22,458)
Pension Expense(26,034)(24,875)
Other(20,147)(16,462)
Total Deferred Tax Liabilities(444,193)(411,680)
Deferred Income Taxes$(267,745)$(237,273)
Schedule of Unrecognized Tax Benefits Roll Forward
The following table summarizes the activity for unrecognized tax benefits for the years ended December 31, 2024, 2023 and 2022:
(in thousands)202420232022
Balance on January 1$1,489 $923 $827 
Increases (Decreases) for tax positions taken during a prior period
(189)596 44 
Increases for tax positions taken during the current period188 163 260 
Decreases due to settlements with taxing authorities— — — 
Decreases as a result of a lapse of applicable statutes of limitations(363)(193)(208)
Balance on December 31$1,125 $1,489 $923 
v3.25.0.1
Commitments and Contingencies (Tables)
 12 Months Ended
Dec. 31, 2024
Commitments and Contingencies Disclosure [Abstract]  
Schedule of Commitments Contractual Obligation Maturities
Our future commitments as of December 31, 2024 were as follows:
(in thousands)Coal Purchase
Commitments		Land
 Easement
Payments		Other Commitments
2025$24,192 $1,897 $1,817 
202624,416 1,902 1,518 
202725,127 1,941 661 
202825,859 1,981 544 
202927,102 2,021 272 
Beyond 2029314,713 52,261 5,758 
Total$441,409 $62,003 $10,570 
v3.25.0.1
Accumulated Other Comprehensive Income (Loss) (Tables)
 12 Months Ended
Dec. 31, 2024
Equity [Abstract]  
Schedule of Changes in Accumulated Other Comprehensive Income (Loss)
The following table shows the changes in accumulated other comprehensive Income (loss) for the years ended December 31, 2024, 2023 and 2022:
(in thousands)Pension and Other Postretirement BenefitsNet Unrealized Gain (Losses) on Available-for-Sale SecuritiesTotal
Balance, December 31, 2021
$(6,537)$13 $(6,524)
Other Comprehensive Income (Loss) Before Reclassifications, net of tax7,331 (433)6,898 
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss)540 
(1)
1 
(2)
541 
Total Other Comprehensive Income (Loss)7,871 (432)7,439 
Balance, December 31, 2022
1,334 (419)915 
Other Comprehensive Income Before Reclassifications, net of tax
59 180 239 
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss)(18)
(1)
12 
(2)
(6)
Total Other Comprehensive Income
41 192 233 
Balance, December 31, 2023
1,375 (227)1,148 
Other Comprehensive Income Before Reclassifications, net of tax501 407 908 
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss)(1,503)
(1)
(21)
(2)
(1,524)
Total Other Comprehensive Income (Loss)
(1,002)386 (616)
Balance, December 31, 2024
$373 $159 $532 
(1) Included in the computation of net periodic pension and other postretirement benefit costs. See Note 11 for further information.
(2) Included in other income (expense), net on the accompanying consolidated statements of income.
v3.25.0.1
Share-Based Payments (Tables)
 12 Months Ended
Dec. 31, 2024
Share-Based Payment Arrangement [Abstract]  
Schedule of Nonvested Restricted Stock Shares Activity
The following is a summary of restricted stock award activity for the year ended December 31, 2024:
SharesWeighted-Average
Grant-Date
Fair Value
Nonvested, Beginning of Year148,913 $56.48 
Granted52,425 85.25 
Vested(57,771)52.78 
Forfeited(150)66.39 
Nonvested, End of Year143,417 $68.47 
Schedule of Share-Based Payment Award, Employee Stock Purchase Plan, Valuation Assumptions The grant-date fair value of the TSR component of the stock performance awards granted during the years ended December 31, 2024, 2023 and 2022 was determined using a Monte Carlo fair value simulation model incorporating the following assumptions:
202420232022
Risk-free interest rate4.16 %4.15 %1.52 %
Expected term (in years)3.003.003.00
Expected volatility35.10 %34.00 %32.00 %
Dividend yield2.40 %2.50 %2.90 %
Schedule of Share-based Compensation Arrangements by Share-based Payment Award
The following is a summary of stock performance award activity for the year ended December 31, 2024 (share amounts reflect awards at target):
 SharesWeighted-Average
Grant-Date
Fair Value
Nonvested, Beginning of Year194,200 $50.33 
Granted43,400 94.45 
Vested(92,800)42.06 
Forfeited— — 
Nonvested, End of Year144,800 $68.85 
v3.25.0.1
Earnings Per Share (Tables)
 12 Months Ended
Dec. 31, 2024
Earnings Per Share [Abstract]  
Schedule of Earnings Per Share, Basic and Diluted
The following includes the computation of the denominator for basic and diluted weighted-average shares outstanding for the years ended December 31, 2024, 2023 and 2022:
(in thousands)202420232022
Weighted Average Common Shares Outstanding – Basic41,778 41,668 41,586 
Effect of Dilutive Securities:
Stock Performance Awards196 269 248 
Restricted Stock Awards96 100 95 
Employee Stock Purchase Plan Shares and Other2 2 2 
Dilutive Effect of Potential Common Shares294 371 345 
Weighted Average Common Shares Outstanding – Diluted42,072 42,039 41,931 
v3.25.0.1
Derivative Instruments (Tables)
 12 Months Ended
Dec. 31, 2024
Derivative Instruments and Hedging Activities Disclosure [Abstract]  
Schedule of Notional Amounts and Fair Value of Derivative Instruments The following presents the notional amounts and fair value of our derivative instruments as of December 31, 2024 and 2023:
(in thousands)20242023
Megawatt hours of electricity167187
Derivative Liabilities:
Other Current Liabilities$1,989 $4,210 
Other Noncurrent Liabilities— — 
Total Derivative Liabilities$1,989 $4,210 
v3.25.0.1
Fair Value Measurements (Tables)
 12 Months Ended
Dec. 31, 2024
Fair Value Disclosures [Abstract]  
Schedule of Fair Value, Assets and Liabilities Measured on Recurring Basis
The following tables present our assets and liabilities measured at fair value on a recurring basis as of December 31, 2024 and 2023 classified by the input method used to measure fair value:
Level 1Level 2Level 3
December 31, 2024
Assets
Investments:
Money Market Funds$596 $— $— 
Mutual Funds10,653 — — 
Corporate Debt Securities— 1,628 — 
Government Debt Securities— 61,131 — 
Total Assets11,249 62,759 — 
Liabilities
Derivative Instruments— 1,989 — 
Total Liabilities$— $1,989 $— 
(in thousands)Level 1Level 2Level 3
December 31, 2023
Assets
Investments:
Money Market Funds$3,125 $— $— 
Mutual Funds7,771 — — 
Corporate Debt Securities— 1,579 — 
Government Debt Securities— 7,724 — 
Total Assets$10,896 $9,303 $— 
Liabilities
Derivative Instruments$— $4,210 $— 
Total Liabilities$— $4,210 $— 
Schedule of Fair Value of Assets and Liabilities The following reflects the carrying value and estimated fair value of these assets and liabilities as of December 31, 2024 and 2023:
 December 31, 2024December 31, 2023
(in thousands)Carrying
Amount		Fair ValueCarrying
Amount		Fair Value
Assets:
Cash and Cash Equivalents$294,651 $294,651 $230,373 $230,373 
Total294,651 294,651 230,373 230,373 
Liabilities:
Short-Term Debt69,615 69,615 81,422 81,422 
Long-Term Debt943,734 806,826 824,059 710,839 
Total$1,013,349 $876,441 $905,481 $792,261 
v3.25.0.1
Schedule I - Condensed Financial Information of Registrant (Tables)
 12 Months Ended
Dec. 31, 2024
Condensed Financial Information Disclosure [Abstract]  
Condensed Balance Sheet
SCHEDULE I - CONDENSED FINANCIAL INFORMATION OF REGISTRANT
OTTER TAIL CORPORATION (PARENT COMPANY)
CONDENSED BALANCE SHEETS
December 31,
(in thousands)20242023
Assets
Current Assets
Cash and Cash Equivalents$291,575 $228,137 
Accounts Receivable from Subsidiaries5,642 2,555 
Interest Receivable from Subsidiaries117 117 
Notes Receivable from Subsidiaries4,706 — 
Other3,538 977 
Total Current Assets305,578 231,786 
Investments in Subsidiaries2,006,239 1,725,584 
Notes Receivable from Subsidiaries78,900 78,900 
Deferred Income Taxes69,781 65,244 
Other Assets109,057 50,795 
Total Assets$2,569,555 $2,152,309 
Liabilities and Stockholders' Equity
Current Liabilities
Accounts Payable to Subsidiaries$7 $7 
Notes Payable to Subsidiaries752,625 568,672 
Other19,100 15,320 
Total Current Liabilities771,732 583,999 
Other Noncurrent Liabilities49,424 45,455 
Commitments and Contingencies
Capitalization
Long-Term Debt79,900 79,849 
Common Stockholders' Equity1,668,499 1,443,006 
Total Capitalization1,748,399 1,522,855 
Total Liabilities and Stockholders' Equity$2,569,555 $2,152,309 
Condensed Income Statement
OTTER TAIL CORPORATION (PARENT COMPANY)
CONDENSED STATEMENTS OF INCOME
Years Ended December 31,
(in thousands)202420232022
Income
Equity Income in Earnings of Subsidiaries$304,525 $294,467 $296,833 
Interest Income from Subsidiaries3,107 2,898 3,382 
Other Income15,085 10,496 466 
Total Income322,717 307,861 300,681 
Expense
Nonelectric Selling, General, and Administrative Expenses23,016 12,816 17,269 
Interest Expense3,599 3,813 4,066 
Interest Expense from Subsidiaries5 6 5 
Nonservice Cost Components of Postretirement Benefits970 1,063 1,023 
Total Expense27,590 17,698 22,363 
Income Before Income Taxes295,127 290,163 278,318 
Income Tax Benefit6,535 4,028 5,866 
Net Income$301,662 $294,191 $284,184 
Condensed Cash Flow Statement
OTTER TAIL CORPORATION (PARENT COMPANY)
CONDENSED STATEMENTS OF CASH FLOWS
Years Ended December 31,
(in thousands)202420232022
Cash Flows from Operating Activities
Net Cash Provided by Operating Activities$76,333 $77,139 $28,807 
Cash Flows from Investing Activities
Investment in Subsidiaries(55,000)(40,000)(50,000)
Purchases of Investments and Other Assets
(53,085)(1,754)(3,175)
Other, net1,394 1,686 1,480 
Net Cash Used in Investing Activities(106,691)(40,068)(51,695)
Cash Flows from Financing Activities
Net (Repayments) Borrowings on Short-Term Debt— — (22,637)
Borrowings from Subsidiaries179,247 148,308 236,926 
Payments for Shares Withheld for Employee Tax Obligations(6,457)(3,088)(2,942)
Dividends Paid(78,265)(73,061)(68,755)
Other, net(729)(339)(461)
Net Cash Provided by Financing Activities
93,796 71,820 142,131 
Net Change in Cash and Cash Equivalents63,438 108,891 119,243 
Cash and Cash Equivalents at Beginning of Period228,137 119,246 3 
Cash and Cash Equivalents at End of Period$291,575 $228,137 $119,246 
Schedule of Related Party Transactions
Outstanding receivables from and payables to OTC's subsidiaries as of December 31, 2024 and 2023 are as follows:
(in thousands)Accounts
Receivable		Interest
Receivable		Current
Notes
Receivable		Long-Term
Notes
Receivable		Accounts
Payable
Current
Notes
Payable
December 31, 2024
Otter Tail Power Company$5,223 $— $— $— $7 $— 
Northern Pipe Products, Inc.36 7 — 5,000 — 66,170 
Vinyltech Corporation— 17 — 11,500 — 90,764 
BTD Manufacturing, Inc.— 78 — 52,000 — 5,662 
T.O. Plastics, Inc.42 15 4,706 10,400 — — 
Varistar Corporation— — — — — 590,029 
Otter Tail Assurance Limited341 — — — — — 
Total
$5,642 $117 $4,706 $78,900 $7 $752,625 
December 31, 2023
Otter Tail Power Company$2,415 $— $— $— $7 $— 
Northern Pipe Products, Inc.— 7 — 5,000 — 56,917 
Vinyltech Corporation14 17 — 11,500 — 98,016 
BTD Manufacturing, Inc.— 78 — 52,000 — 6,291 
T.O. Plastics, Inc.36 15 — 10,400 — 980 
Varistar Corporation— — — — — 406,468 
Otter Tail Assurance Limited90 — — — — — 
Total
$2,555 $117 $— $78,900 $7 $568,672 
Schedule of Cash Dividends Paid to Parent by Subsidiaries
Dividends paid to OTC (the Parent) from its subsidiaries were as follows:
(in thousands)202420232022
Cash Dividends Paid to Parent by Subsidiaries$78,191 $72,982 $68,680 
v3.25.0.1
Summary of Significant Accounting Policies - Narrative (Details)
$ in Millions
 12 Months Ended
Dec. 31, 2024
USD ($)
segment
plan
foundation
plant
reportingUnit
Dec. 31, 2023
USD ($)
foundation
Dec. 31, 2022
USD ($)
foundation
Property, Plant and Equipment [Line Items]      
Number of reportable segments | segment 3    
Number of jointly owned electric generation plants | plant 2    
Number of reporting units | reportingUnit 3    
Amortization period 10 years    
Amortization expense $ 3.0 $ 1.3 $ 1.4
Number of plans offered | plan 2    
Deferred compensation liability, company contributions $ 29.1 24.6  
Deferred compensation arrangement, contributions by employer 1.3 1.2 0.9
Gains or (losses) recognized due to changes in payment obligations (3.3) $ (3.3) $ 3.1
Coyote Creek Mining Company, L.L.C. (CCMC) | Lignite Sales Agreement | OTP      
Property, Plant and Equipment [Line Items]      
Variable interest entity, reporting entity involvement, maximum loss exposure, amount $ 40.0    
Variable interest entity, reporting entity involvement, maximum loss exposure (as a percent) 35.00%    
Contribution Obligation | Subsidiaries      
Property, Plant and Equipment [Line Items]      
Number of foundations transacting with | foundation 2 2  
Contribution obligations $ 5.5 $ 5.5  
Contribution Obligation Paid | Subsidiaries      
Property, Plant and Equipment [Line Items]      
Number of foundations transacting with | foundation 2 2 2
Amounts of transaction $ 5.5 $ 4.3 $ 4.5
Minimum      
Property, Plant and Equipment [Line Items]      
Intangible asset, useful life 15 years    
Operating lease, term 1 year    
Maximum      
Property, Plant and Equipment [Line Items]      
Intangible asset, useful life 20 years    
Operating lease, term 10 years    
Electric Plant in Service      
Property, Plant and Equipment [Line Items]      
Interest costs capitalized $ 1.9 $ 1.9 $ 0.9
v3.25.0.1
Summary of Significant Accounting Policies - Schedule of Inventories (Details) - USD ($)
$ in Thousands
Dec. 31, 2024
Dec. 31, 2023
Accounting Policies [Abstract]    
Finished Goods $ 43,345 $ 47,614
Work in Process 22,637 26,354
Raw Material, Fuel and Supplies 82,903 75,733
Total Inventories $ 148,885 $ 149,701
v3.25.0.1
Summary of Significant Accounting Policies - Schedule of Estimated Service Lives (Details)
 12 Months Ended
Dec. 31, 2024
Minimum | Equipment | Electric Plant in Service  
Public Utility, Property, Plant and Equipment [Line Items]  
Production, useful life 21 years
Transmission, useful life 51 years
Distribution, useful life 10 years
General, useful life 5 years
Minimum | Equipment | Nonelectric Property, Plant and Equipment  
Public Utility, Property, Plant and Equipment [Line Items]  
Useful life 2 years
Minimum | Buildings and Leasehold Improvements | Nonelectric Property, Plant and Equipment  
Public Utility, Property, Plant and Equipment [Line Items]  
Useful life 2 years
Maximum | Equipment | Electric Plant in Service  
Public Utility, Property, Plant and Equipment [Line Items]  
Production, useful life 114 years
Transmission, useful life 80 years
Distribution, useful life 72 years
General, useful life 56 years
Maximum | Equipment | Nonelectric Property, Plant and Equipment  
Public Utility, Property, Plant and Equipment [Line Items]  
Useful life 20 years
Maximum | Buildings and Leasehold Improvements | Nonelectric Property, Plant and Equipment  
Public Utility, Property, Plant and Equipment [Line Items]  
Useful life 40 years
v3.25.0.1
Summary of Significant Accounting Policies - Schedule of Capitalized Cost and Related Accumulated Amortization (Details) - USD ($)
$ in Thousands
Dec. 31, 2024
Dec. 31, 2023
Accounting Policies [Abstract]    
Cloud Computing Costs $ 15,741 $ 12,782
Accumulated Amortization (3,796) (1,505)
Cloud Computing Costs, net $ 11,945 $ 11,277
v3.25.0.1
Segment Information - Narrative (Details)
 12 Months Ended
Dec. 31, 2024
segment
Concentration Risk [Line Items]  
Number of reportable segments 3
Number of operating segments 2
Revenue from Contract with Customer Benchmark | Customer Concentration Risk | Two Customers | Electric  
Concentration Risk [Line Items]  
Concentration risk (as a percent) 19.00%
Revenue from Contract with Customer Benchmark | Customer Concentration Risk | Two Customers | Manufacturing  
Concentration Risk [Line Items]  
Concentration risk (as a percent) 36.00%
Revenue from Contract with Customer Benchmark | Customer Concentration Risk | Two Customers | Plastics  
Concentration Risk [Line Items]  
Concentration risk (as a percent) 52.00%
Revenue from Contract with Customer Benchmark | Customer Concentration Risk | One Customer  
Concentration Risk [Line Items]  
Concentration risk (as a percent) 11.00%
v3.25.0.1
Segment Information - Schedule of Segment Profit or Loss (Details) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2024
Dec. 31, 2023
Dec. 31, 2022
Segment Reporting Information [Line Items]      
Operating Revenue $ 1,330,548 $ 1,349,166 $ 1,460,209
Operating and Maintenance Expenses 190,422 191,263 181,378
Depreciation and Amortization 107,121 97,954 92,597
Property Taxes 15,662 16,614 17,742
Interest Expense 41,815 37,677 36,016
Selling, General, and Administrative Expenses 80,065 72,663 69,718
Income Tax Expense (Benefit) 65,230 69,298 73,351
Net Income 301,662 294,191 284,184
Electric      
Segment Reporting Information [Line Items]      
Operating Revenue 524,515 528,359 549,699
Manufacturing      
Segment Reporting Information [Line Items]      
Operating Revenue 342,592 402,781 397,983
Plastics      
Segment Reporting Information [Line Items]      
Operating Revenue 463,441 418,026 512,527
Operating Segments      
Segment Reporting Information [Line Items]      
Interest Expense 41,322 36,761 35,331
Income Tax Expense (Benefit) 71,995 73,104 79,074
Net Income 305,391 293,626 296,298
Operating Segments | Electric      
Segment Reporting Information [Line Items]      
Operating Revenue 524,515 528,359 549,699
Production Fuel and Purchased Power 122,506 138,631 165,391
Operating and Maintenance Expenses 190,422 191,263 181,378
Depreciation and Amortization 82,136 75,330 72,050
Property Taxes 15,662 16,614 17,742
Interest Expense 38,216 33,864 31,950
Income Tax Expense (Benefit) (1,544) 1,648 5,065
Other Segment Items (13,846) (13,415) (3,851)
Net Income 90,963 84,424 79,974
Operating Segments | Manufacturing      
Segment Reporting Information [Line Items]      
Operating Revenue 342,592 402,781 397,983
Depreciation and Amortization 20,393 18,495 16,202
Interest Expense 2,516 2,295 2,796
Cost of Goods Sold 283,390 324,245 327,228
Selling, General, and Administrative Expenses 40,110 49,396 41,690
Income Tax Expense (Benefit) 2,895 5,390 5,321
Other Segment Items 0 1 (2)
Net Income 13,681 21,454 20,950
Operating Segments | Plastics      
Segment Reporting Information [Line Items]      
Operating Revenue 463,441 418,026 512,527
Depreciation and Amortization 4,494 4,027 4,205
Interest Expense 590 602 585
Cost of Goods Sold 166,628 143,521 227,571
Selling, General, and Administrative Expenses 24,908 20,103 20,378
Income Tax Expense (Benefit) 70,644 66,066 68,688
Other Segment Items (76) (14) (69)
Net Income $ 200,747 $ 187,748 $ 195,374
v3.25.0.1
Segment Information - Schedule of Capital Expenditures and Identifiable Assets (Details) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2024
Dec. 31, 2023
Dec. 31, 2022
Segment Reporting Information [Line Items]      
Capital Expenditures $ 358,650 $ 287,134 $ 171,134
Identifiable Assets 3,652,082 3,242,568  
Operating Segments | Electric      
Segment Reporting Information [Line Items]      
Capital Expenditures 301,454 240,695 147,869
Identifiable Assets 2,785,522 2,533,831  
Operating Segments | Manufacturing      
Segment Reporting Information [Line Items]      
Capital Expenditures 32,159 23,284 17,954
Identifiable Assets 254,445 251,343  
Operating Segments | Plastics      
Segment Reporting Information [Line Items]      
Capital Expenditures 24,749 23,029 5,245
Identifiable Assets 186,043 164,179  
Corporate      
Segment Reporting Information [Line Items]      
Capital Expenditures 288 126 $ 66
Identifiable Assets $ 426,072 $ 293,215  
v3.25.0.1
Segment Information - Schedule of Reconciliation to Consolidated Amounts (Details) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2024
Dec. 31, 2023
Dec. 31, 2022
Segment Reporting Information [Line Items]      
Depreciation and Amortization $ 107,121 $ 97,954 $ 92,597
Interest Expense 41,815 37,677 36,016
Income Tax Expense (Benefit) 65,230 69,298 73,351
Net Income (Loss) 301,662 294,191 284,184
Operating Segments      
Segment Reporting Information [Line Items]      
Interest Expense 41,322 36,761 35,331
Income Tax Expense (Benefit) 71,995 73,104 79,074
Net Income (Loss) 305,391 293,626 296,298
Operating Segments | Electric      
Segment Reporting Information [Line Items]      
Depreciation and Amortization 82,136 75,330 72,050
Interest Expense 38,216 33,864 31,950
Income Tax Expense (Benefit) (1,544) 1,648 5,065
Net Income (Loss) 90,963 84,424 79,974
Operating Segments | Manufacturing      
Segment Reporting Information [Line Items]      
Depreciation and Amortization 20,393 18,495 16,202
Interest Expense 2,516 2,295 2,796
Income Tax Expense (Benefit) 2,895 5,390 5,321
Net Income (Loss) 13,681 21,454 20,950
Operating Segments | Plastics      
Segment Reporting Information [Line Items]      
Depreciation and Amortization 4,494 4,027 4,205
Interest Expense 590 602 585
Income Tax Expense (Benefit) 70,644 66,066 68,688
Net Income (Loss) 200,747 187,748 195,374
Corporate      
Segment Reporting Information [Line Items]      
Depreciation and Amortization 98 102 140
Interest Expense 493 916 685
Income Tax Expense (Benefit) (6,765) (3,806) (5,723)
Net Income (Loss) $ (3,729) $ 565 $ (12,114)
v3.25.0.1
Revenue (Details) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2024
Dec. 31, 2023
Dec. 31, 2022
Disaggregation of Revenue [Line Items]      
Total Operating Revenue $ 1,330,548 $ 1,349,166 $ 1,460,209
Total Operating Revenues from Contracts with Customers 1,329,973 1,353,476 1,469,475
Electric      
Disaggregation of Revenue [Line Items]      
Total Operating Revenue 524,515 528,359 549,699
Electric Segment - ARP Revenues 575 (4,310) (9,266)
Manufacturing      
Disaggregation of Revenue [Line Items]      
Total Operating Revenue 342,592 402,781 397,983
Plastics      
Disaggregation of Revenue [Line Items]      
Total Operating Revenue 463,441 418,026 512,527
Total Retail | Electric      
Disaggregation of Revenue [Line Items]      
Total Operating Revenue 453,214 455,840 470,300
Retail: Residential | Electric      
Disaggregation of Revenue [Line Items]      
Total Operating Revenue 133,408 135,570 143,888
Retail: Commercial and Industrial | Electric      
Disaggregation of Revenue [Line Items]      
Total Operating Revenue 311,968 312,551 318,494
Retail: Other | Electric      
Disaggregation of Revenue [Line Items]      
Total Operating Revenue 7,838 7,719 7,918
Transmission | Electric      
Disaggregation of Revenue [Line Items]      
Total Operating Revenue 53,517 52,555 52,213
Wholesale | Electric      
Disaggregation of Revenue [Line Items]      
Total Operating Revenue 11,077 12,459 18,539
Other | Electric      
Disaggregation of Revenue [Line Items]      
Total Operating Revenue 6,707 7,505 8,647
Metal Parts and Tooling | Manufacturing      
Disaggregation of Revenue [Line Items]      
Total Operating Revenue 303,077 351,267 338,865
Plastic Products and Tooling | Manufacturing      
Disaggregation of Revenue [Line Items]      
Total Operating Revenue 32,210 41,395 49,080
Scrap Metal | Manufacturing      
Disaggregation of Revenue [Line Items]      
Total Operating Revenue $ 7,305 $ 10,119 $ 10,038
v3.25.0.1
Receivables - Schedule of Receivables (Details) - USD ($)
$ in Thousands
Dec. 31, 2024
Dec. 31, 2023
Dec. 31, 2022
Receivables [Abstract]      
Trade $ 112,169 $ 129,257  
Other 13,799 9,084  
Unbilled Receivables 21,916 21,324  
Total Receivables 147,884 159,665  
Less Allowance for Credit Losses 1,920 2,522 $ 1,648
Receivables, net of allowance for credit losses $ 145,964 $ 157,143  
v3.25.0.1
Receivables - Schedule of Allowance for Credit Loss (Details) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2024
Dec. 31, 2023
Accounts Receivable, Allowance for Credit Loss [Roll Forward]    
Beginning Balance $ 2,522 $ 1,648
Additions Charged to Expense 1,242 2,014
Reductions for Amounts Written Off, Net of Recoveries (1,844) (1,140)
Ending Balance $ 1,920 $ 2,522
v3.25.0.1
Investments - Schedule of Investments (Details) - USD ($)
$ in Thousands
Dec. 31, 2024
Dec. 31, 2023
Long-term Investments    
Corporate-Owned Life Insurance Policies $ 47,895 $ 42,287
Mutual Funds 10,653 7,771
Money Market Funds 596 3,125
Other Investments 27 30
Total Long-term Investments 121,177 62,516
Total Investments 121,930 62,516
Government Debt Securities    
Short-term Investments    
Government Debt Securities 753 0
Long-term Investments    
Debt Securities 60,378 7,724
Corporate Debt Securities    
Long-term Investments    
Debt Securities $ 1,628 $ 1,579
v3.25.0.1
Investments - Narrative (Details) - USD ($)
$ in Millions
 1 Months Ended 12 Months Ended
Apr. 30, 2024
Dec. 31, 2024
Dec. 31, 2023
Investments, Debt and Equity Securities [Abstract]      
Payments to acquire U.S treasuries $ 50.1    
Investment income   $ 19.8 $ 15.2
v3.25.0.1
Investments - Schedule of Amortized Costs and Fair Value of Debt Securities, Available-for-Sale (Details)
$ in Thousands
Dec. 31, 2024
USD ($)
Debt Securities, Available-for-Sale [Line Items]  
Amortized Cost $ 62,520
Gross Unrealized Gains 433
Gross Unrealized (Losses) (194)
Fair Value 62,759
Government Debt Securities  
Debt Securities, Available-for-Sale [Line Items]  
Amortized Cost 60,891
Gross Unrealized Gains 424
Gross Unrealized (Losses) (184)
Fair Value 61,131
Corporate Debt Securities  
Debt Securities, Available-for-Sale [Line Items]  
Amortized Cost 1,629
Gross Unrealized Gains 9
Gross Unrealized (Losses) (10)
Fair Value $ 1,628
v3.25.0.1
Investments - Schedule of Investments Classified by Contractual Maturity Date (Details)
$ in Thousands
Dec. 31, 2024
USD ($)
Investments, Debt and Equity Securities [Abstract]  
Due in one year or less $ 753
Due in one to five years 62,006
Total $ 62,759
v3.25.0.1
Regulatory Matters - Schedule of Regulatory Assets and Liabilities (Details) - USD ($)
$ in Thousands
Dec. 31, 2024
Dec. 31, 2023
Schedule of Regulatory Assets and Liabilities [Line Items]    
Regulatory assets - current $ 9,962 $ 16,127
Regulatory assets, long-term 98,673 95,715
Regulatory liabilities, current 29,307 25,408
Regulatory liabilities, long -term 288,928 276,547
Deferred Income Taxes    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Regulatory liabilities, current 0 0
Regulatory liabilities, long -term 130,387 136,022
Plant Removal Obligations    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Regulatory liabilities, current 0 0
Regulatory liabilities, long -term 126,263 117,030
Fuel Clause Adjustments    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Regulatory liabilities, current 11,432 11,350
Regulatory liabilities, long -term $ 0 0
Regulatory liabilities - remaining recovery/refund period 1 year  
Alternative Revenue Program Riders    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Regulatory liabilities, current $ 14,255 6,885
Regulatory liabilities, long -term $ 0 0
Regulatory liabilities - remaining recovery/refund period 1 year  
North Dakota PTC Refunds    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Regulatory liabilities, current $ 0 0
Regulatory liabilities, long -term 20,099 12,011
Pension and Other Postretirement Benefit Plans    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Regulatory liabilities, current 2,547 6,138
Regulatory liabilities, long -term 10,758 11,307
Other    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Regulatory liabilities, current 1,073 1,035
Regulatory liabilities, long -term 1,421 177
Pension and Other Postretirement Benefit Plans    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Regulatory assets - current 0 154
Regulatory assets, long-term 88,161 86,134
Alternative Revenue Program Riders    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Regulatory assets - current 4,257 3,719
Regulatory assets, long-term $ 195 158
Regulatory assets - Remaining recovery/refund period 2 years  
Deferred Income Taxes    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Regulatory assets - current $ 0 0
Regulatory assets, long-term 8,944 6,940
Fuel Clause Adjustments    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Regulatory assets - current 2,218 7,294
Regulatory assets, long-term $ 0 0
Regulatory assets - Remaining recovery/refund period 1 year  
Derivative Instruments    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Regulatory assets - current $ 1,989 4,210
Regulatory assets, long-term $ 0 0
Regulatory assets - Remaining recovery/refund period 1 year  
Other    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Regulatory assets - current $ 1,498 750
Regulatory assets, long-term $ 1,373 $ 2,483
v3.25.0.1
Regulatory Matters - Narrative (Details) - North Dakota Rate Case - NDPSC - USD ($)
$ in Millions
Dec. 30, 2024
Jul. 03, 2024
Nov. 02, 2023
Schedule of Regulatory Assets and Liabilities [Line Items]      
Requested rate increase, amount     $ 17.4
Requested rate increase, percent (as a percent)     8.40%
Requested return on base rate, percent (as a percent)     7.85%
Requested return on equity, percent (as a percent)     10.60%
Requested equity ratio, percent (as a percent)     53.50%
Revenue, interim rate increase, amount     $ 12.4
Revenue, interim rate increase, percentage (as a percent)     6.00%
Requested rate increase, amended, amount   $ 22.5  
Requested rate increase, amended, amount (as a percent)   10.90%  
Approved rate, revenue requirement $ 225.6    
Approved return on base rate, percent (as a percent) 7.53%    
Approved return on equity, percent (as a percent) 10.10%    
Approved equity ratio, percent (as a percent) 53.50%    
Approved rate increase, amount $ 13.1    
Approved rate increase, percent (as a percent) 6.18%    
Approved rate, revenue requirement reduced $ 3.0    
Earnings sharing, percentage (as a percent) 70.00%    
Earnings sharing, return on equity in excess of, percentage (as a percent) 10.20%    
Earnings sharing, retained, percentage (as a percent) 30.00%    
v3.25.0.1
Property, Plant and Equipment - Schedule of Property, Plant and Equipment (Details) - USD ($)
$ in Thousands
Dec. 31, 2024
Dec. 31, 2023
Electric Plant in Service    
Production $ 1,469,008 $ 1,412,826
Transmission 820,415 777,613
Distribution 726,159 654,704
General 165,361 144,738
Electric Plant in Service 3,180,943 2,989,881
Construction Work in Progress 231,890 137,212
Total Gross Electric Plant 3,412,833 3,127,093
Less Accumulated Depreciation 899,049 851,148
Net Electric Plant 2,513,784 2,275,945
Nonelectric Property, Plant and Equipment    
Nonelectric Property, Plant and Equipment 362,565 311,924
Construction Work in Progress 40,536 38,062
Total Gross Nonelectric Property, Plant and Equipment 403,101 349,986
Less Accumulated Depreciation 224,425 207,556
Net Nonelectric Property, Plant and Equipment 178,676 142,430
Net Property, Plant and Equipment 2,692,460 2,418,375
Equipment    
Nonelectric Property, Plant and Equipment    
Nonelectric Property, Plant and Equipment 260,307 233,571
Buildings and Leasehold Improvements    
Nonelectric Property, Plant and Equipment    
Nonelectric Property, Plant and Equipment 88,680 64,753
Land    
Nonelectric Property, Plant and Equipment    
Nonelectric Property, Plant and Equipment $ 13,578 $ 13,600
v3.25.0.1
Property, Plant and Equipment - Narrative (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2024
Dec. 31, 2023
Dec. 31, 2022
Property, Plant and Equipment [Abstract]      
Depreciation expense $ 99.4 $ 90.8 $ 84.4
v3.25.0.1
Property, Plant and Equipment - Schedule of Jointly Owned Utility Plants (Details) - USD ($)
$ in Thousands
Dec. 31, 2024
Dec. 31, 2023
Big Stone Plant    
Jointly Owned Utility Plant Interests [Line Items]    
Electric Plant in Service $ 345,990 $ 341,683
Construction Work in Progress 459 820
Accumulated Depreciation (135,065) (126,904)
Net Plant $ 211,384 $ 215,599
Big Stone Plant | OTP    
Jointly Owned Utility Plant Interests [Line Items]    
Ownership Percentage 53.90% 53.90%
Coyote Station    
Jointly Owned Utility Plant Interests [Line Items]    
Electric Plant in Service $ 188,066 $ 188,656
Construction Work in Progress 813 104
Accumulated Depreciation (118,268) (115,306)
Net Plant $ 70,611 $ 73,454
Coyote Station | OTP    
Jointly Owned Utility Plant Interests [Line Items]    
Ownership Percentage 35.00% 35.00%
Big Stone South–Ellendale 345 kV line    
Jointly Owned Utility Plant Interests [Line Items]    
Electric Plant in Service $ 106,185 $ 106,185
Construction Work in Progress 0 0
Accumulated Depreciation (8,445) (7,181)
Net Plant $ 97,740 $ 99,004
Big Stone South–Ellendale 345 kV line | OTP    
Jointly Owned Utility Plant Interests [Line Items]    
Ownership Percentage 50.00% 50.00%
Fargo–Monticello 345 kV line    
Jointly Owned Utility Plant Interests [Line Items]    
Electric Plant in Service $ 78,184 $ 78,184
Construction Work in Progress 0 0
Accumulated Depreciation (12,247) (11,238)
Net Plant $ 65,937 $ 66,946
Fargo–Monticello 345 kV line | OTP    
Jointly Owned Utility Plant Interests [Line Items]    
Ownership Percentage 14.20% 14.20%
Big Stone South–Brookings 345 kV line    
Jointly Owned Utility Plant Interests [Line Items]    
Electric Plant in Service $ 53,167 $ 53,170
Construction Work in Progress 0 0
Accumulated Depreciation (5,822) (5,207)
Net Plant $ 47,345 $ 47,963
Big Stone South–Brookings 345 kV line | OTP    
Jointly Owned Utility Plant Interests [Line Items]    
Ownership Percentage 50.00% 50.00%
Brookings–Southeast Twin Cities 345 kV line    
Jointly Owned Utility Plant Interests [Line Items]    
Electric Plant in Service $ 28,013 $ 26,409
Construction Work in Progress 1,131 83
Accumulated Depreciation (3,941) (3,617)
Net Plant $ 25,203 $ 22,875
Brookings–Southeast Twin Cities 345 kV line | OTP    
Jointly Owned Utility Plant Interests [Line Items]    
Ownership Percentage 4.80% 4.80%
Bemidji–Grand Rapids 230 kV line    
Jointly Owned Utility Plant Interests [Line Items]    
Electric Plant in Service $ 16,331 $ 16,331
Construction Work in Progress 0 0
Accumulated Depreciation (3,693) (3,568)
Net Plant $ 12,638 $ 12,763
Bemidji–Grand Rapids 230 kV line | OTP    
Jointly Owned Utility Plant Interests [Line Items]    
Ownership Percentage 14.80% 14.80%
Jamestown– Ellendale 345 kV line    
Jointly Owned Utility Plant Interests [Line Items]    
Electric Plant in Service $ 0 $ 0
Construction Work in Progress 5,509 1,121
Accumulated Depreciation 0 0
Net Plant $ 5,509 $ 1,121
Jamestown– Ellendale 345 kV line | OTP    
Jointly Owned Utility Plant Interests [Line Items]    
Ownership Percentage 50.00% 50.00%
Big Stone South–Alexandria 345 kV line    
Jointly Owned Utility Plant Interests [Line Items]    
Electric Plant in Service $ 0 $ 0
Construction Work in Progress 2,418 555
Accumulated Depreciation 0 0
Net Plant $ 2,418 $ 555
Big Stone South–Alexandria 345 kV line | OTP    
Jointly Owned Utility Plant Interests [Line Items]    
Ownership Percentage 40.00% 40.00%
Alexandria–Big Oaks 345 kV line    
Jointly Owned Utility Plant Interests [Line Items]    
Electric Plant in Service $ 0 $ 0
Construction Work in Progress 417 343
Accumulated Depreciation 0 0
Net Plant $ 417 $ 343
Alexandria–Big Oaks 345 kV line | OTP    
Jointly Owned Utility Plant Interests [Line Items]    
Ownership Percentage 14.20% 14.20%
Oslo - Lake Ardoch 115 kV line    
Jointly Owned Utility Plant Interests [Line Items]    
Electric Plant in Service $ 0  
Construction Work in Progress 2,646  
Accumulated Depreciation 0  
Net Plant $ 2,646  
Oslo - Lake Ardoch 115 kV line | OTP    
Jointly Owned Utility Plant Interests [Line Items]    
Ownership Percentage 72.00%  
v3.25.0.1
Intangible Assets - Schedule of Goodwill by Segment (Details) - USD ($)
$ in Thousands
Dec. 31, 2024
Dec. 31, 2023
Goodwill [Line Items]    
Goodwill $ 37,572 $ 37,572
Manufacturing    
Goodwill [Line Items]    
Goodwill 18,270 18,270
Plastics    
Goodwill [Line Items]    
Goodwill $ 19,302 $ 19,302
v3.25.0.1
Intangible Assets - Schedule of Components of Intangible Assets (Details) - USD ($)
$ in Thousands
Dec. 31, 2024
Dec. 31, 2023
Finite-Lived Intangible Assets [Line Items]    
Gross Amount $ 22,517 $ 22,517
Accumulated Amortization 16,774 15,674
Net Carrying Amount 5,743 6,843
Customer Relationships    
Finite-Lived Intangible Assets [Line Items]    
Gross Amount 22,491 22,491
Accumulated Amortization 16,766 15,667
Net Carrying Amount 5,725 6,824
Other    
Finite-Lived Intangible Assets [Line Items]    
Gross Amount 26 26
Accumulated Amortization 8 7
Net Carrying Amount $ 18 $ 19
v3.25.0.1
Intangible Assets - Narrative (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2024
Dec. 31, 2023
Dec. 31, 2022
Goodwill and Intangible Assets Disclosure [Abstract]      
Amortization expense $ 1.1 $ 1.1 $ 1.1
v3.25.0.1
Intangible Assets - Schedule of Intangible Assets Future Amortization Expenses (Details)
$ in Thousands
Dec. 31, 2024
USD ($)
Goodwill and Intangible Assets Disclosure [Abstract]  
2025 $ 1,100
2026 1,092
2027 1,090
2028 554
2029 $ 285
v3.25.0.1
Leases - Schedule of Lease Costs (Details) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2024
Dec. 31, 2023
Dec. 31, 2022
Lease Cost      
Operating Lease Cost $ 6,688 $ 6,309 $ 5,606
Variable Lease Cost 1,460 1,433 1,386
Short-Term Lease Cost 2,746 2,525 1,517
Total Lease Cost 10,894 10,267 8,509
Lease Cash Flows      
Operating Cash Flows from Operating Leases $ 6,762 $ 6,424 $ 5,592
v3.25.0.1
Leases - Schedule of Supplemental Balance Sheet (Details) - USD ($)
$ in Thousands
Dec. 31, 2024
Dec. 31, 2023
Leases [Abstract]    
Right of Use Lease Asset $ 28,179 $ 16,788
Lease Liabilities    
Current 4,776 5,756
Long-Term 23,567 11,258
Total Lease Liabilities $ 28,343 $ 17,014
Operating Lease, Right-of-Use Asset, Statement of Financial Position [Extensible List] Other Noncurrent Assets Other Noncurrent Assets
Operating Lease, Liability, Current, Statement of Financial Position [Extensible List] Other Current Liabilities Other Current Liabilities
Operating Lease, Liability, Noncurrent, Statement of Financial Position [Extensible List] Other Noncurrent Liabilities Other Noncurrent Liabilities
v3.25.0.1
Leases - Narrative (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2024
Dec. 31, 2023
Leases [Abstract]    
Right-of-use asset obtained in exchange for operating lease liability $ 17.6 $ 3.6
v3.25.0.1
Leases - Schedule of Operating Lease Liability Maturities (Details) - USD ($)
$ in Thousands
Dec. 31, 2024
Dec. 31, 2023
Leases [Abstract]    
2025 $ 6,350  
2026 5,638  
2027 4,785  
2028 3,604  
2029 2,750  
Thereafter 12,921  
Total Lease Payments 36,048  
Less: Interest 7,705  
Present Value of Lease Liabilities $ 28,343 $ 17,014
v3.25.0.1
Leases - Schedule of Weighted-Average Remaining Lease Term and Discount Rate (Details)
Dec. 31, 2024
Dec. 31, 2023
Leases [Abstract]    
Weighted-Average Remaining Lease Term (in years) 7 years 10 months 24 days 3 years 4 months 24 days
Weighted-Average Discount Rate 6.37% 5.40%
v3.25.0.1
Short-Term and Long-Term Borrowings - Schedule of Debt (Details) - USD ($)
$ in Thousands
Dec. 31, 2024
Dec. 31, 2023
Debt Instrument [Line Items]    
Short-Term Debt $ 69,615 $ 81,422
Long-Term Debt 943,734 824,059
Total 1,013,349 905,481
OTC    
Debt Instrument [Line Items]    
Short-Term Debt 0 0
Long-Term Debt 79,900 79,849
Total 79,900 79,849
OTP    
Debt Instrument [Line Items]    
Short-Term Debt 69,615 81,422
Long-Term Debt 863,834 744,210
Total $ 933,449 $ 825,632
v3.25.0.1
Short-Term and Long-Term Borrowings - Schedule of Lines of Credit (Details) - USD ($)
Dec. 31, 2024
Dec. 31, 2023
Line of Credit Facility [Line Items]    
Line Limit $ 390,000,000  
Amount Outstanding 69,615,000 $ 81,422,000
Letters of Credit 8,772,000  
Amount Available 311,613,000 249,446,000
OTC Credit Agreement    
Line of Credit Facility [Line Items]    
Line Limit 170,000,000  
Amount Outstanding 0  
Letters of Credit 0  
Amount Available 170,000,000 170,000,000
OTP Credit Agreement    
Line of Credit Facility [Line Items]    
Line Limit 220,000,000  
Amount Outstanding 69,615,000  
Letters of Credit 8,772,000  
Amount Available $ 141,613,000 $ 79,446,000
v3.25.0.1
Short-Term and Long-Term Borrowings - Narrative (Details) - USD ($)
 12 Months Ended
Mar. 28, 2024
Dec. 31, 2024
Dec. 11, 2024
Dec. 31, 2023
Debt Instrument [Line Items]        
Line of credit facility, maximum borrowing capacity   $ 390,000,000    
Debt, weighted average interest rate (as a percent)   5.61%   6.70%
Long-term debt   $ 943,734,000   $ 824,059,000
Long-term debt   $ 947,000,000   827,000,000
Debt instrument, redemption price, percentage of principal amount redeemed (as a percent) 10.00%      
Debt instrument, redemption price (as a percent) 100.00% 100.00%    
Senior Notes        
Debt Instrument [Line Items]        
Long-term debt $ 120,000,000.0      
Minimum        
Debt Instrument [Line Items]        
Debt instrument, basis spread on variable rate (as a percent)   0.00%    
Debt to total capitalization ratio   0.60    
Interest and dividend coverage ratio   1.50    
Maximum        
Debt Instrument [Line Items]        
Debt instrument, basis spread on variable rate (as a percent)   2.00%    
Debt to total capitalization ratio   0.65    
OTC Credit Agreement        
Debt Instrument [Line Items]        
Line of credit facility, maximum borrowing capacity   $ 170,000,000    
Line of credit facility, increased current borrowing capacity, accordion provision     $ 290,000,000  
OTP Credit Agreement        
Debt Instrument [Line Items]        
Line of credit facility, maximum borrowing capacity   220,000,000    
Line of credit facility, increased current borrowing capacity, accordion provision     300,000,000  
Series 2024A Senior Unsecured Notes        
Debt Instrument [Line Items]        
Long-term debt   $ 60,000,000   0
Interest rate (as a percent)   5.48%    
Series 2024A Senior Unsecured Notes | Senior Notes        
Debt Instrument [Line Items]        
Long-term debt $ 60,000,000.0      
Interest rate (as a percent) 5.48%      
Series 2024B Senior Unsecured Notes        
Debt Instrument [Line Items]        
Long-term debt   $ 60,000,000   $ 0
Interest rate (as a percent)   5.77%    
Series 2024B Senior Unsecured Notes | Senior Notes        
Debt Instrument [Line Items]        
Long-term debt $ 60,000,000.0      
Interest rate (as a percent) 5.77%      
Revolving Credit Facility | OTC Credit Agreement        
Debt Instrument [Line Items]        
Line of credit facility, maximum borrowing capacity     170,000,000  
Revolving Credit Facility | OTP Credit Agreement        
Debt Instrument [Line Items]        
Line of credit facility, maximum borrowing capacity     220,000,000  
Letter of Credit | OTC Credit Agreement        
Debt Instrument [Line Items]        
Line of credit facility, maximum borrowing capacity     40,000,000  
Letter of Credit | OTP Credit Agreement        
Debt Instrument [Line Items]        
Line of credit facility, maximum borrowing capacity     $ 50,000,000  
v3.25.0.1
Short-Term and Long-Term Borrowings - Schedule of Outstanding Long-term Debt (Details) - USD ($)
$ in Thousands
Dec. 31, 2024
Dec. 31, 2023
Debt Instrument [Line Items]    
Long-term debt $ 947,000 $ 827,000
Unamortized Long-Term Debt Issuance Costs 3,266 2,941
Total Long-Term Debt Net of Unamortized Debt Issuance Costs $ 943,734 824,059
Guaranteed Senior Notes    
Debt Instrument [Line Items]    
Rate (as a percent) 3.55%  
Long-term debt $ 80,000 80,000
Series 2007C Senior Unsecured Notes    
Debt Instrument [Line Items]    
Rate (as a percent) 6.37%  
Long-term debt $ 42,000 42,000
Series 2013A Senior Unsecured Notes    
Debt Instrument [Line Items]    
Rate (as a percent) 4.68%  
Long-term debt $ 60,000 60,000
Series 2019A Senior Unsecured Notes    
Debt Instrument [Line Items]    
Rate (as a percent) 3.07%  
Long-term debt $ 10,000 10,000
Series 2020A Senior Unsecured Notes    
Debt Instrument [Line Items]    
Rate (as a percent) 3.22%  
Long-term debt $ 10,000 10,000
Series 2020B Senior Unsecured Notes    
Debt Instrument [Line Items]    
Rate (as a percent) 3.22%  
Long-term debt $ 40,000 40,000
Series 2021A Senior Unsecured Notes    
Debt Instrument [Line Items]    
Rate (as a percent) 2.74%  
Long-term debt $ 40,000 40,000
Series 2024A Senior Unsecured Notes    
Debt Instrument [Line Items]    
Rate (as a percent) 5.48%  
Long-term debt $ 60,000 0
Series 2007D Senior Unsecured Notes    
Debt Instrument [Line Items]    
Rate (as a percent) 6.47%  
Long-term debt $ 50,000 50,000
Series 2019B Senior Unsecured Notes    
Debt Instrument [Line Items]    
Rate (as a percent) 3.52%  
Long-term debt $ 26,000 26,000
Series 2020C Senior Unsecured Notes    
Debt Instrument [Line Items]    
Rate (as a percent) 3.62%  
Long-term debt $ 10,000 10,000
Series 2013B Senior Unsecured Notes    
Debt Instrument [Line Items]    
Rate (as a percent) 5.47%  
Long-term debt $ 90,000 90,000
Series 2018A Senior Unsecured Notes    
Debt Instrument [Line Items]    
Rate (as a percent) 4.07%  
Long-term debt $ 100,000 100,000
Series 2019C Senior Unsecured Notes    
Debt Instrument [Line Items]    
Rate (as a percent) 3.82%  
Long-term debt $ 64,000 64,000
Series 2020D Senior Unsecured Notes    
Debt Instrument [Line Items]    
Rate (as a percent) 3.92%  
Long-term debt $ 15,000 15,000
Series 2021B Senior Unsecured Notes    
Debt Instrument [Line Items]    
Rate (as a percent) 3.69%  
Long-term debt $ 100,000 100,000
Series 2022A Senior Unsecured Notes    
Debt Instrument [Line Items]    
Rate (as a percent) 3.77%  
Long-term debt $ 90,000 90,000
Series 2024B Senior Unsecured Notes    
Debt Instrument [Line Items]    
Rate (as a percent) 5.77%  
Long-term debt $ 60,000 $ 0
v3.25.0.1
Short-Term and Long-Term Borrowings - Schedule of Maturities of Long-term Debt (Details)
$ in Thousands
Dec. 31, 2024
USD ($)
Debt Disclosure [Abstract]  
2025 $ 0
2026 80,000
2027 42,000
2028 0
2029 $ 70,000
v3.25.0.1
Employee Postretirement Benefits - Narrative (Details)
$ in Thousands
 12 Months Ended
Dec. 31, 2024
USD ($)
year
Dec. 31, 2023
USD ($)
Dec. 31, 2022
USD ($)
Defined Benefit Plan Disclosure [Line Items]      
Payment for pension benefits $ 0 $ 0 $ 20,000
Defined contribution plan, cost $ 9,300 7,800 $ 6,700
Pension Benefits (Pension Plan)      
Defined Benefit Plan Disclosure [Line Items]      
Age for compensation | year 65    
Age below which compensation is reduced | year 62    
Vesting period 5 years    
Investment redemption, notice period 95 days    
Subscriptions, lock-up period 3 years    
Accumulated benefit obligation $ 288,500 288,800  
Decrease in benefit obligation for change in discount rate 4,700 2,200  
Decrease from plan participant census data 3,000    
Actual returns 15,000    
Expected returns 25,500    
Obligation impact $ 10,500    
Other Postretirement Benefits Plan      
Defined Benefit Plan Disclosure [Line Items]      
Health insurance benefits, requisite age 55 years    
Health insurance benefits, requisite service period 10 years    
Decrease in benefit obligation for change in discount rate $ 200 1,300  
Decrease from plan participant census data 900    
Increase in benefit obligation for change in expected healthcare cost trends 400    
Executive Survivor and Supplemental Retirement Plan      
Defined Benefit Plan Disclosure [Line Items]      
Accumulated benefit obligation 35,300 35,800  
Decrease in benefit obligation for change in discount rate $ 200 $ 100  
v3.25.0.1
Employee Postretirement Benefits - Schedule of Target Asset Allocation Ranges By Asset Class (Details) - Pension Benefits (Pension Plan)
Dec. 31, 2024
Dec. 31, 2023
Defined Benefit Plan Disclosure [Line Items]    
Actual Allocation 100.00% 100.00%
Return Enhancement    
Defined Benefit Plan Disclosure [Line Items]    
Actual Allocation 41.00% 48.00%
Return Enhancement | 35 - 60% | Minimum    
Defined Benefit Plan Disclosure [Line Items]    
Permitted Range 35.00%  
Return Enhancement | 35 - 60% | Maximum    
Defined Benefit Plan Disclosure [Line Items]    
Permitted Range 60.00%  
Risk Management    
Defined Benefit Plan Disclosure [Line Items]    
Actual Allocation 59.00% 51.00%
Risk Management | 40 - 80% | Minimum    
Defined Benefit Plan Disclosure [Line Items]    
Permitted Range 40.00%  
Risk Management | 40 - 80% | Maximum    
Defined Benefit Plan Disclosure [Line Items]    
Permitted Range 80.00%  
Alternatives    
Defined Benefit Plan Disclosure [Line Items]    
Actual Allocation 0.00% 1.00%
Alternatives | 0 - 20% | Minimum    
Defined Benefit Plan Disclosure [Line Items]    
Permitted Range 0.00%  
Alternatives | 0 - 20% | Maximum    
Defined Benefit Plan Disclosure [Line Items]    
Permitted Range 20.00%  
v3.25.0.1
Employee Postretirement Benefits - Schedule of Allocation of Plan Assets (Details) - USD ($)
$ in Thousands
Dec. 31, 2024
Dec. 31, 2023
Defined Benefit Plan Disclosure [Line Items]    
Plan Assets $ 327,275 $ 330,479
Level 1    
Defined Benefit Plan Disclosure [Line Items]    
Plan Assets 326,214 328,961
Level 2    
Defined Benefit Plan Disclosure [Line Items]    
Plan Assets 0 0
Level 3    
Defined Benefit Plan Disclosure [Line Items]    
Plan Assets 0 0
NAV    
Defined Benefit Plan Disclosure [Line Items]    
Plan Assets 1,061 1,518
Equity Funds    
Defined Benefit Plan Disclosure [Line Items]    
Plan Assets 116,889 127,159
Equity Funds | Level 1    
Defined Benefit Plan Disclosure [Line Items]    
Plan Assets 116,889 127,159
Equity Funds | Level 2    
Defined Benefit Plan Disclosure [Line Items]    
Plan Assets 0 0
Equity Funds | Level 3    
Defined Benefit Plan Disclosure [Line Items]    
Plan Assets 0 0
Equity Funds | NAV    
Defined Benefit Plan Disclosure [Line Items]    
Plan Assets 0 0
Fixed Income Funds    
Defined Benefit Plan Disclosure [Line Items]    
Plan Assets 175,310 167,604
Fixed Income Funds | Level 1    
Defined Benefit Plan Disclosure [Line Items]    
Plan Assets 175,310 167,604
Fixed Income Funds | Level 2    
Defined Benefit Plan Disclosure [Line Items]    
Plan Assets 0 0
Fixed Income Funds | Level 3    
Defined Benefit Plan Disclosure [Line Items]    
Plan Assets 0 0
Fixed Income Funds | NAV    
Defined Benefit Plan Disclosure [Line Items]    
Plan Assets 0 0
Hybrid Funds    
Defined Benefit Plan Disclosure [Line Items]    
Plan Assets 10,106 10,980
Hybrid Funds | Level 1    
Defined Benefit Plan Disclosure [Line Items]    
Plan Assets 10,106 10,980
Hybrid Funds | Level 2    
Defined Benefit Plan Disclosure [Line Items]    
Plan Assets 0 0
Hybrid Funds | Level 3    
Defined Benefit Plan Disclosure [Line Items]    
Plan Assets 0 0
Hybrid Funds | NAV    
Defined Benefit Plan Disclosure [Line Items]    
Plan Assets 0 0
U.S. Treasury Securities    
Defined Benefit Plan Disclosure [Line Items]    
Plan Assets 23,909 23,218
U.S. Treasury Securities | Level 1    
Defined Benefit Plan Disclosure [Line Items]    
Plan Assets 23,909 23,218
U.S. Treasury Securities | Level 2    
Defined Benefit Plan Disclosure [Line Items]    
Plan Assets 0 0
U.S. Treasury Securities | Level 3    
Defined Benefit Plan Disclosure [Line Items]    
Plan Assets 0 0
U.S. Treasury Securities | NAV    
Defined Benefit Plan Disclosure [Line Items]    
Plan Assets 0 0
SEI Energy Debt Collective Fund    
Defined Benefit Plan Disclosure [Line Items]    
Plan Assets 1,061 1,518
SEI Energy Debt Collective Fund | Level 1    
Defined Benefit Plan Disclosure [Line Items]    
Plan Assets 0 0
SEI Energy Debt Collective Fund | Level 2    
Defined Benefit Plan Disclosure [Line Items]    
Plan Assets 0 0
SEI Energy Debt Collective Fund | Level 3    
Defined Benefit Plan Disclosure [Line Items]    
Plan Assets 0 0
SEI Energy Debt Collective Fund | NAV    
Defined Benefit Plan Disclosure [Line Items]    
Plan Assets $ 1,061 $ 1,518
v3.25.0.1
Employee Postretirement Benefits - Schedule of Changes in Fair Value of Plan Assets and Plan's Benefit Obligations (Details) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2024
Dec. 31, 2023
Dec. 31, 2022
Change in Fair Value of Plan Assets:      
Fair Value of Plan Assets, beginning balance $ 330,479    
Fair Value of Plan Assets, ending balance 327,275 $ 330,479  
Pension Benefits (Pension Plan)      
Change in Fair Value of Plan Assets:      
Fair Value of Plan Assets, beginning balance 330,479 313,797  
Actual Return on Plan Assets 14,976 34,196  
Company Contributions 0 0  
Benefit Payments (18,180) (17,514)  
Participant Premium Payments 0 0  
Fair Value of Plan Assets, ending balance 327,275 330,479 $ 313,797
Change in Benefit Obligation:      
Projected Benefit Obligation, beginning balance 318,801 308,055  
Service Cost 3,886 3,698 6,576
Interest Cost 17,189 16,436 12,344
Benefit Payments (18,180) (17,514)  
Participant Premium Payments 0 0  
Plan Amendments 0 0  
Actuarial (Gain) Loss (7,686) 8,126  
Projected Benefit Obligation, ending balance 314,010 318,801 308,055
Funded Status 13,265 11,678  
Amounts Recognized in Consolidated Balance Sheets at December 31:      
Noncurrent Assets 13,265 11,678  
Current Liabilities 0 0  
Noncurrent Liabilities 0 0  
Net Asset (Liability) 13,265 11,678  
Pension Benefits (ESSRP)      
Change in Fair Value of Plan Assets:      
Fair Value of Plan Assets, beginning balance 0 0  
Actual Return on Plan Assets 0 0  
Company Contributions 2,694 2,197  
Benefit Payments (2,694) (2,197)  
Participant Premium Payments 0 0  
Fair Value of Plan Assets, ending balance 0 0 0
Change in Benefit Obligation:      
Projected Benefit Obligation, beginning balance 35,780 35,624  
Service Cost 0 72 195
Interest Cost 1,897 1,889 1,341
Benefit Payments (2,694) (2,197)  
Participant Premium Payments 0 0  
Plan Amendments 0 0  
Actuarial (Gain) Loss 331 392  
Projected Benefit Obligation, ending balance 35,314 35,780 35,624
Funded Status (35,314) (35,780)  
Amounts Recognized in Consolidated Balance Sheets at December 31:      
Noncurrent Assets 0 0  
Current Liabilities (2,700) (2,679)  
Noncurrent Liabilities (32,614) (33,101)  
Net Asset (Liability) (35,314) (35,780)  
Postretirement Benefits      
Change in Fair Value of Plan Assets:      
Fair Value of Plan Assets, beginning balance 0 0  
Actual Return on Plan Assets 0 0  
Company Contributions 1,568 3,167  
Benefit Payments (3,734) (8,900)  
Participant Premium Payments 2,166 5,733  
Fair Value of Plan Assets, ending balance 0 0 0
Change in Benefit Obligation:      
Projected Benefit Obligation, beginning balance 30,145 49,947  
Service Cost 490 565 1,338
Interest Cost 1,600 2,416 2,041
Benefit Payments (3,734) (8,900)  
Participant Premium Payments 2,166 5,733  
Plan Amendments 0 (17,493)  
Actuarial (Gain) Loss (664) (2,123)  
Projected Benefit Obligation, ending balance 30,003 30,145 $ 49,947
Funded Status (30,003) (30,145)  
Amounts Recognized in Consolidated Balance Sheets at December 31:      
Noncurrent Assets 0 0  
Current Liabilities (2,618) (2,469)  
Noncurrent Liabilities (27,385) (27,676)  
Net Asset (Liability) $ (30,003) $ (30,145)  
v3.25.0.1
Employee Postretirement Benefits - Schedule of Assumptions Used to Determine Benefit Obligations (Details)
 12 Months Ended
Dec. 31, 2024
Dec. 31, 2023
Participants Age 40 to 49, Non-Union    
Defined Benefit Plan Disclosure [Line Items]    
Long-Term Rate of Compensation Increase 3.50%  
Participants Age 50 and Older, Non-Union    
Defined Benefit Plan Disclosure [Line Items]    
Long-Term Rate of Compensation Increase 3.00%  
Pension Benefits (Pension Plan)    
Defined Benefit Plan Disclosure [Line Items]    
Discount Rate 5.70% 5.57%
Pension Benefits (Pension Plan) | Participants up to Age 39    
Defined Benefit Plan Disclosure [Line Items]    
Long-Term Rate of Compensation Increase 4.50% 4.50%
Pension Benefits (Pension Plan) | Participants Ages 40 to 49    
Defined Benefit Plan Disclosure [Line Items]    
Long-Term Rate of Compensation Increase 4.50% 4.50%
Pension Benefits (Pension Plan) | Participants Age 50 and Older    
Defined Benefit Plan Disclosure [Line Items]    
Long-Term Rate of Compensation Increase 3.75% 3.75%
Pension Benefits (ESSRP)    
Defined Benefit Plan Disclosure [Line Items]    
Discount Rate 5.60% 5.53%
Long-Term Rate of Compensation Increase 3.00% 3.00%
Postretirement Benefits    
Defined Benefit Plan Disclosure [Line Items]    
Discount Rate 5.61% 5.53%
Healthcare Cost Immediate Trend Rate 6.44% 6.97%
Healthcare Cost Ultimate Trend Rate 4.00% 4.00%
Year the Rate Reaches the Ultimate Trend Rate 2048 2048
v3.25.0.1
Employee Postretirement Benefits - Schedule of Components of Net Periodic Benefit Cost (Details) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2024
Dec. 31, 2023
Dec. 31, 2022
Defined Benefit Plan Disclosure [Line Items]      
Net Periodic Benefit Cost $ (6,600) $ (7,487) $ 5,913
Pension Benefits (Pension Plan)      
Defined Benefit Plan Disclosure [Line Items]      
Service Cost 3,886 3,698 6,576
Interest Cost 17,189 16,436 12,344
Expected Return on Assets (25,518) (25,914) (23,684)
Amortization of Prior Service Cost 0 0 0
Amortization of Net Actuarial Loss 158 0 7,865
Net Periodic Benefit Cost (4,285) (5,780) 3,101
Pension Benefits (ESSRP)      
Defined Benefit Plan Disclosure [Line Items]      
Service Cost 0 72 195
Interest Cost 1,897 1,889 1,341
Expected Return on Assets 0 0 0
Amortization of Prior Service Cost 0 0 0
Amortization of Net Actuarial Loss 0 0 567
Net Periodic Benefit Cost 1,897 1,961 2,103
Postretirement Benefits      
Defined Benefit Plan Disclosure [Line Items]      
Service Cost 490 565 1,338
Interest Cost 1,600 2,416 2,041
Expected Return on Assets 0 0 0
Amortization of Prior Service Cost (6,302) (6,649) (5,733)
Amortization of Net Actuarial Loss 0 0 3,063
Net Periodic Benefit Cost $ (4,212) $ (3,668) $ 709
v3.25.0.1
Employee Postretirement Benefits - Schedule of Composition of Periodic Benefit Cost (Details) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2024
Dec. 31, 2023
Dec. 31, 2022
Retirement Benefits [Abstract]      
Net Periodic Benefit Cost $ (6,600) $ (7,487) $ 5,913
Net Amount Amortized Due to the Effect of Regulation 1,367 1,225 1,121
Net Periodic Benefit Cost Recognized $ (5,233) $ (6,262) $ 7,034
v3.25.0.1
Employee Postretirement Benefits - Schedule of Weighted-Average Assumptions Used to Determine Net Periodic Benefit Cost (Details)
 12 Months Ended
Dec. 31, 2024
Dec. 31, 2023
Dec. 31, 2022
Pension Benefits (Pension Plan)      
Defined Benefit Plan Disclosure [Line Items]      
Discount Rate 5.57% 5.51% 3.03%
Long-Term Rate of Return on Plan Assets 7.00% 7.00% 6.30%
Pension Benefits (Pension Plan) | Participants up to Age 39      
Defined Benefit Plan Disclosure [Line Items]      
Long-Term Rate of Compensation Increase 4.50% 4.50% 4.50%
Pension Benefits (Pension Plan) | Participants Ages 40 to 49      
Defined Benefit Plan Disclosure [Line Items]      
Long-Term Rate of Compensation Increase 4.00% 3.50% 3.50%
Pension Benefits (Pension Plan) | Participants Age 50 and Older      
Defined Benefit Plan Disclosure [Line Items]      
Long-Term Rate of Compensation Increase 3.38% 2.75% 2.75%
Pension Benefits (ESSRP)      
Defined Benefit Plan Disclosure [Line Items]      
Discount Rate 5.53% 5.51% 2.93%
Long-Term Rate of Compensation Increase 3.00% 3.00% 3.00%
Postretirement Benefits      
Defined Benefit Plan Disclosure [Line Items]      
Discount Rate 5.53% 5.52% 3.01%
v3.25.0.1
Employee Postretirement Benefits - Schedule of Amounts Not Yet Recognized as Components of Net Periodic Benefit Cost (Details) - USD ($)
$ in Thousands
Dec. 31, 2024
Dec. 31, 2023
Pension Benefits (Pension Plan)    
Regulatory Assets (Liabilities):    
Unrecognized Prior Service Cost $ 0 $ 0
Unrecognized Actuarial Loss 87,868 85,227
Net Regulatory Assets (Liabilities) 87,868 85,227
Accumulated Other Comprehensive Income (Loss):    
Unrecognized Prior Service Cost 0 0
Unrecognized Actuarial Gain (Loss) 1,937 1,994
Total Accumulated Other Comprehensive Income (Loss) 1,937 1,994
Pension Benefits (ESSRP)    
Regulatory Assets (Liabilities):    
Unrecognized Prior Service Cost 0 0
Unrecognized Actuarial Loss 292 1,061
Net Regulatory Assets (Liabilities) 292 1,061
Accumulated Other Comprehensive Income (Loss):    
Unrecognized Prior Service Cost 0 0
Unrecognized Actuarial Gain (Loss) (2,502) (1,403)
Total Accumulated Other Comprehensive Income (Loss) (2,502) (1,403)
Postretirement Benefits    
Regulatory Assets (Liabilities):    
Unrecognized Prior Service Cost (12,703) (18,845)
Unrecognized Actuarial Loss 1,121 1,759
Net Regulatory Assets (Liabilities) (11,582) (17,086)
Accumulated Other Comprehensive Income (Loss):    
Unrecognized Prior Service Cost 339 498
Unrecognized Actuarial Gain (Loss) 732 707
Total Accumulated Other Comprehensive Income (Loss) $ 1,071 $ 1,205
v3.25.0.1
Employee Postretirement Benefits - Schedule of Expected Benefit Payments (Details)
$ in Thousands
Dec. 31, 2024
USD ($)
Defined Benefit Plan Disclosure [Line Items]  
2025 $ 24,661
2026 25,317
2027 25,919
2028 26,346
2029 27,007
2030-2034 137,569
Projected Pension Plan Benefit Payments  
Defined Benefit Plan Disclosure [Line Items]  
2025 19,274
2026 19,804
2027 20,284
2028 20,859
2029 21,420
2030-2034 111,275
Projected ESSRP Benefit Payments  
Defined Benefit Plan Disclosure [Line Items]  
2025 2,769
2026 2,899
2027 3,059
2028 2,999
2029 3,070
2030-2034 14,275
Projected Postretirement Benefit Payments  
Defined Benefit Plan Disclosure [Line Items]  
2025 2,618
2026 2,614
2027 2,576
2028 2,488
2029 2,517
2030-2034 $ 12,019
v3.25.0.1
Asset Retirement Obligations - Narrative (Details) - USD ($)
 12 Months Ended
Dec. 31, 2024
Dec. 31, 2023
Asset Retirement Obligation Disclosure [Abstract]    
Asset retirement obligation, legally restricted assets, fair value $ 0 $ 0
Asset retirement obligation, current 100,000 100,000
Asset retirement obligations, noncurrent 42,100,000 36,400,000
Additional liabilities for new obligations $ 2,991,000 $ 4,506,000
v3.25.0.1
Asset Retirement Obligations - Schedule of Change in Asset Retirement Obligations (Details) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2024
Dec. 31, 2023
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]    
Beginning Balance $ 36,477 $ 25,182
New Obligations Recognized 2,991 4,506
Adjustments Due to Revisions in Cash Flow Estimates 1,098 8,394
Accrued Accretion 1,676 1,191
Settlements (79) (2,796)
Ending Balance $ 42,163 $ 36,477
v3.25.0.1
Income Taxes - Schedule of Income Tax Expense (Benefit) (Details) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2024
Dec. 31, 2023
Dec. 31, 2022
Current      
Federal Income Taxes $ 36,238 $ 41,253 $ 31,949
State Income Taxes 6,533 15,126 9,568
Deferred      
Federal Income Taxes 13,078 9,832 22,480
State Income Taxes 9,979 3,676 9,943
Tax Credits      
North Dakota Wind Tax Credit Amortization, Net of Federal Tax (586) (586) (586)
Investment Tax Credit Amortization (12) (3) (3)
Income Taxes at Effective Tax Rate $ 65,230 $ 69,298 $ 73,351
v3.25.0.1
Income Taxes - Schedule of Effective Income Tax Rate (Details) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2024
Dec. 31, 2023
Dec. 31, 2022
Income Tax Disclosure [Abstract]      
Income Taxes at Federal Statutory Rate $ 77,047 $ 76,332 $ 75,082
Increases (Decreases) in Tax from:      
State Taxes on Income, Net of Federal Tax 14,360 14,429 15,049
Production Tax Credits (PTCs) (20,106) (17,394) (14,985)
Amortization of Excess Deferred Income Taxes (2,788) (2,205) (1,625)
North Dakota Wind Tax Credit Amortization, Net of Federal Tax (586) (586) (586)
Other, Net (2,697) (1,278) 416
Income Taxes at Effective Tax Rate $ 65,230 $ 69,298 $ 73,351
Income Taxes at Federal Statutory Rate 21.00% 21.00% 21.00%
Increases (Decreases) in Tax from:      
State Taxes on Income, Net of Federal Tax 3.90% 4.00% 4.20%
Production Tax Credits (PTCs) (5.50%) (4.80%) (4.20%)
Amortization of Excess Deferred Income Taxes (0.80%) (0.60%) (0.50%)
North Dakota Wind Tax Credit Amortization, Net of Federal Tax (0.20%) (0.20%) (0.20%)
Other, Net (0.60%) (0.30%) 0.20%
Income Taxes at Effective Tax Rate 17.80% 19.10% 20.50%
v3.25.0.1
Income Taxes - Schedule of Deferred Tax Assets and Liabilities (Details) - USD ($)
$ in Thousands
Dec. 31, 2024
Dec. 31, 2023
Deferred Tax Assets    
Employee Benefits $ 37,456 $ 39,959
Regulatory Liabilities 52,664 56,479
Tax Credit Carryforwards 18,268 21,836
Cost of Removal 35,374 32,993
Asset Retirement Obligations 10,948 9,494
Net Operating Loss Carryforward 2,289 2,336
Other 19,449 11,310
Total Deferred Tax Assets 176,448 174,407
Deferred Tax Liabilities    
Differences Related to Property (375,120) (347,885)
Retirement Benefits Regulatory Asset (22,892) (22,458)
Pension Expense (26,034) (24,875)
Other (20,147) (16,462)
Total Deferred Tax Liabilities (444,193) (411,680)
Deferred Income Taxes $ (267,745) $ (237,273)
v3.25.0.1
Income Taxes - Narrative (Details)
$ in Millions
 12 Months Ended
Dec. 31, 2024
USD ($)
Operating Loss Carryforwards [Line Items]  
Period for unrecognized tax benefits not expected change 12 months
State and Local Jurisdiction | Tax Years 2029 to 2037  
Operating Loss Carryforwards [Line Items]  
Net operating loss carryforwards $ 2.3
State and Local Jurisdiction | Tax Years 2040 to 2043  
Operating Loss Carryforwards [Line Items]  
State tax credits $ 18.3
v3.25.0.1
Income Taxes - Schedule of Unrecognized Tax Benefit Activity (Details) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2024
Dec. 31, 2023
Dec. 31, 2022
Unrecognized Tax Benefits [Roll Forward]      
Balance on January 1 $ 1,489 $ 923 $ 827
Decreases for tax positions taken during a prior period (189)    
Increases for tax positions taken during a prior period   596 44
Increases for tax positions taken during the current period 188 163 260
Decreases due to settlements with taxing authorities 0 0 0
Decreases as a result of a lapse of applicable statutes of limitations (363) (193) (208)
Balance on December 31 $ 1,125 $ 1,489 $ 923
v3.25.0.1
Commitments and Contingencies - Narrative (Details)
$ in Thousands
 12 Months Ended 24 Months Ended
Dec. 31, 2024
USD ($)
agreement
Dec. 31, 2023
USD ($)
Dec. 31, 2022
USD ($)
Dec. 31, 2026
USD ($)
Jun. 30, 2024
regionalTransmissionOrganization
Other Commitments [Line Items]          
Capacity and energy requirement costs $ 6,000 $ 5,600 $ 13,100    
Coal purchase commitment, number of agreements | agreement 2        
Coal purchase commitment costs $ 44,700 43,700 45,100    
Land easement non-lease payments expense 1,800 1,800 1,400    
Payments to acquire productive assets 358,650 287,134 171,134    
Pre-tax benefit 366,892 $ 363,489 $ 357,535    
Number of regional transmission organizations with show cause proceedings | regionalTransmissionOrganization         4
Solar | Forecast          
Other Commitments [Line Items]          
Payments to acquire productive assets       $ 23,600  
Commitments, termination fee       $ 5,000  
OTP | Federal Energy Regulatory Commission          
Other Commitments [Line Items]          
Pre-tax benefit 2,500        
Estimated liability of refund obligation $ 500        
OTP | OTP Land Easements          
Other Commitments [Line Items]          
Long-term purchase commitment, period 99 years        
Long-term purchase commitment, amount $ 62,000        
OTP | Other Commitments          
Other Commitments [Line Items]          
Long-term purchase commitment, amount $ 10,600        
v3.25.0.1
Commitments and Contingencies - Schedule of Commitments Contractual Obligation Maturities (Details) - OTP
$ in Thousands
Dec. 31, 2024
USD ($)
Other Commitments  
2025 $ 1,897
2026 1,902
2027 1,941
2028 1,981
2029 2,021
Beyond 2029 52,261
Land easement payments, total 62,003
Coal Purchase Commitments  
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]  
2025 24,192
2026 24,416
2027 25,127
2028 25,859
2029 27,102
Beyond 2029 314,713
Commitments other than operating leases, total 441,409
Other Commitments  
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]  
2025 1,817
2026 1,518
2027 661
2028 544
2029 272
Beyond 2029 5,758
Commitments other than operating leases, total $ 10,570
v3.25.0.1
Stockholders' Equity (Details) - USD ($)
$ in Thousands
 12 Months Ended
May 03, 2024
Dec. 31, 2024
Dec. 31, 2023
Class of Stock [Line Items]      
Total capitalization   $ 2,612,233 $ 2,267,065
OTP      
Class of Stock [Line Items]      
Equity to total capitalization ratio (as a percent)   53.10%  
Net assets restricted from distribution   $ 834,500  
Minimum | Minnesota Public Utilities Commission | OTP      
Class of Stock [Line Items]      
Public utilities, requested equity capital structure (as a percent)   47.20%  
Maximum | OTP      
Class of Stock [Line Items]      
Total capitalization   $ 2,200,000  
Maximum | Minnesota Public Utilities Commission | OTP      
Class of Stock [Line Items]      
Public utilities, requested equity capital structure (as a percent)   57.70%  
Second Shelf Registration      
Class of Stock [Line Items]      
Shelf registration (in shares) 1,500,000    
Number of shares issued (in shares)   70,469  
Number of shares available for grant (in shares)   1,429,531  
Cumulative Preferred Shares      
Class of Stock [Line Items]      
Preferred stock, shares authorized (in shares)   1,500,000  
Preferred stock, shares outstanding (in shares)   0 0
Cumulative Preference Shares      
Class of Stock [Line Items]      
Preferred stock, shares authorized (in shares)   1,000,000  
Preferred stock, shares outstanding (in shares)   0 0
v3.25.0.1
Accumulated Other Comprehensive Income (Loss) (Details) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2024
Dec. 31, 2023
Dec. 31, 2022
AOCI Attributable to Parent, Net of Tax [Roll Forward]      
Beginning balance $ 1,443,006 $ 1,217,317 $ 990,777
Other Comprehensive Income (Loss) Before Reclassifications, net of tax 908 239 6,898
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss) (1,524) (6) 541
Total Other Comprehensive Income (Loss) (616) 233 7,439
Ending balance 1,668,499 1,443,006 1,217,317
Pension and Other Postretirement Benefits      
AOCI Attributable to Parent, Net of Tax [Roll Forward]      
Beginning balance 1,375 1,334 (6,537)
Other Comprehensive Income (Loss) Before Reclassifications, net of tax 501 59 7,331
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss) (1,503) (18) 540
Total Other Comprehensive Income (Loss) (1,002) 41 7,871
Ending balance 373 1,375 1,334
Net Unrealized Gain (Losses) on Available-for-Sale Securities      
AOCI Attributable to Parent, Net of Tax [Roll Forward]      
Beginning balance (227) (419) 13
Other Comprehensive Income (Loss) Before Reclassifications, net of tax 407 180 (433)
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss) (21) 12 1
Total Other Comprehensive Income (Loss) 386 192 (432)
Ending balance 159 (227) (419)
Accumulated Other Comprehensive Income (Loss)      
AOCI Attributable to Parent, Net of Tax [Roll Forward]      
Beginning balance 1,148 915 (6,524)
Total Other Comprehensive Income (Loss) (616) 233 7,439
Ending balance $ 532 $ 1,148 $ 915
v3.25.0.1
Share-Based Payments - Narrative (Details)
 1 Months Ended 12 Months Ended
Apr. 30, 2023
shares
Dec. 31, 2024
USD ($)
measure
$ / shares
shares
Dec. 31, 2023
USD ($)
$ / shares
shares
Dec. 31, 2022
USD ($)
$ / shares
shares
Restricted Stock Awards        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Award cliff vesting period   4 years    
Granted (in dollars per share) | $ / shares   $ 85.25 $ 68.03 $ 59.95
Fair value of awards vested   $ 5,100,000 $ 3,100,000 $ 3,000,000.0
Unrecognized compensation costs   $ 3,700,000    
Unrecognized compensation costs, period of recognition   1 year 6 months    
Restricted Stock Awards | Minimum        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Award vesting period   3 years    
Restricted Stock Awards | Maximum        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Award vesting period   4 years    
Stock Performance Awards        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Award vesting period   3 years    
Granted (in dollars per share) | $ / shares   $ 94.45 $ 61.97 $ 54.91
Fair value of awards vested   $ 12,300,000 $ 5,300,000 $ 5,100,000
Unrecognized compensation costs   $ 500,000    
Unrecognized compensation costs, period of recognition   7 months 24 days    
Number of performance measures | measure   2    
Expected volatility rate measurement period   5 years    
Stock Performance Awards | Minimum        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Percentage of target amount as actual payment (as a percent)   0.00%    
Stock Performance Awards | Maximum        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Percentage of target amount as actual payment (as a percent)   150.00%    
The 1999 Employee Stock Purchase Plan        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Share-based payment awards, number of shares authorized (in shares) | shares   1,400,000    
Discount from market price rate (as a percent)   15.00%    
Purchase period   6 months    
Maximum number of shares available to be purchased per participant (in shares) | shares   2,000    
Maximum employee subscription amount   $ 25,000    
Shares holding period   18 months    
Share-based payment arrangement, expense   $ 400,000 $ 300,000 $ 300,000
Shares issued under the plan (in shares) | shares   31,252 26,348 26,420
Number of shares available for grant (in shares) | shares   206,115    
The 1999 Employee Stock Purchase Plan | Minimum        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Employee monthly withholding amounts   $ 10    
The 1999 Employee Stock Purchase Plan | Maximum        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Employee monthly withholding amounts   2,000    
The 2023 Stock Incentive Plan        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Share-based payment awards, number of shares authorized (in shares) | shares 979,891      
Share-based payment arrangement, expense   $ 9,100,000 $ 7,400,000 $ 6,600,000
Number of shares available for grant (in shares) | shares   828,761    
Number of additional shares authorized (in shares) | shares 500,000      
Share-based payment arrangement, expense, tax benefit   $ 2,700,000 $ 1,600,000 $ 1,700,000
v3.25.0.1
Share-Based Payments - Schedule of Restricted Stock Awards and Stock Performance Awards (Details) - $ / shares
 12 Months Ended
Dec. 31, 2024
Dec. 31, 2023
Dec. 31, 2022
Restricted Stock Awards      
Shares      
Beginning of year (in shares) 148,913    
Granted (in shares) 52,425    
Vested (in shares) (57,771)    
Forfeited (in shares) (150)    
End of year (in shares) 143,417 148,913  
Weighted-Average Grant-Date Fair Value      
Beginning of year (in dollars per share) $ 56.48    
Granted (in dollars per share) 85.25 $ 68.03 $ 59.95
Vested (in dollars per share) 52.78    
Forfeited (in dollars per share) 66.39    
End of year (in dollars per share) $ 68.47 $ 56.48  
Stock Performance Awards      
Shares      
Beginning of year (in shares) 194,200    
Granted (in shares) 43,400    
Vested (in shares) (92,800)    
Forfeited (in shares) 0    
End of year (in shares) 144,800 194,200  
Weighted-Average Grant-Date Fair Value      
Beginning of year (in dollars per share) $ 50.33    
Granted (in dollars per share) 94.45 $ 61.97 $ 54.91
Vested (in dollars per share) 42.06    
Forfeited (in dollars per share) 0    
End of year (in dollars per share) $ 68.85 $ 50.33  
v3.25.0.1
Share-Based Payments - Schedule of Weighted-Average Assumptions (Details) - Stock Performance Awards
 12 Months Ended
Dec. 31, 2024
Dec. 31, 2023
Dec. 31, 2022
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Risk-free interest rate 4.16% 4.15% 1.52%
Expected term (in years) 3 years 3 years 3 years
Expected volatility 35.10% 34.00% 32.00%
Dividend yield 2.40% 2.50% 2.90%
v3.25.0.1
Earnings Per Share (Details) - shares
shares in Thousands
 12 Months Ended
Dec. 31, 2024
Dec. 31, 2023
Dec. 31, 2022
Earnings Per Share, Diluted, by Common Class, Including Two Class Method [Line Items]      
Weighted Average Common Shares Outstanding - Basic (in shares) 41,778 41,668 41,586
Effect of Dilutive Securities:      
Dilutive Effect of Potential Common Shares (in shares) 294 371 345
Weighted Average Common Shares Outstanding – Diluted (in shares) 42,072 42,039 41,931
Stock Performance Awards      
Effect of Dilutive Securities:      
Effect of Dilutive Securities (in shares) 196 269 248
Restricted Stock Awards      
Effect of Dilutive Securities:      
Effect of Dilutive Securities (in shares) 96 100 95
Employee Stock Purchase Plan Shares and Other      
Effect of Dilutive Securities:      
Effect of Dilutive Securities (in shares) 2 2 2
v3.25.0.1
Derivative Instruments - Schedule of Notional Amounts and Fair Value (Details) - Swap
$ in Thousands
 12 Months Ended
Dec. 31, 2024
USD ($)
MWh
Dec. 31, 2023
USD ($)
MWh
Derivative Instruments and Hedging Activities Disclosures [Line Items]    
Megawatt hours of electricity | MWh 167,000 187,000
Derivative Liabilities:    
Other Current Liabilities $ 1,989 $ 4,210
Other Noncurrent Liabilities 0 0
Total Derivative Liabilities $ 1,989 $ 4,210
v3.25.0.1
Derivative Instruments - Narrative (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2024
Dec. 31, 2023
Swap    
Derivative Instruments and Hedging Activities Disclosures [Line Items]    
Loss on derivatives, net $ 3.5 $ 16.5
v3.25.0.1
Fair Value Measurements - Schedule of Fair Value, Assets Measured on Recurring Basis (Details) - USD ($)
$ in Thousands
Dec. 31, 2024
Dec. 31, 2023
Assets    
Debt securities $ 62,759  
Corporate Debt Securities    
Assets    
Debt securities 1,628  
Level 1 | Fair Value, Recurring    
Assets    
Money Market Funds 596 $ 3,125
Mutual Funds 10,653 7,771
Total Assets 11,249 10,896
Liabilities    
Derivative Instruments 0 0
Total Liabilities 0 0
Level 1 | Fair Value, Recurring | Corporate Debt Securities    
Assets    
Debt securities 0 0
Level 1 | Fair Value, Recurring | Government Debt Securities    
Assets    
Debt securities 0 0
Level 2 | Fair Value, Recurring    
Assets    
Money Market Funds 0 0
Mutual Funds 0 0
Total Assets 62,759 9,303
Liabilities    
Derivative Instruments 1,989 4,210
Total Liabilities 1,989 4,210
Level 2 | Fair Value, Recurring | Corporate Debt Securities    
Assets    
Debt securities 1,628 1,579
Level 2 | Fair Value, Recurring | Government Debt Securities    
Assets    
Debt securities 61,131 7,724
Level 3 | Fair Value, Recurring    
Assets    
Money Market Funds 0 0
Mutual Funds 0 0
Total Assets 0 0
Liabilities    
Derivative Instruments 0 0
Total Liabilities 0 0
Level 3 | Fair Value, Recurring | Corporate Debt Securities    
Assets    
Debt securities 0 0
Level 3 | Fair Value, Recurring | Government Debt Securities    
Assets    
Debt securities $ 0 $ 0
v3.25.0.1
Fair Value Measurements - Schedule of Fair Value of Assets and Liabilities (Details) - USD ($)
$ in Thousands
Dec. 31, 2024
Dec. 31, 2023
Carrying Amount    
Assets:    
Cash and Cash Equivalents $ 294,651 $ 230,373
Total Assets 294,651 230,373
Liabilities:    
Short-Term Debt 69,615 81,422
Long-Term Debt 943,734 824,059
Total Liabilities 1,013,349 905,481
Fair Value    
Assets:    
Cash and Cash Equivalents 294,651 230,373
Total Assets 294,651 230,373
Liabilities:    
Short-Term Debt 69,615 81,422
Long-Term Debt 806,826 710,839
Total Liabilities $ 876,441 $ 792,261
v3.25.0.1
Schedule I - Condensed Financial Information of Registrant - CONDENSED BALANCE SHEETS, PARENT COMPANY (Details) - USD ($)
$ in Thousands
Dec. 31, 2024
Dec. 31, 2023
Dec. 31, 2022
Dec. 31, 2021
Current Assets        
Cash and Cash Equivalents $ 294,651 $ 230,373    
Accounts Receivable from Subsidiaries 145,964 157,143    
Other 30,579 16,826    
Total Current Assets 630,041 570,170    
Investments in Subsidiaries 121,177 62,516    
Deferred Income Taxes 176,448 174,407    
Other Assets 66,416 51,377    
Total Assets 3,652,082 3,242,568    
Current Liabilities        
Accounts Payable to Subsidiaries 113,574 94,428    
Other 45,582 43,775    
Total Current Liabilities 309,790 309,757    
Other Noncurrent Liabilities 98,397 75,977    
Commitments and Contingencies (Note 14)    
Capitalization        
Long-Term Debt 943,734 824,059    
Total Shareholders' Equity 1,668,499 1,443,006 $ 1,217,317 $ 990,777
Total Capitalization 2,612,233 2,267,065    
Total Liabilities and Shareholders' Equity 3,652,082 3,242,568    
OTC        
Current Assets        
Cash and Cash Equivalents 291,575 228,137    
Other 3,538 977    
Total Current Assets 305,578 231,786    
Deferred Income Taxes 69,781 65,244    
Other Assets 109,057 50,795    
Total Assets 2,569,555 2,152,309    
Current Liabilities        
Other 19,100 15,320    
Total Current Liabilities 771,732 583,999    
Other Noncurrent Liabilities 49,424 45,455    
Commitments and Contingencies (Note 14)    
Capitalization        
Long-Term Debt 79,900 79,849    
Total Shareholders' Equity 1,668,499 1,443,006    
Total Capitalization 1,748,399 1,522,855    
Total Liabilities and Shareholders' Equity 2,569,555 2,152,309    
OTC | Subsidiaries        
Current Assets        
Accounts Receivable from Subsidiaries 5,642 2,555    
Interest Receivable from Subsidiaries 117 117    
Notes Receivable from Subsidiaries 4,706 0    
Investments in Subsidiaries 2,006,239 1,725,584    
Notes Receivable from Subsidiaries 78,900 78,900    
Current Liabilities        
Accounts Payable to Subsidiaries 7 7    
Notes Payable to Subsidiaries $ 752,625 $ 568,672    
v3.25.0.1
Schedule I - Condensed Financial Information of Registrant - CONDENSED STATEMENTS OF INCOME, PARENT COMPANY (Details) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2024
Dec. 31, 2023
Dec. 31, 2022
Income      
Other Income $ 18,848 $ 12,650 $ 2,037
Expense      
Nonelectric Selling, General, and Administrative Expenses 80,065 72,663 69,718
Interest Expense 41,815 37,677 36,016
Nonservice Cost Components of Postretirement Benefits (9,609) (10,597) (1,075)
Income Before Income Taxes 366,892 363,489 357,535
Income Tax Benefit (65,230) (69,298) (73,351)
Net Income 301,662 294,191 284,184
OTC      
Income      
Other Income 15,085 10,496 466
Total Income 322,717 307,861 300,681
Expense      
Nonelectric Selling, General, and Administrative Expenses 23,016 12,816 17,269
Nonservice Cost Components of Postretirement Benefits 970 1,063 1,023
Total Expense 27,590 17,698 22,363
Income Before Income Taxes 295,127 290,163 278,318
Income Tax Benefit 6,535 4,028 5,866
Net Income 301,662 294,191 284,184
OTC | Subsidiaries      
Income      
Equity Income in Earnings of Subsidiaries 304,525 294,467 296,833
Interest Income from Subsidiaries 3,107 2,898 3,382
Expense      
Interest Expense 5 6 5
OTC | Nonrelated Party      
Expense      
Interest Expense $ 3,599 $ 3,813 $ 4,066
v3.25.0.1
Schedule I - Condensed Financial Information of Registrant - CONDENSED STATEMENTS OF CASH FLOWS, PARENT COMPANY (Details) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2024
Dec. 31, 2023
Dec. 31, 2022
Cash Flows from Operating Activities      
Net Cash Provided by Operating Activities $ 452,731 $ 404,499 $ 389,309
Cash Flows from Investing Activities      
Investment in Subsidiaries (61,573) (8,378) (8,283)
Net Cash Used in Investing Activities (411,374) (289,287) (175,071)
Cash Flows from Financing Activities      
Net (Repayments) Borrowings on Short-Term Debt (11,807) 73,218 (82,959)
Payments for Shares Withheld for Employee Tax Obligations (6,457) (3,088) (2,942)
Dividends Paid (78,266) (73,061) (68,755)
Other, net (549) (904) (2,123)
Net Cash Provided by (Used in) Financing Activities 22,921 (3,835) (96,779)
Net Change in Cash and Cash Equivalents 64,278 111,377 117,459
Cash and Cash Equivalents at Beginning of Period 230,373 118,996 1,537
Cash and Cash Equivalents at End of Period 294,651 230,373 118,996
OTC      
Cash Flows from Operating Activities      
Net Cash Provided by Operating Activities 76,333 77,139 28,807
Cash Flows from Investing Activities      
Other, net 1,394 1,686 1,480
Net Cash Used in Investing Activities (106,691) (40,068) (51,695)
Cash Flows from Financing Activities      
Net (Repayments) Borrowings on Short-Term Debt 0 0 (22,637)
Borrowings from Subsidiaries 179,247 148,308 236,926
Payments for Shares Withheld for Employee Tax Obligations (6,457) (3,088) (2,942)
Dividends Paid (78,265) (73,061) (68,755)
Other, net (729) (339) (461)
Net Cash Provided by (Used in) Financing Activities 93,796 71,820 142,131
Net Change in Cash and Cash Equivalents 63,438 108,891 119,243
Cash and Cash Equivalents at Beginning of Period 228,137 119,246 3
Cash and Cash Equivalents at End of Period 291,575 228,137 119,246
OTC | Subsidiaries      
Cash Flows from Investing Activities      
Investment in Subsidiaries (55,000) (40,000) (50,000)
Purchases of Investments and Other Assets $ (53,085) $ (1,754) $ (3,175)
v3.25.0.1
Schedule I - Condensed Financial Information of Registrant - Schedule of Related Party Transactions (Details) - USD ($)
$ in Thousands
Dec. 31, 2024
Dec. 31, 2023
Related Party Transaction [Line Items]    
Accounts Receivable $ 145,964 $ 157,143
Accounts Payable 113,574 94,428
Otter Tail Corporation | Subsidiaries    
Related Party Transaction [Line Items]    
Accounts Receivable 5,642 2,555
Interest Receivable 117 117
Current Notes Receivable 4,706 0
Long-Term Notes Receivable 78,900 78,900
Accounts Payable 7 7
Current Notes Payable 752,625 568,672
Otter Tail Corporation | Subsidiaries | Otter Tail Power Company    
Related Party Transaction [Line Items]    
Accounts Receivable 5,223 2,415
Interest Receivable 0 0
Current Notes Receivable 0 0
Long-Term Notes Receivable 0 0
Accounts Payable 7 7
Current Notes Payable 0 0
Otter Tail Corporation | Subsidiaries | Northern Pipe Products, Inc.    
Related Party Transaction [Line Items]    
Accounts Receivable 36 0
Interest Receivable 7 7
Current Notes Receivable 0 0
Long-Term Notes Receivable 5,000 5,000
Accounts Payable 0 0
Current Notes Payable 66,170 56,917
Otter Tail Corporation | Subsidiaries | Vinyltech Corporation    
Related Party Transaction [Line Items]    
Accounts Receivable 0 14
Interest Receivable 17 17
Current Notes Receivable 0 0
Long-Term Notes Receivable 11,500 11,500
Accounts Payable 0 0
Current Notes Payable 90,764 98,016
Otter Tail Corporation | Subsidiaries | BTD Manufacturing, Inc.    
Related Party Transaction [Line Items]    
Accounts Receivable 0 0
Interest Receivable 78 78
Current Notes Receivable 0 0
Long-Term Notes Receivable 52,000 52,000
Accounts Payable 0 0
Current Notes Payable 5,662 6,291
Otter Tail Corporation | Subsidiaries | T.O. Plastics, Inc.    
Related Party Transaction [Line Items]    
Accounts Receivable 42 36
Interest Receivable 15 15
Current Notes Receivable 4,706 0
Long-Term Notes Receivable 10,400 10,400
Accounts Payable 0 0
Current Notes Payable 0 980
Otter Tail Corporation | Subsidiaries | Varistar Corporation    
Related Party Transaction [Line Items]    
Accounts Receivable 0 0
Interest Receivable 0 0
Current Notes Receivable 0 0
Long-Term Notes Receivable 0 0
Accounts Payable 0 0
Current Notes Payable 590,029 406,468
Otter Tail Corporation | Subsidiaries | Otter Tail Assurance Limited    
Related Party Transaction [Line Items]    
Accounts Receivable 341 90
Interest Receivable 0 0
Current Notes Receivable 0 0
Long-Term Notes Receivable 0 0
Accounts Payable 0 0
Current Notes Payable $ 0 $ 0
v3.25.0.1
Schedule I - Condensed Financial Information of Registrant - Schedule of Dividends (Details) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2024
Dec. 31, 2023
Dec. 31, 2022
Condensed Financial Information Disclosure [Abstract]      
Cash Dividends Paid to Parent by Subsidiaries $ 78,191 $ 72,982 $ 68,680
v3.25.0.1
Schedule II - Valuation and Qualifying Accounts and Reserves (Details) - Allowance for Credit Losses - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2024
Dec. 31, 2023
Dec. 31, 2022
SEC Schedule, 12-09, Movement in Valuation Allowances and Reserves [Roll Forward]      
Balance, January 1 $ 2,522 $ 1,648 $ 1,836
Charged to Cost and Expenses 1,242 2,014 909
Deductions (1,844) (1,140) (1,097)
Balance, December 31 $ 1,920 $ 2,522 $ 1,648