OTTER TAIL CORP, 10-Q filed on 11/5/2025
Quarterly Report
v3.25.3
Cover - shares
 9 Months Ended
Sep. 30, 2025
Oct. 31, 2025
Cover [Abstract]    
Document Type 10-Q  
Document Quarterly Report true  
Document Period End Date Sep. 30, 2025  
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 $5.00 per share  
Trading Symbol OTTR  
Security Exchange Name NASDAQ  
Entity Current Reporting Status Yes  
Entity Interactive Data Current Yes  
Entity Filer Category Large Accelerated Filer  
Entity Small Business false  
Entity Emerging Growth Company false  
Entity Shell Company false  
Entity Common Stock, Shares Outstanding   41,905,520
Entity Central Index Key 0001466593  
Amendment Flag false  
Document Fiscal Period Focus Q3  
Document Fiscal Year Focus 2025  
Current Fiscal Year End Date --12-31  
v3.25.3
CONSOLIDATED BALANCE SHEETS (unaudited) - USD ($)
$ in Thousands
Sep. 30, 2025
Dec. 31, 2024
Current Assets    
Cash and Cash Equivalents $ 325,786 $ 294,651
Receivables, net of allowance for credit losses 166,694 145,964
Inventories 155,765 148,885
Investments 53,877 753
Regulatory Assets 9,433 9,962
Other Current Assets 25,688 29,826
Total Current Assets 737,243 630,041
Noncurrent Assets    
Investments 77,662 121,177
Property, Plant and Equipment, net of accumulated depreciation 2,820,689 2,692,460
Regulatory Assets 97,936 98,673
Intangible Assets, net of accumulated amortization 4,917 5,743
Goodwill 37,572 37,572
Other Noncurrent Assets 67,804 66,416
Total Noncurrent Assets 3,106,580 3,022,041
Total Assets 3,843,823 3,652,082
Current Liabilities    
Short-Term Debt 0 69,615
Accounts Payable 95,441 113,574
Accrued Salaries and Wages 32,197 34,398
Accrued Taxes 21,267 17,314
Regulatory Liabilities 21,753 29,307
Other Current Liabilities 35,702 45,582
Total Current Liabilities 206,360 309,790
Noncurrent Liabilities    
Pension Benefit Liability 32,001 32,614
Other Postretirement Benefits Liability 26,502 27,385
Regulatory Liabilities 296,216 288,928
Deferred Income Taxes 288,013 267,745
Deferred Tax Credits 14,513 14,990
Other Noncurrent Liabilities 105,330 98,397
Total Noncurrent Liabilities 762,575 730,059
Commitments and Contingencies (Note 9)
Capitalization    
Long-Term Debt 1,043,437 943,734
Shareholders' Equity    
Common Shares: 50,000,000 shares authorized, $5 par value; 41,905,520 and 41,827,967 outstanding at September 30, 2025 and December 31, 2024 209,528 209,140
Additional Paid-In Capital 433,368 429,089
Retained Earnings 1,187,813 1,029,738
Accumulated Other Comprehensive Income 742 532
Total Shareholders' Equity 1,831,451 1,668,499
Total Capitalization 2,874,888 2,612,233
Total Liabilities and Shareholders' Equity $ 3,843,823 $ 3,652,082
v3.25.3
CONSOLIDATED BALANCE SHEETS (unaudited) (Parentheticals) - $ / shares
Sep. 30, 2025
Dec. 31, 2024
Shareholders' Equity    
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) 41,905,520 41,827,967
v3.25.3
CONSOLIDATED STATEMENTS OF INCOME (unaudited) - USD ($)
shares in Thousands, $ in Thousands
 3 Months Ended 9 Months Ended
Sep. 30, 2025
Sep. 30, 2024
Sep. 30, 2025
Sep. 30, 2024
Operating Revenues        
Total Operating Revenues $ 325,563 $ 338,033 $ 995,959 $ 1,027,437
Operating Expenses        
Electric Production Fuel 25,442 14,991 56,055 45,009
Electric Purchased Power 9,495 10,735 55,862 42,507
Electric Operating and Maintenance Expenses 41,144 43,737 136,830 136,367
Cost of Products Sold (excluding depreciation) 101,198 111,444 311,551 342,962
Nonelectric Selling, General, and Administrative Expenses 17,792 18,829 56,434 55,896
Depreciation and Amortization 29,554 27,051 88,376 79,579
Electric Property Taxes 4,333 3,705 12,788 11,691
Total Operating Expenses 228,958 230,492 717,896 714,011
Operating Income 96,605 107,541 278,063 313,426
Other Income and (Expense)        
Interest Expense (11,790) (11,173) (35,063) (31,225)
Nonservice Components of Postretirement Benefits 304 2,367 2,441 7,197
Other Income (Expense), net 5,990 5,421 15,231 14,491
Income Before Income Taxes 91,109 104,156 260,672 303,889
Income Tax Expense 12,817 18,677 36,553 57,077
Net Income $ 78,292 $ 85,479 $ 224,119 $ 246,812
Weighted-Average Common Shares Outstanding:        
Basic (in shares) 41,877 41,800 41,859 41,770
Diluted (in shares) 42,138 42,081 42,106 42,068
Earnings Per Share:        
Basic (in dollars per share) $ 1.87 $ 2.04 $ 5.35 $ 5.91
Diluted (in dollars per share) $ 1.86 $ 2.03 $ 5.32 $ 5.87
Electric        
Operating Revenues        
Total Operating Revenues $ 138,597 $ 130,380 $ 417,048 $ 384,696
Product Sales        
Operating Revenues        
Total Operating Revenues $ 186,966 $ 207,653 $ 578,911 $ 642,741
v3.25.3
CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME (unaudited) - USD ($)
$ in Thousands
 3 Months Ended 9 Months Ended
Sep. 30, 2025
Sep. 30, 2024
Sep. 30, 2025
Sep. 30, 2024
Statement of Comprehensive Income [Abstract]        
Net Income $ 78,292 $ 85,479 $ 224,119 $ 246,812
Other Comprehensive Income (Loss):        
Unrealized Gain on Available-for-Sale Securities, net of tax expense of $5, $331, $71 and $255 20 1,244 245 963
Pension and Other Postretirement Benefits, net of tax benefit of $4, $11, $12 and $47 (12) (28) (35) (132)
Total Other Comprehensive Income (Loss) 8 1,216 210 831
Total Comprehensive Income $ 78,300 $ 86,695 $ 224,329 $ 247,643
v3.25.3
CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME (unaudited) (Parentheticals) - USD ($)
$ in Thousands
 3 Months Ended 9 Months Ended
Sep. 30, 2025
Sep. 30, 2024
Sep. 30, 2025
Sep. 30, 2024
Statement of Comprehensive Income [Abstract]        
Unrealized Gain on Available-for-Sale Securities, tax expense $ (5) $ (331) $ (71) $ (255)
Pension and Other Postretirement Benefits, net of tax benefit (expense) $ 4 $ 11 $ 12 $ 47
v3.25.3
CONSOLIDATED STATEMENTS OF SHAREHOLDERS' EQUITY (unaudited) - USD ($)
$ in Thousands
Total
Common Shares
Additional Paid-In Capital
Retained Earnings
Accumulated Other Comprehensive Income
Beginning balance (in shares) at Dec. 31, 2023   41,710,521      
Balance, Beginning of Period at Dec. 31, 2023 $ 1,443,006 $ 208,553 $ 426,963 $ 806,342 $ 1,148
Increase (Decrease) in Stockholders' Equity [Roll Forward]          
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 Purchase Plan Expenses (250)   (250)    
Stock Compensation Expense 8,082   8,082    
Net Income 246,812     246,812  
Other Comprehensive Income 831       831
Common Dividends (58,693)     (58,693)  
Ending balance (in shares) at Sep. 30, 2024   41,827,967      
Balance, End of Period at Sep. 30, 2024 1,633,331 $ 209,140 427,751 994,461 1,979
Beginning balance (in shares) at Jun. 30, 2024   41,814,425      
Balance, Beginning of Period at Jun. 30, 2024 1,565,652 $ 209,072 427,264 928,553 763
Increase (Decrease) in Stockholders' Equity [Roll Forward]          
Stock Issued Under Share-Based Compensation Plans, net of shares withheld for employee taxes (in shares)   13,542      
Stock Issued Under Share-Based Compensation Plans, net of shares withheld for employee taxes (704) $ 68 (772)    
Stock Compensation Expense 1,259   1,259    
Net Income 85,479     85,479  
Other Comprehensive Income 1,216       1,216
Common Dividends (19,571)     (19,571)  
Ending balance (in shares) at Sep. 30, 2024   41,827,967      
Balance, End of Period at Sep. 30, 2024 $ 1,633,331 $ 209,140 427,751 994,461 1,979
Beginning balance (in shares) at Dec. 31, 2024 41,827,967 41,827,967      
Balance, Beginning of Period at Dec. 31, 2024 $ 1,668,499 $ 209,140 429,089 1,029,738 532
Increase (Decrease) in Stockholders' Equity [Roll Forward]          
Stock Issued Under Share-Based Compensation Plans, net of shares withheld for employee taxes (in shares)   77,553      
Stock Issued Under Share-Based Compensation Plans, net of shares withheld for employee taxes (3,134) $ 388 (3,522)    
Stock Purchase Plan Expenses (305)   (305)    
Stock Compensation Expense 8,106   8,106    
Net Income 224,119     224,119  
Other Comprehensive Income 210       210
Common Dividends $ (66,044)     (66,044)  
Ending balance (in shares) at Sep. 30, 2025 41,905,520 41,905,520      
Balance, End of Period at Sep. 30, 2025 $ 1,831,451 $ 209,528 433,368 1,187,813 742
Beginning balance (in shares) at Jun. 30, 2025   41,904,370      
Balance, Beginning of Period at Jun. 30, 2025 1,774,462 $ 209,522 432,664 1,131,542 734
Increase (Decrease) in Stockholders' Equity [Roll Forward]          
Stock Issued Under Share-Based Compensation Plans, net of shares withheld for employee taxes (in shares)   1,150      
Stock Issued Under Share-Based Compensation Plans, net of shares withheld for employee taxes 0 $ 6 (6)    
Stock Compensation Expense 710   710    
Net Income 78,292     78,292  
Other Comprehensive Income 8       8
Common Dividends $ (22,021)     (22,021)  
Ending balance (in shares) at Sep. 30, 2025 41,905,520 41,905,520      
Balance, End of Period at Sep. 30, 2025 $ 1,831,451 $ 209,528 $ 433,368 $ 1,187,813 $ 742
v3.25.3
CONSOLIDATED STATEMENTS OF SHAREHOLDERS' EQUITY (unaudited) (Parentheticals) - $ / shares
 3 Months Ended 9 Months Ended
Sep. 30, 2025
Sep. 30, 2024
Sep. 30, 2025
Sep. 30, 2024
Statement of Stockholders' Equity [Abstract]        
Common Dividends (in dollars per share) $ 0.5250 $ 0.4675 $ 1.5750 $ 1.4025
v3.25.3
CONSOLIDATED STATEMENTS OF CASH FLOWS (unaudited) - USD ($)
$ in Thousands
 9 Months Ended
Sep. 30, 2025
Sep. 30, 2024
Operating Activities    
Net Income $ 224,119 $ 246,812
Adjustments to Reconcile Net Income to Net Cash Provided by Operating Activities:    
Depreciation and Amortization 88,376 79,579
Deferred Tax Credits (477) (559)
Deferred Income Taxes 14,527 8,840
Investment Gains (5,519) (5,259)
Stock Compensation Expense 8,106 8,082
Other, Net (3,636) (2,167)
Changes in Operating Assets and Liabilities:    
Receivables (20,730) (29,130)
Inventories (2,837) (2,198)
Regulatory Assets (1,429) 7,209
Other Assets 4,985 (2,785)
Accounts Payable (8,702) 3,180
Accrued and Other Liabilities (3,530) (5,745)
Regulatory Liabilities (363) 24,083
Pension and Other Postretirement Benefits (3,941) (7,167)
Net Cash Provided by Operating Activities 288,949 322,775
Investing Activities    
Capital Expenditures (213,329) (259,750)
Proceeds from Disposal of Noncurrent Assets 4,957 6,684
Purchases of Investments and Other Assets (7,642) (59,100)
Net Cash Used in Investing Activities (216,014) (312,166)
Financing Activities    
Net Repayments of Short-Term Debt (69,615) (14,021)
Proceeds from Issuance of Long-Term Debt 100,000 120,000
Dividends Paid (66,044) (58,693)
Payments for Shares Withheld for Employee Tax Obligations (3,134) (6,457)
Other, net (3,007) (1,791)
Net Cash (Used In) Provided by Financing Activities (41,800) 39,038
Net Change in Cash and Cash Equivalents 31,135 49,647
Cash and Cash Equivalents at Beginning of Period 294,651 230,373
Cash and Cash Equivalents at End of Period 325,786 280,020
Supplemental Disclosure of Noncash Investing Activities    
Accrued Property, Plant and Equipment Additions $ 13,009 $ 9,396
v3.25.3
Summary of Significant Accounting Policies
 9 Months Ended
Sep. 30, 2025
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 polyvinyl chloride (PVC) pipe products. We classify our business into three segments: Electric, Manufacturing and Plastics.
Basis of Presentation
The unaudited consolidated financial statements included herein have been prepared pursuant to the rules and regulations of the SEC for interim reporting. Accordingly, they do not include all the information and footnotes required by generally accepted accounting principles. In the opinion of management, we have included all adjustments, including normal recurring accruals, necessary for a fair presentation of the consolidated financial statements for the periods presented. The consolidated financial statements and condensed notes thereto should be read in conjunction with the consolidated financial statements and notes included in our Annual Report on Form 10-K for the fiscal year ended December 31, 2024.
Because of the seasonality of our businesses and other factors, earnings for the three and nine months ended September 30, 2025 should not be taken as an indication of earnings for all or any part of the balance of the current year or as an indication of earnings for future years.
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.
Reclassifications
Short-term investments in the amount of $0.8 million were previously included in other current assets as of December 31, 2024. This amount has been reclassified to maintain consistency and comparability between the periods presented and are now presented separately on the consolidated balance sheets. The reclassification had no impact on previously reported current or total assets, current or total liabilities, or total shareholders' equity.
Recent Accounting Pronouncements
Income Taxes. In December 2023, the Financial Accounting Standards Board (FASB) issued amended authoritative guidance codified in Accounting Standards Codification (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 ending 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 new 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 of the new standard is permitted. We anticipate adopting the updated standard in our Form 10-K for the year ending December 31, 2027.
Measurement of Credit Losses. In July 2025, the FASB issued amended authoritative guidance codified in ASC 326, Financial Instruments - Credit Losses. The amended guidance provides a practical expedient permitting an entity to assume that conditions on the balance sheet date remain unchanged over the life of the asset when estimating expected credit losses for current accounts receivable and current contract assets. The updated standard is effective for our annual and interim periods beginning in 2026 and is to be applied on a prospective basis. We are currently evaluating the impact that the updated standard will have on our consolidated financial statements, but we do not anticipate it will have a material effect on our future financial position or operating results.
Software Costs. In September 2025, the FASB issued amended authoritative guidance codified in ASC 350, Intangibles – Goodwill and Other. The amended guidance updates the cost capitalization threshold for internal-use software development costs by removing all references to software project development stages and providing new guidance on how to evaluate whether the probable-to-complete recognition threshold has been met. The updated standard is effective for our annual and interim periods beginning in 2028. Early adoption of the amended guidance is permitted. The amended guidance can be applied on a prospective,
modified, or retrospective basis. We are currently evaluating the impact that the updated standard will have on our consolidated financial statements, but we do not anticipate it will have a material effect on our future financial position or operating results.
v3.25.3
Segment Information
 9 Months Ended
Sep. 30, 2025
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 internal reporting and review processes. Segment net income is the primary measure of segment profit or loss used by our chief operating decision maker in assessing segment performance and allocating resources to our segments.
Segment Profit or Loss
Information about each segment, including significant expenses and net income of each segment, for the three and nine months ended September 30, 2025 and 2024 are as follows:
Electric Segment
Three Months Ended September 30,Nine Months Ended September 30,
(in thousands)2025202420252024
Operating Revenue$138,597 $130,380 $417,048 $384,696 
Production Fuel and Purchased Power34,937 25,726 111,917 87,516 
Operating and Maintenance Expenses41,144 43,737 136,830 136,367 
Depreciation and Amortization22,433 20,741 67,087 61,014 
Property Taxes4,333 3,705 12,788 11,691 
Interest Expense10,891 10,274 32,370 28,534 
Income Tax (Benefit) Expense(701)948 (9,178)723 
Other Segment Items(1)
(1,748)(3,281)(5,977)(10,635)
Net Income$27,308 $28,530 $71,211 $69,486 
(1) Other segment items includes nonservice components of postretirement benefits, allowance for funds used during construction and other expenses (income).
Manufacturing Segment
Three Months Ended September 30,Nine Months Ended September 30,
(in thousands)2025202420252024
Operating Revenue$76,951 $79,896 $237,363 $275,961 
Cost of Goods Sold61,283 68,360 193,108 224,595 
Selling, General, and Administrative Expenses9,978 8,853 31,071 31,667 
Interest Expense633 664 1,882 1,865 
Income Tax Expense1,141 (154)2,371 3,563 
Other Segment Items— (1)1 — 
Net Income$3,916 $2,174 $8,930 $14,271 
Plastics Segment
Three Months Ended September 30,Nine Months Ended September 30,
(in thousands)2025202420252024
Operating Revenue$110,015 $127,757 $341,548 $366,780 
Cost of Goods Sold44,139 47,056 131,155 129,867 
Selling, General, and Administrative Expenses6,919 6,956 20,526 17,777 
Interest Expense146 146 539 444 
Income Tax Expense15,317 19,166 49,294 56,938 
Other Segment Items(1)(46)(4)(75)
Net Income$43,495 $54,479 $140,038 $161,829 
Capital Expenditures and Identifiable Assets
The following provides capital expenditures for each reportable segment and our corporate cost center for the nine months ended September 30, 2025 and 2024:
Nine Months Ended September 30,
(in thousands)20252024
Capital Expenditures
Electric$199,618 $215,056 
Manufacturing6,763 25,034 
Plastics6,336 19,580 
Corporate612 80 
Total$213,329 $259,750 
The following provides the identifiable assets by segment and corporate assets as of September 30, 2025 and December 31, 2024:
(in thousands)September 30,
2025		December 31, 2024
Identifiable Assets
Electric$2,924,509 $2,785,522 
Manufacturing251,906 254,445 
Plastics210,180 186,043 
Corporate457,228 426,072 
Total$3,843,823 $3,652,082 
Corporate assets consist primarily of cash and cash equivalents, investments, and prepaid expenses.
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 three and nine months ended September 30, 2025 and 2024:
Three Months Ended September 30,Nine Months Ended September 30,
(in thousands)2025202420252024
Depreciation and Amortization
Electric$22,433 $20,741 $67,087 $61,014 
Manufacturing5,411 5,199 16,357 15,199 
Plastics1,636 1,086 4,771 3,293 
Corporate74 25 161 73 
Total
$29,554 $27,051 $88,376 $79,579 
Interest Expense
Total Interest Expense of Reportable Segments$11,670 $11,084 $34,791 $30,843 
Corporate Interest Expense120 89 272 382 
Total
$11,790 $11,173 $35,063 $31,225 
Income Tax Expense (Benefit)
Total Income Tax Expense of Reportable Segments$15,757 $19,960 $42,487 $61,224 
Corporate Income Tax Benefit(2,940)(1,283)(5,934)(4,147)
Total
$12,817 $18,677 $36,553 $57,077 
Net Income
Total Net Income of Reportable Segments$74,719 $85,183 $220,179 $245,586 
Corporate Net Income
3,573 296 3,940 1,226 
Total
$78,292 $85,479 $224,119 $246,812 
v3.25.3
Revenue
 9 Months Ended
Sep. 30, 2025
Revenue from Contract with Customer [Abstract]  
Revenue Revenue
The following presents our operating revenues from external customers, in total and by amounts arising from contracts with customers and alternative revenue program (ARP) arrangements, disaggregated by revenue source and segment for the three and nine months ended September 30, 2025 and 2024:
Three Months Ended September 30,Nine Months Ended September 30,
(in thousands)2025202420252024
Operating Revenues
Electric Segment
Retail: Residential$32,106 $34,150 $104,939 $99,971 
Retail: Commercial and Industrial77,507 74,602 244,105 225,827 
Retail: Other2,029 2,050 6,100 5,875 
  Total Retail111,642 110,802 355,144 331,673 
Transmission14,934 15,152 40,297 39,805 
Wholesale10,379 3,139 16,418 8,273 
Other1,642 1,287 5,189 4,945 
Total Electric Segment138,597 130,380 417,048 384,696 
Manufacturing Segment
Metal Parts and Tooling67,649 69,904 207,868 245,972 
Plastic Products and Tooling7,710 8,537 24,255 23,989 
Scrap Metal1,592 1,455 5,240 6,000 
Total Manufacturing Segment76,951 79,896 237,363 275,961 
Plastics Segment
PVC Pipe110,015 127,757 341,548 366,780 
Total Operating Revenue325,563 338,033 995,959 1,027,437 
Less: Non-contract Revenues Included Above
Electric Segment - ARP Revenues(323)647 305 413 
Total Operating Revenues from Contracts with Customers$325,886 $337,386 $995,654 $1,027,024 
v3.25.3
Select Balance Sheet Information
 9 Months Ended
Sep. 30, 2025
Organization, Consolidation and Presentation of Financial Statements [Abstract]  
Select Balance Sheet Information Select Balance Sheet Information
Receivables and Allowance for Credit Losses
Receivables as of September 30, 2025 and December 31, 2024 are as follows:
(in thousands)September 30,
2025		December 31,
2024
Receivables
Trade$140,890 $112,169 
Other10,593 13,799 
Unbilled Receivables16,968 21,916 
Total Receivables168,451 147,884 
Less: Allowance for Credit Losses1,757 1,920 
Receivables, net of allowance for credit losses$166,694 $145,964 
The following is a summary of activity in the allowance for credit losses for the nine months ended September 30, 2025 and 2024:
(in thousands)20252024
Beginning Balance, January 1$1,920 $2,522 
Additions Charged to Expense1,134 904 
Reductions for Amounts Written Off, Net of Recoveries(1,297)(1,476)
Ending Balance, September 30
$1,757 $1,950 
Inventories
Inventories consist of the following as of September 30, 2025 and December 31, 2024:
(in thousands)September 30,
2025		December 31,
2024
Finished Goods
$39,146 $43,345 
Work in Process24,174 22,637 
Raw Material, Fuel and Supplies
92,445 82,903 
Total Inventories$155,765 $148,885 
Investments
The following is a summary of our investments as of September 30, 2025 and December 31, 2024:
(in thousands)September 30,
2025		December 31,
2024
Short-term Investments
Government Debt Securities
$53,482 $753 
Corporate Debt Securities
395 — 
Total Short-term Investments
53,877 753 
Long-term Investments
Corporate-Owned Life Insurance Policies50,027 47,895 
Government Debt Securities
9,022 60,378 
Corporate Debt Securities
895 1,628 
Mutual Funds16,454 10,653 
Money Market Funds1,237 596 
Other Investments27 27 
Total Long-term Investments
77,662 121,177 
Total Investments$131,539 $121,930 
Debt Securities. The following table summarizes the amortized cost and fair value of available-for-sale debt securities and the corresponding amounts of gross unrealized gains and losses as of September 30, 2025 and December 31, 2024:
September 30, 2025
(in thousands)
Amortized Cost
Gross Unrealized Gains
Gross Unrealized (Losses)
Fair Value
Government Debt Securities$61,967 $586 $(49)$62,504 
Corporate Debt Securities1,280 12 (2)1,290 
Total
$63,247 $598 $(51)$63,794 
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 
As of September 30, 2025 and December 31, 2024, no unrealized losses on debt securities were deemed to be other-than-temporary.
The following table summarizes the fair value of available-for-sale debt securities by contractual maturity date as of September 30, 2025:
(in thousands)September 30, 2025
Due in one year or less
$53,877 
Due in one to five years
9,917 
Total$63,794 
Equity Securities. The amount of net unrealized gains and losses during the nine months ended September 30, 2025 and 2024 on marketable equity securities still held as of September 30, 2025 and 2024 was not material.
Property, Plant and Equipment
Major classes of property, plant and equipment as of September 30, 2025 and December 31, 2024 include:
(in thousands)September 30,
2025		December 31,
2024
Electric Plant  
Electric Plant in Service$3,234,585 $3,180,943 
Construction Work in Progress334,457 231,890 
Total Gross Electric Plant3,569,042 3,412,833 
Less Accumulated Depreciation and Amortization921,038 899,049 
Net Electric Plant2,648,004 2,513,784 
Nonelectric Property, Plant and Equipment
Nonelectric Property, Plant and Equipment in Service401,193 362,565 
Construction Work in Progress12,223 40,536 
Total Gross Nonelectric Property, Plant and Equipment413,416 403,101 
Less Accumulated Depreciation and Amortization240,731 224,425 
Net Nonelectric Property, Plant and Equipment172,685 178,676 
Net Property, Plant and Equipment$2,820,689 $2,692,460 
v3.25.3
Regulatory Matters
 9 Months Ended
Sep. 30, 2025
Regulatory Assets and Liabilities Disclosure [Abstract]  
Regulatory Matters Regulatory Matters
Regulatory Assets and Liabilities
The following presents our current and long-term regulatory assets and liabilities as of September 30, 2025 and December 31, 2024 and the period we expect to recover or refund such amounts:
Period ofSeptember 30, 2025December 31, 2024
(in thousands)Recovery/RefundCurrentLong-TermCurrentLong-Term
Regulatory Assets
Pension and Other Postretirement Benefit Plans1
Various$— $87,182 $— $88,161 
Alternative Revenue Program Riders2
Up to 2 years
4,610 146 4,257 195 
Deferred Income Taxes1
Asset lives— 9,070 — 8,944 
Fuel Clause Adjustments1
Up to 1 year
3,229 — 2,218 — 
Derivative Instruments1
Up to 2 years
795 512 1,989 — 
Other1
Various799 1,026 1,498 1,373 
Total Regulatory Assets$9,433 $97,936 $9,962 $98,673 
Regulatory Liabilities
Deferred Income TaxesAsset lives$— $124,832 $— $130,387 
Plant Removal ObligationsAsset lives— 132,438 — 126,263 
Fuel Clause Adjustments
Up to 1 year
5,197 — 11,432 — 
Alternative Revenue Program Riders
Up to 1 year
10,888 — 14,255 — 
North Dakota PTC RefundsAsset lives— 26,348 — 20,099 
Pension and Other Postretirement Benefit PlansVarious2,547 10,350 2,547 10,758 
OtherVarious3,121 2,248 1,073 1,421 
Total Regulatory Liabilities$21,753 $296,216 $29,307 $288,928 
1Costs subject to recovery without a rate of return.
2Amounts eligible for recovery includes an incentive or rate of return.
South Dakota Rate Case
On June 4, 2025, Otter Tail Power (OTP) filed a request with the South Dakota Public Utilities Commission (SDPUC) for an increase in revenue recoverable under general rates in South Dakota. In its filing, OTP requested a net increase in annual revenue of $5.7 million, or 12.50%, based on an allowed rate of return on rate base of 8.29% and an allowed rate of return on equity (ROE) of 10.80% on an equity ratio of 53.54% of total capital. Through this proceeding, OTP has proposed changes to the mechanism of certain cost and investment recovery, with recovery moving from riders into base rates. If the SDPUC does not issue its decision on OTP's request within 180 days, OTP can increase rates on an interim basis beginning December 1, 2025. These interim rate revenues, when collected, are subject to potential refund until the finalization of the rate case.
Minnesota Rate Case
On October 31, 2025, OTP filed a request with the Minnesota Public Utilities Commission (MPUC) for an increase in revenue recoverable under general rates in Minnesota. In its filing, OTP requested a net increase in annual revenue of $44.8 million, or 17.7%, based on an allowed rate of return on rate base of 7.92% and an allowed ROE of 10.65% on an equity ratio of 53.5% of total capital. The request includes, among other items, accelerated recovery of the remaining investment of the jurisdictionally allocated share of Coyote Station. The accelerated recovery request is equal to $4.3 million annually. The request is driven by the MPUC’s order in OTP’s most recent Integrated Resource Plan to discontinue serving Minnesota customers with capacity and energy from Coyote Station by December 2031. If this part of the request is granted, we anticipate the amounts collected would be deferred and recognized over the remaining estimated useful life of the plant, which extends until 2041.
The filing also included an interim rate request for a net increase in annual revenue of $31.8 million, or 12.6%. We anticipate interim rates will commence on January 1, 2026, and will be subject to potential refund until the finalization of the rate case.
v3.25.3
Short-Term and Long-Term Borrowings
 9 Months Ended
Sep. 30, 2025
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 September 30, 2025 and December 31, 2024:
Short-Term Debt
The following is a summary of our lines of credit as of September 30, 2025 and December 31, 2024:
September 30, 2025December 31, 2024
(in thousands)Borrowing LimitAmount OutstandingLetters
of Credit		Amount AvailableAmount Available
OTC Credit Agreement$170,000 $— $— $170,000 $170,000 
OTP Credit Agreement220,000 — 10,461 209,539 141,613 
Total$390,000 $— $10,461 $379,539 $311,613 
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 the Secured Overnight Financing Rate (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. There were no outstanding borrowings on either credit facility as of September 30, 2025. The weighted-average interest rate on all outstanding borrowings as of December 31, 2024 was 5.61%.
Long-Term Debt
The following is a summary of outstanding long-term debt by borrower as of September 30, 2025 and December 31, 2024: 
(in thousands)
BorrowerDebt InstrumentRateMaturitySeptember 30,
2025		December 31,
2024
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 Notes3.07 %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 60,000 
OTPSeries 2025A Senior Unsecured Notes5.49 %03/27/3550,000 — 
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 60,000 
OTPSeries 2025B Senior Unsecured Notes5.98 %06/05/5550,000 — 
Total1,047,000 947,000 
Less:Unamortized Long-Term Debt Issuance Costs3,563 3,266 
Total Long-Term Debt, Net of Unamortized Debt Issuance Costs$1,043,437 $943,734 
On March 27, 2025, OTP entered into a Note Purchase Agreement pursuant to which OTP issued, in a private placement transaction, $100.0 million of senior unsecured notes consisting of (a) $50.0 million of 5.49% Series 2025A Senior Unsecured Notes due March 27, 2035, and (b) $50.0 million of 5.98% Series 2025B Senior Unsecured Notes due June 5, 2055. The Series 2025A Notes were issued on March 27, 2025, upon entering into the agreement. The Series 2025B Notes were issued on June 5, 2025.
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 2025A Notes then outstanding on or after December 27, 2034, or the Series 2025B Notes then outstanding on or after December 5, 2054, 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.
Financial Covenants
Certain of OTC's and OTP's short- and long-term debt agreements require the borrower, whether OTC or OTP, to maintain certain financial covenants, including a maximum debt to total capitalization ratio of 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 September 30, 2025, OTC and OTP were in compliance with these financial covenants.
v3.25.3
Employee Postretirement Benefits
 9 Months Ended
Sep. 30, 2025
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 (the ESSRP), both accounted for as defined benefit pension plans, and a postretirement healthcare plan accounted for as an other postretirement benefit plan.
The following tables include the components of net periodic benefit cost (income) related to our defined benefit pension plans and other postretirement benefits for the three and nine months ended September 30, 2025 and 2024:
Three Months Ended September 30,
Pension Benefits (Pension Plan)Pension Benefits (ESSRP)Postretirement Benefits
(in thousands)202520242025202420252024
Service Cost$876 $972 $— $— $123 $123 
Interest Cost4,325 4,297 474 474 403 399 
Expected Return on Assets(6,191)(6,380)— — — — 
Amortization of Prior Service Cost— — — — (948)(1,576)
Amortization of Net Actuarial Loss335 40 — — — — 
Net Periodic Benefit Cost (Income)$(655)$(1,071)$474 $474 $(422)$(1,054)
Nine Months Ended September 30,
Pension Benefits (Pension Plan)Pension Benefits (ESSRP)Postretirement Benefits
(in thousands)202520242025202420252024
Service Cost$2,627 $2,915 $— $— $368 $368 
Interest Cost12,977 12,891 1,421 1,422 1,208 1,199 
Expected Return on Assets(18,573)(19,139)— — — — 
Amortization of Prior Service Cost— — — — (2,846)(4,727)
Amortization of Net Actuarial Loss1,006 119 — — — — 
Net Periodic Benefit Cost (Income)$(1,963)$(3,214)$1,421 $1,422 $(1,270)$(3,160)
The following table includes the impact of regulation on the recognition of periodic benefit cost (income) arising from pension and other postretirement benefits for the three and nine months ended September 30, 2025 and 2024:
Three Months Ended September 30,Nine Months Ended September 30,
(in thousands)2025202420252024
Net Periodic Benefit Cost (Income)$(603)$(1,651)$(1,812)$(4,952)
Net Amount Amortized Due to the Effect of Regulation1,298 379 2,366 1,038 
Net Periodic Benefit Cost (Income) Recognized$695 $(1,272)$554 $(3,914)
We had no minimum funding requirements for our Pension Plan or any other postretirement benefit plans as of December 31, 2024. We did not make any contributions to our Pension Plan during the nine months ended September 30, 2025 and 2024.
v3.25.3
Income Taxes
 9 Months Ended
Sep. 30, 2025
Income Tax Disclosure [Abstract]  
Income Taxes Income Taxes
The Company's effective tax rate was 14.1% and 17.9% for the three months ended September 30, 2025 and 2024 and 14.0% and 18.8% for the nine months ended September 30, 2025 and 2024. These rates differ from the federal statutory rate of 21% primarily due to the impact of production tax credits (PTCs) associated with the energy generation of our wind and solar assets, partially offset by the impact of state taxes. The decrease in our effective tax rate for each period was primarily driven by an increase in PTCs from our wind generation assets. In late 2024, we completed our first wind repowering project and additional projects were completed in 2025. The completion of these repowering projects resulted in the commencement of PTCs associated with the generation from the facilities.
One Big Beautiful Bill Act
On July 4, 2025, broad spending and tax law legislation referred to as the One Big Beautiful Bill Act (OBBBA) was enacted in the U.S. The aspects of the law that impact our financial position and may impact our future investment opportunities include certain changes to corporate income taxes and modifications to existing renewable energy credits.
The OBBBA includes changes to corporate income tax rules and regulations, including reinstating 100% bonus depreciation, immediate expensing of domestic research and development costs, and modifications to the business interest expense limitation.
The effects of changes in tax laws and regulations are required to be recognized in our consolidated financial statements in the period of enactment. Accordingly, as of September 30, 2025, we recognized a reduction to our current year income tax payable in the amount of $7.7 million, with a corresponding increase to our deferred income tax liability, as a result of electing to deduct previously deferred research and development costs in the current year. We also anticipate electing bonus depreciation for eligible assets in our 2025 corporate income tax return, which will result in a reduction of our current year income tax payable and an increase to our deferred income tax liability.
The OBBBA also alters the timing and eligibility of certain tax credits for renewable energy projects. Wind and solar projects that begin construction by July 4, 2026 are eligible for technology-neutral tax credits (production tax credits or investment tax credits). Projects that begin construction after July 4, 2026 must be in service by December 31, 2027 to qualify for technology-neutral tax credits. For projects that begin construction after December 31, 2025, new provisions restrict tax credit eligibility for those projects involving material assistance or effective control by a Foreign Entity of Concern, as defined in the legislation, which includes entities linked to China, Russia, Iran or North Korea.
v3.25.3
Commitments and Contingencies
 9 Months Ended
Sep. 30, 2025
Commitments and Contingencies Disclosure [Abstract]  
Commitments and Contingencies Commitments and Contingencies
Commitments
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, and remains subject to certain conditions to close, including regulatory and other approvals. Under certain conditions, 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
Self-Funding of Transmission Upgrades for Generator Interconnections. The Federal Energy Regulatory Commission (FERC) has granted transmission owners within Midcontinent Independent System Operator, Inc. (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) 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 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
Class Action Lawsuits and Related Matters. Beginning in August of 2024, a series of putative federal class action lawsuits consolidated under the caption In re: PVC Pipe Antitrust Litigation (Case No. 1:24-cv-07639) were filed in the United States District Court for the Northern District of Illinois against Northern Pipe Products, Vinyltech Corporation, Otter Tail Corporation and more than twenty other PVC pipe manufacturers, as well as Oil Price Information Systems, LLC (OPIS), a reporting service that provides pricing and market intelligence in various industries, including the PVC pipe industry, during the relevant period. The Court has allowed three putative classes to file complaints: a Direct Purchaser Class, a Non-Converter Seller Purchaser Class and an End-User Class.
In July 2025, the Court preliminarily approved a settlement agreement among the Direct Purchaser Class, the Non-Converter Seller Purchaser Class and OPIS. The settlement agreement resolved claims against OPIS and provides for its cooperation with the plaintiffs.
In August of 2025, the three putative classes each filed a first or an amended complaint alleging, among other things, that beginning in January 2017 or January 2020, depending on the class, the defendants and alleged co-conspirators conspired to fix, raise, maintain and stabilize the price of PVC municipal pipe, PVC plumping pipe, PVC electrical pipe and PVC pipe fittings in violation of U.S. federal and state antitrust laws. The complaints allege that PVC pipe manufacturers improperly exchanged confidential information through OPIS and engaged in other indirect and direct communications with each other. Plaintiffs are seeking treble damages, injunctive relief, pre- and post-judgment interest, costs and attorneys' fees on behalf of the putative classes.
On October 30, 2025, the defendants filed motions to dismiss.
In August 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 has responded to the subpoena and intends to comply with its obligations thereunder. On October 7, 2025, the DOJ filed a motion to intervene and for a partial stay of document discovery for a period of six months in In Re: PVC Pipe Antitrust Litigation, which the Court granted on October 10, 2025.
On September 26, 2025, a putative nation-wide class action complaint (Case No. S-257310) was filed in the Supreme Court of British Columbia, Canada against Northern Pipe, Vinyltech Corporation, Otter Tail Corporation and several other PVC pipe manufacturers, as well as OPIS. The complaint alleges that the defendants, beginning in 2021, conspired to fix, raise, maintain, and stabilize the price of PVC pipe through an information exchange, OPIS, breaching Canada's Competition Act, and creating tortious liability. The plaintiffs seek general damages, injunctive relief, pre- and post-judgment interest, punitive damages, cost, and attorneys' fees on behalf of the putative class.
The Company believes there are factual and legal defenses to the allegations in the complaints and is defending itself accordingly. There remains considerable uncertainty regarding the timing or ultimate resolution of these matters. 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 in the United States and Canada or the DOJ investigation. The resolution of these matters could have a material impact on the Company’s financial position, operating results and liquidity, and it is reasonably possible that our estimate of a loss arising from these matters could change in the near term.
On May 20, 2025, the Otter Tail Corporation Board of Directors received a letter from counsel submitted on behalf of a shareholder, demanding the Board investigate and take legal action against certain current and former directors and officers of the Company. The derivative demand letter includes alleged securities law violations and breach of fiduciary duties and unjust enrichment against certain current and former officers and directors of the company in connection with the matters at issue in the pending civil antitrust cases. At this time, we are unable to determine the likelihood of any outcome related to this matter.
Other Contingencies. We are involved in claims, legal proceedings, investigations 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 or range of 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 September 30, 2025, other than those discussed above, will not be material.
v3.25.3
Shareholders' Equity
 9 Months Ended
Sep. 30, 2025
Equity [Abstract]  
Shareholders' Equity Shareholders' Equity
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. No new debt or equity has been issued pursuant to the registration statement. The registration statement expires in May 2027.
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 newly issued common shares or common shares purchased on the open market. During the nine months ended September 30, 2025, we issued 75,173 shares under this plan. We repurchased a sufficient number of shares on the open market to satisfy all issuances under the plan; accordingly, no proceeds were received as a result of the issuance of these shares. As of September 30, 2025, there were 1,354,358 shares 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 OTC's shareholders is from dividends paid or distributions made by OTC's subsidiaries. As a result of certain statutory limitations or regulatory or financing agreements, the amount of distributions allowed to be made by OTC's subsidiaries or the amount of dividends paid by OTC could be restricted. Both the OTC Credit Agreement and the 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 September 30, 2025, 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 or 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% based on OTP’s current capital structure requirements. As of September 30, 2025, OTP’s equity-to-total-capitalization ratio, including short-term debt, was 55.1% and its net assets restricted from distribution totaled approximately $861 million. Under the MPUC order, total capitalization for OTP cannot exceed $2.2 billion.
v3.25.3
Accumulated Other Comprehensive Income (Loss)
 9 Months Ended
Sep. 30, 2025
Equity [Abstract]  
Accumulated Other Comprehensive Income (Loss) Accumulated Other Comprehensive Income (Loss)
The following presents the changes in accumulated other comprehensive income (loss) for the three and nine months ended September 30, 2025 and 2024:
Three Months Ended September 30,
20252024
(in thousands)Pension and Other Postretirement Benefits
Net Unrealized Gains (Losses) on Available-for-Sale Securities
TotalPension and Other Postretirement BenefitsNet Unrealized Gains (Losses) on Available-for-Sale SecuritiesTotal
Balance, Beginning of Period$350 $384 $734 $1,271 $(508)$763 
Other Comprehensive Income Before Reclassifications, net of tax
— 37 37 — 1,233 1,233 
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss)(12)
(1)
(17)
(2)
(29)(28)
(1)
11 
(2)
(17)
Total Other Comprehensive Income (Loss)
(12)20 8 (28)1,244 1,216 
Balance, End of Period$338 $404 $742 $1,243 $736 $1,979 
Nine Months Ended September 30,
20252024
(in thousands)Pension and Other Postretirement Benefits
Net Unrealized Gains (Losses) on Available-for-Sale Securities
TotalPension and Other Postretirement BenefitsNet Unrealized Gains (Losses) on Available-for-Sale SecuritiesTotal
Balance, Beginning of Period$373 $159 $532 $1,375 $(227)$1,148 
Other Comprehensive Income Before Reclassifications, net of tax
— 272 272 — 989 989 
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss)(35)
(1)
(27)
(2)
(62)(132)
(1)
(26)
(2)
(158)
Total Other Comprehensive Income (Loss)(35)245 210 (132)963 831 
Balance, End of Period$338 $404 $742 $1,243 $736 $1,979 
(1) Included in the computation of net periodic pension and other postretirement benefit costs. See Note 7.
(2) Included in other income (expense), net on the accompanying consolidated statements of income.
v3.25.3
Share-Based Payments
 9 Months Ended
Sep. 30, 2025
Share-Based Payment Arrangement [Abstract]  
Share-Based Payments Share-Based Payments
Stock Compensation Expense
Stock-based compensation expense arising from our employee stock purchase plan and share-based compensation plans recognized within operating expenses in the consolidated statements of income amounted to $0.7 million and $1.3 million for the three months ended September 30, 2025 and 2024 and $8.1 million for the nine months ended September 30, 2025 and 2024.
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 or 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 stock award activity for the nine months ended September 30, 2025:
SharesWeighted-Average
Grant-Date
Fair Value
Nonvested, January 1, 2025
143,417 $68.47 
Granted57,185 74.22 
Vested(47,593)59.74 
Forfeited(6,600)68.72 
Nonvested, September 30, 2025
146,409 $73.55 
The fair value of vested awards was $3.6 million and $5.1 million during the nine months ended September 30, 2025 and 2024.
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 and ii) ROE. 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 stock performance awards granted during the nine months ended September 30, 2025 and 2024 was determined using a Monte Carlo fair value simulation model incorporating the following assumptions:
20252024
Risk-free interest rate4.28 %4.16 %
Expected term (in years)33
Expected volatility30.30 %35.10 %
Dividend yield2.50 %2.40 %
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. Dividend yield was estimated based on historical and future yield estimates.
The following is a summary of stock performance award activity for the nine months ended September 30, 2025 (share amounts reflect awards at target):
 SharesWeighted-Average
Grant-Date
Fair Value
Nonvested, January 1, 2025
144,800 $68.85 
Granted57,000 73.90 
Vested(49,000)53.93 
Forfeited— — 
Nonvested, September 30, 2025
152,800 $75.52 
The fair value of vested awards was $5.5 million and $12.3 million during the nine months ended September 30, 2025 and 2024, respectively.
v3.25.3
Earnings Per Share
 9 Months Ended
Sep. 30, 2025
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 time- and performance-based stock awards and employee stock purchase plan shares.
The following includes the computation of the denominator for basic and diluted weighted-average shares outstanding for the three and nine months ended September 30, 2025 and 2024:
Three Months Ended September 30,Nine Months Ended September 30,
(in thousands)2025202420252024
Weighted-Average Common Shares Outstanding – Basic41,877 41,800 41,859 41,770 
Effect of Dilutive Securities:
Stock Performance Awards164 188 150 199 
Restricted Stock Awards94 91 95 97 
Employee Stock Purchase Plan
3 2 2 2 
Dilutive Effect of Potential Common Shares261 281 247 298 
Weighted-Average Common Shares Outstanding – Diluted42,138 42,081 42,106 42,068 
The number of shares excluded from diluted weighted-average common shares outstanding because such shares were anti-dilutive was not material for the three and nine months ended September 30, 2025 and 2024
v3.25.3
Derivative Instruments
 9 Months Ended
Sep. 30, 2025
Derivative Instruments and Hedging Activities Disclosure [Abstract]  
Derivative Instruments Derivative Instruments
OTP enters into derivative instruments to manage its exposure to future market energy price variability and reduce price volatility for our retail customers. These derivative instruments are not designated as qualifying hedging transactions but provide for an economic hedge against future market energy price variability. The instruments are recorded at fair value on the consolidated balance sheets. In accordance with ratemaking 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 September 30, 2025, OTP had multiple outstanding pay-fixed, receive-variable swap agreements with various settlement dates extending to December 31, 2026. The following presents the notional amounts and fair value of our derivative instruments as of September 30, 2025 and December 31, 2024:
(in thousands)
September 30,
2025		December 31,
2024
Megawatt hours of electricity
347167
Derivative Assets:
Other Current Assets
$1,285 $— 
Other Noncurrent Assets
119 — 
Total Derivative Assets
1,404 — 
Derivative Liabilities:
Other Current Liabilities
795 1,989 
Other Noncurrent Liabilities
512 — 
Total Derivative Liabilities
$1,307 $1,989 
During the nine months ended September 30, 2025 and 2024, contracts matured and were settled resulting in losses of $2.6 million and $2.7 million, 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.
v3.25.3
Fair Value Measurements
 9 Months Ended
Sep. 30, 2025
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 September 30, 2025 and December 31, 2024 classified by the input method used to measure fair value:
(in thousands)Level 1Level 2Level 3
September 30, 2025
Assets:
Investments:
Money Market Funds$1,237 $— $— 
Mutual Funds16,454 — — 
Corporate Debt Securities— 1,290 — 
Government Debt Securities
— 62,504 — 
Derivative Instruments— 1,404 — 
Total Assets17,691 65,198 — 
Liabilities:
Derivative Instruments— 1,307 — 
Total Liabilities$— $1,307 $— 
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 $— 
Level 1 fair value measurements are based on quoted prices (unadjusted) in active markets for identical assets 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 service 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 September 30, 2025 and December 31, 2024:
 September 30, 2025December 31, 2024
(in thousands)Carrying
Amount		Fair ValueCarrying
Amount		Fair Value
Assets:
Cash and Cash Equivalents$325,786 $325,786 $294,651 $294,651 
Total325,786 325,786 294,651 294,651 
Liabilities:
Short-Term Debt— — 69,615 69,615 
Long-Term Debt1,043,437 930,550 943,734 806,826 
Total$1,043,437 $930,550 $1,013,349 $876,441 
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 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.
v3.25.3
Insider Trading Arrangements
 3 Months Ended
Sep. 30, 2025
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
v3.25.3
Summary of Significant Accounting Policies (Policies)
 9 Months Ended
Sep. 30, 2025
Accounting Policies [Abstract]  
Basis of Presentation
Basis of Presentation
The unaudited consolidated financial statements included herein have been prepared pursuant to the rules and regulations of the SEC for interim reporting. Accordingly, they do not include all the information and footnotes required by generally accepted accounting principles. In the opinion of management, we have included all adjustments, including normal recurring accruals, necessary for a fair presentation of the consolidated financial statements for the periods presented. The consolidated financial statements and condensed notes thereto should be read in conjunction with the consolidated financial statements and notes included in our Annual Report on Form 10-K for the fiscal year ended December 31, 2024.
Because of the seasonality of our businesses and other factors, earnings for the three and nine months ended September 30, 2025 should not be taken as an indication of earnings for all or any part of the balance of the current year or as an indication of earnings for future years.
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.
Reclassifications
Reclassifications
Short-term investments in the amount of $0.8 million were previously included in other current assets as of December 31, 2024. This amount has been reclassified to maintain consistency and comparability between the periods presented and are now presented separately on the consolidated balance sheets. The reclassification had no impact on previously reported current or total assets, current or total liabilities, or total shareholders' equity.
Recent Accounting Pronouncements
Recent Accounting Pronouncements
Income Taxes. In December 2023, the Financial Accounting Standards Board (FASB) issued amended authoritative guidance codified in Accounting Standards Codification (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 ending 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 new 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 of the new standard is permitted. We anticipate adopting the updated standard in our Form 10-K for the year ending December 31, 2027.
Measurement of Credit Losses. In July 2025, the FASB issued amended authoritative guidance codified in ASC 326, Financial Instruments - Credit Losses. The amended guidance provides a practical expedient permitting an entity to assume that conditions on the balance sheet date remain unchanged over the life of the asset when estimating expected credit losses for current accounts receivable and current contract assets. The updated standard is effective for our annual and interim periods beginning in 2026 and is to be applied on a prospective basis. We are currently evaluating the impact that the updated standard will have on our consolidated financial statements, but we do not anticipate it will have a material effect on our future financial position or operating results.
Software Costs. In September 2025, the FASB issued amended authoritative guidance codified in ASC 350, Intangibles – Goodwill and Other. The amended guidance updates the cost capitalization threshold for internal-use software development costs by removing all references to software project development stages and providing new guidance on how to evaluate whether the probable-to-complete recognition threshold has been met. The updated standard is effective for our annual and interim periods beginning in 2028. Early adoption of the amended guidance is permitted. The amended guidance can be applied on a prospective,
modified, or retrospective basis. We are currently evaluating the impact that the updated standard will have on our consolidated financial statements, but we do not anticipate it will have a material effect on our future financial position or operating results.
v3.25.3
Segment Information (Tables)
 9 Months Ended
Sep. 30, 2025
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 three and nine months ended September 30, 2025 and 2024 are as follows:
Electric Segment
Three Months Ended September 30,Nine Months Ended September 30,
(in thousands)2025202420252024
Operating Revenue$138,597 $130,380 $417,048 $384,696 
Production Fuel and Purchased Power34,937 25,726 111,917 87,516 
Operating and Maintenance Expenses41,144 43,737 136,830 136,367 
Depreciation and Amortization22,433 20,741 67,087 61,014 
Property Taxes4,333 3,705 12,788 11,691 
Interest Expense10,891 10,274 32,370 28,534 
Income Tax (Benefit) Expense(701)948 (9,178)723 
Other Segment Items(1)
(1,748)(3,281)(5,977)(10,635)
Net Income$27,308 $28,530 $71,211 $69,486 
(1) Other segment items includes nonservice components of postretirement benefits, allowance for funds used during construction and other expenses (income).
Manufacturing Segment
Three Months Ended September 30,Nine Months Ended September 30,
(in thousands)2025202420252024
Operating Revenue$76,951 $79,896 $237,363 $275,961 
Cost of Goods Sold61,283 68,360 193,108 224,595 
Selling, General, and Administrative Expenses9,978 8,853 31,071 31,667 
Interest Expense633 664 1,882 1,865 
Income Tax Expense1,141 (154)2,371 3,563 
Other Segment Items— (1)1 — 
Net Income$3,916 $2,174 $8,930 $14,271 
Plastics Segment
Three Months Ended September 30,Nine Months Ended September 30,
(in thousands)2025202420252024
Operating Revenue$110,015 $127,757 $341,548 $366,780 
Cost of Goods Sold44,139 47,056 131,155 129,867 
Selling, General, and Administrative Expenses6,919 6,956 20,526 17,777 
Interest Expense146 146 539 444 
Income Tax Expense15,317 19,166 49,294 56,938 
Other Segment Items(1)(46)(4)(75)
Net Income$43,495 $54,479 $140,038 $161,829 
Capital Expenditures and Identifiable Assets
The following provides capital expenditures for each reportable segment and our corporate cost center for the nine months ended September 30, 2025 and 2024:
Nine Months Ended September 30,
(in thousands)20252024
Capital Expenditures
Electric$199,618 $215,056 
Manufacturing6,763 25,034 
Plastics6,336 19,580 
Corporate612 80 
Total$213,329 $259,750 
The following provides the identifiable assets by segment and corporate assets as of September 30, 2025 and December 31, 2024:
(in thousands)September 30,
2025		December 31, 2024
Identifiable Assets
Electric$2,924,509 $2,785,522 
Manufacturing251,906 254,445 
Plastics210,180 186,043 
Corporate457,228 426,072 
Total$3,843,823 $3,652,082 
Included below is a reconciliation of certain segment information and our unallocated corporate costs to consolidated amounts for the three and nine months ended September 30, 2025 and 2024:
Three Months Ended September 30,Nine Months Ended September 30,
(in thousands)2025202420252024
Depreciation and Amortization
Electric$22,433 $20,741 $67,087 $61,014 
Manufacturing5,411 5,199 16,357 15,199 
Plastics1,636 1,086 4,771 3,293 
Corporate74 25 161 73 
Total
$29,554 $27,051 $88,376 $79,579 
Interest Expense
Total Interest Expense of Reportable Segments$11,670 $11,084 $34,791 $30,843 
Corporate Interest Expense120 89 272 382 
Total
$11,790 $11,173 $35,063 $31,225 
Income Tax Expense (Benefit)
Total Income Tax Expense of Reportable Segments$15,757 $19,960 $42,487 $61,224 
Corporate Income Tax Benefit(2,940)(1,283)(5,934)(4,147)
Total
$12,817 $18,677 $36,553 $57,077 
Net Income
Total Net Income of Reportable Segments$74,719 $85,183 $220,179 $245,586 
Corporate Net Income
3,573 296 3,940 1,226 
Total
$78,292 $85,479 $224,119 $246,812 
v3.25.3
Revenue (Tables)
 9 Months Ended
Sep. 30, 2025
Revenue from Contract with Customer [Abstract]  
Schedule of Disaggregation of Revenue
The following presents our operating revenues from external customers, in total and by amounts arising from contracts with customers and alternative revenue program (ARP) arrangements, disaggregated by revenue source and segment for the three and nine months ended September 30, 2025 and 2024:
Three Months Ended September 30,Nine Months Ended September 30,
(in thousands)2025202420252024
Operating Revenues
Electric Segment
Retail: Residential$32,106 $34,150 $104,939 $99,971 
Retail: Commercial and Industrial77,507 74,602 244,105 225,827 
Retail: Other2,029 2,050 6,100 5,875 
  Total Retail111,642 110,802 355,144 331,673 
Transmission14,934 15,152 40,297 39,805 
Wholesale10,379 3,139 16,418 8,273 
Other1,642 1,287 5,189 4,945 
Total Electric Segment138,597 130,380 417,048 384,696 
Manufacturing Segment
Metal Parts and Tooling67,649 69,904 207,868 245,972 
Plastic Products and Tooling7,710 8,537 24,255 23,989 
Scrap Metal1,592 1,455 5,240 6,000 
Total Manufacturing Segment76,951 79,896 237,363 275,961 
Plastics Segment
PVC Pipe110,015 127,757 341,548 366,780 
Total Operating Revenue325,563 338,033 995,959 1,027,437 
Less: Non-contract Revenues Included Above
Electric Segment - ARP Revenues(323)647 305 413 
Total Operating Revenues from Contracts with Customers$325,886 $337,386 $995,654 $1,027,024 
v3.25.3
Select Balance Sheet Information (Tables)
 9 Months Ended
Sep. 30, 2025
Organization, Consolidation and Presentation of Financial Statements [Abstract]  
Schedule of Receivables
Receivables as of September 30, 2025 and December 31, 2024 are as follows:
(in thousands)September 30,
2025		December 31,
2024
Receivables
Trade$140,890 $112,169 
Other10,593 13,799 
Unbilled Receivables16,968 21,916 
Total Receivables168,451 147,884 
Less: Allowance for Credit Losses1,757 1,920 
Receivables, net of allowance for credit losses$166,694 $145,964 
Schedule of Activity in Allowance for Credit Losses
The following is a summary of activity in the allowance for credit losses for the nine months ended September 30, 2025 and 2024:
(in thousands)20252024
Beginning Balance, January 1$1,920 $2,522 
Additions Charged to Expense1,134 904 
Reductions for Amounts Written Off, Net of Recoveries(1,297)(1,476)
Ending Balance, September 30
$1,757 $1,950 
Schedule of Inventories
Inventories consist of the following as of September 30, 2025 and December 31, 2024:
(in thousands)September 30,
2025		December 31,
2024
Finished Goods
$39,146 $43,345 
Work in Process24,174 22,637 
Raw Material, Fuel and Supplies
92,445 82,903 
Total Inventories$155,765 $148,885 
Schedule of Investments
The following is a summary of our investments as of September 30, 2025 and December 31, 2024:
(in thousands)September 30,
2025		December 31,
2024
Short-term Investments
Government Debt Securities
$53,482 $753 
Corporate Debt Securities
395 — 
Total Short-term Investments
53,877 753 
Long-term Investments
Corporate-Owned Life Insurance Policies50,027 47,895 
Government Debt Securities
9,022 60,378 
Corporate Debt Securities
895 1,628 
Mutual Funds16,454 10,653 
Money Market Funds1,237 596 
Other Investments27 27 
Total Long-term Investments
77,662 121,177 
Total Investments$131,539 $121,930 
Schedule of Amortized Costs and Fair Value of Debt Securities, Available-for-Sale The following table summarizes the amortized cost and fair value of available-for-sale debt securities and the corresponding amounts of gross unrealized gains and losses as of September 30, 2025 and December 31, 2024:
September 30, 2025
(in thousands)
Amortized Cost
Gross Unrealized Gains
Gross Unrealized (Losses)
Fair Value
Government Debt Securities$61,967 $586 $(49)$62,504 
Corporate Debt Securities1,280 12 (2)1,290 
Total
$63,247 $598 $(51)$63,794 
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 available-for-sale debt securities by contractual maturity date as of September 30, 2025:
(in thousands)September 30, 2025
Due in one year or less
$53,877 
Due in one to five years
9,917 
Total$63,794 
Schedule of Property, Plant and Equipment
Major classes of property, plant and equipment as of September 30, 2025 and December 31, 2024 include:
(in thousands)September 30,
2025		December 31,
2024
Electric Plant  
Electric Plant in Service$3,234,585 $3,180,943 
Construction Work in Progress334,457 231,890 
Total Gross Electric Plant3,569,042 3,412,833 
Less Accumulated Depreciation and Amortization921,038 899,049 
Net Electric Plant2,648,004 2,513,784 
Nonelectric Property, Plant and Equipment
Nonelectric Property, Plant and Equipment in Service401,193 362,565 
Construction Work in Progress12,223 40,536 
Total Gross Nonelectric Property, Plant and Equipment413,416 403,101 
Less Accumulated Depreciation and Amortization240,731 224,425 
Net Nonelectric Property, Plant and Equipment172,685 178,676 
Net Property, Plant and Equipment$2,820,689 $2,692,460 
v3.25.3
Regulatory Matters (Tables)
 9 Months Ended
Sep. 30, 2025
Regulatory Assets and Liabilities Disclosure [Abstract]  
Schedule of Regulatory Assets and Liabilities
The following presents our current and long-term regulatory assets and liabilities as of September 30, 2025 and December 31, 2024 and the period we expect to recover or refund such amounts:
Period ofSeptember 30, 2025December 31, 2024
(in thousands)Recovery/RefundCurrentLong-TermCurrentLong-Term
Regulatory Assets
Pension and Other Postretirement Benefit Plans1
Various$— $87,182 $— $88,161 
Alternative Revenue Program Riders2
Up to 2 years
4,610 146 4,257 195 
Deferred Income Taxes1
Asset lives— 9,070 — 8,944 
Fuel Clause Adjustments1
Up to 1 year
3,229 — 2,218 — 
Derivative Instruments1
Up to 2 years
795 512 1,989 — 
Other1
Various799 1,026 1,498 1,373 
Total Regulatory Assets$9,433 $97,936 $9,962 $98,673 
Regulatory Liabilities
Deferred Income TaxesAsset lives$— $124,832 $— $130,387 
Plant Removal ObligationsAsset lives— 132,438 — 126,263 
Fuel Clause Adjustments
Up to 1 year
5,197 — 11,432 — 
Alternative Revenue Program Riders
Up to 1 year
10,888 — 14,255 — 
North Dakota PTC RefundsAsset lives— 26,348 — 20,099 
Pension and Other Postretirement Benefit PlansVarious2,547 10,350 2,547 10,758 
OtherVarious3,121 2,248 1,073 1,421 
Total Regulatory Liabilities$21,753 $296,216 $29,307 $288,928 
1Costs subject to recovery without a rate of return.
2Amounts eligible for recovery includes an incentive or rate of return.
v3.25.3
Short-Term and Long-Term Borrowings (Tables)
 9 Months Ended
Sep. 30, 2025
Debt Disclosure [Abstract]  
Schedule of Line of Credit Facilities
The following is a summary of our lines of credit as of September 30, 2025 and December 31, 2024:
September 30, 2025December 31, 2024
(in thousands)Borrowing LimitAmount OutstandingLetters
of Credit		Amount AvailableAmount Available
OTC Credit Agreement$170,000 $— $— $170,000 $170,000 
OTP Credit Agreement220,000 — 10,461 209,539 141,613 
Total$390,000 $— $10,461 $379,539 $311,613 
Schedule of Debt
The following is a summary of outstanding long-term debt by borrower as of September 30, 2025 and December 31, 2024: 
(in thousands)
BorrowerDebt InstrumentRateMaturitySeptember 30,
2025		December 31,
2024
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 Notes3.07 %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 60,000 
OTPSeries 2025A Senior Unsecured Notes5.49 %03/27/3550,000 — 
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 60,000 
OTPSeries 2025B Senior Unsecured Notes5.98 %06/05/5550,000 — 
Total1,047,000 947,000 
Less:Unamortized Long-Term Debt Issuance Costs3,563 3,266 
Total Long-Term Debt, Net of Unamortized Debt Issuance Costs$1,043,437 $943,734 
v3.25.3
Employee Postretirement Benefits (Tables)
 9 Months Ended
Sep. 30, 2025
Retirement Benefits [Abstract]  
Schedule of Net Periodic Benefit Cost (Income)
The following tables include the components of net periodic benefit cost (income) related to our defined benefit pension plans and other postretirement benefits for the three and nine months ended September 30, 2025 and 2024:
Three Months Ended September 30,
Pension Benefits (Pension Plan)Pension Benefits (ESSRP)Postretirement Benefits
(in thousands)202520242025202420252024
Service Cost$876 $972 $— $— $123 $123 
Interest Cost4,325 4,297 474 474 403 399 
Expected Return on Assets(6,191)(6,380)— — — — 
Amortization of Prior Service Cost— — — — (948)(1,576)
Amortization of Net Actuarial Loss335 40 — — — — 
Net Periodic Benefit Cost (Income)$(655)$(1,071)$474 $474 $(422)$(1,054)
Nine Months Ended September 30,
Pension Benefits (Pension Plan)Pension Benefits (ESSRP)Postretirement Benefits
(in thousands)202520242025202420252024
Service Cost$2,627 $2,915 $— $— $368 $368 
Interest Cost12,977 12,891 1,421 1,422 1,208 1,199 
Expected Return on Assets(18,573)(19,139)— — — — 
Amortization of Prior Service Cost— — — — (2,846)(4,727)
Amortization of Net Actuarial Loss1,006 119 — — — — 
Net Periodic Benefit Cost (Income)$(1,963)$(3,214)$1,421 $1,422 $(1,270)$(3,160)
The following table includes the impact of regulation on the recognition of periodic benefit cost (income) arising from pension and other postretirement benefits for the three and nine months ended September 30, 2025 and 2024:
Three Months Ended September 30,Nine Months Ended September 30,
(in thousands)2025202420252024
Net Periodic Benefit Cost (Income)$(603)$(1,651)$(1,812)$(4,952)
Net Amount Amortized Due to the Effect of Regulation1,298 379 2,366 1,038 
Net Periodic Benefit Cost (Income) Recognized$695 $(1,272)$554 $(3,914)
v3.25.3
Accumulated Other Comprehensive Income (Loss) (Tables)
 9 Months Ended
Sep. 30, 2025
Equity [Abstract]  
Schedule of Changes in Accumulated Other Comprehensive Income (Loss)
The following presents the changes in accumulated other comprehensive income (loss) for the three and nine months ended September 30, 2025 and 2024:
Three Months Ended September 30,
20252024
(in thousands)Pension and Other Postretirement Benefits
Net Unrealized Gains (Losses) on Available-for-Sale Securities
TotalPension and Other Postretirement BenefitsNet Unrealized Gains (Losses) on Available-for-Sale SecuritiesTotal
Balance, Beginning of Period$350 $384 $734 $1,271 $(508)$763 
Other Comprehensive Income Before Reclassifications, net of tax
— 37 37 — 1,233 1,233 
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss)(12)
(1)
(17)
(2)
(29)(28)
(1)
11 
(2)
(17)
Total Other Comprehensive Income (Loss)
(12)20 8 (28)1,244 1,216 
Balance, End of Period$338 $404 $742 $1,243 $736 $1,979 
Nine Months Ended September 30,
20252024
(in thousands)Pension and Other Postretirement Benefits
Net Unrealized Gains (Losses) on Available-for-Sale Securities
TotalPension and Other Postretirement BenefitsNet Unrealized Gains (Losses) on Available-for-Sale SecuritiesTotal
Balance, Beginning of Period$373 $159 $532 $1,375 $(227)$1,148 
Other Comprehensive Income Before Reclassifications, net of tax
— 272 272 — 989 989 
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss)(35)
(1)
(27)
(2)
(62)(132)
(1)
(26)
(2)
(158)
Total Other Comprehensive Income (Loss)(35)245 210 (132)963 831 
Balance, End of Period$338 $404 $742 $1,243 $736 $1,979 
(1) Included in the computation of net periodic pension and other postretirement benefit costs. See Note 7.
(2) Included in other income (expense), net on the accompanying consolidated statements of income.
v3.25.3
Share-Based Payments (Tables)
 9 Months Ended
Sep. 30, 2025
Share-Based Payment Arrangement [Abstract]  
Schedule of Nonvested Restricted Stock Shares Activity
The following is a summary of stock award activity for the nine months ended September 30, 2025:
SharesWeighted-Average
Grant-Date
Fair Value
Nonvested, January 1, 2025
143,417 $68.47 
Granted57,185 74.22 
Vested(47,593)59.74 
Forfeited(6,600)68.72 
Nonvested, September 30, 2025
146,409 $73.55 
Schedule of Share-Based Payment Award, Employee Stock Purchase Plan, Valuation Assumptions
The grant-date fair value of stock performance awards granted during the nine months ended September 30, 2025 and 2024 was determined using a Monte Carlo fair value simulation model incorporating the following assumptions:
20252024
Risk-free interest rate4.28 %4.16 %
Expected term (in years)33
Expected volatility30.30 %35.10 %
Dividend yield2.50 %2.40 %
Schedule of Share-based Compensation Arrangements by Share-based Payment Award
The following is a summary of stock performance award activity for the nine months ended September 30, 2025 (share amounts reflect awards at target):
 SharesWeighted-Average
Grant-Date
Fair Value
Nonvested, January 1, 2025
144,800 $68.85 
Granted57,000 73.90 
Vested(49,000)53.93 
Forfeited— — 
Nonvested, September 30, 2025
152,800 $75.52 
v3.25.3
Earnings Per Share (Tables)
 9 Months Ended
Sep. 30, 2025
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 three and nine months ended September 30, 2025 and 2024:
Three Months Ended September 30,Nine Months Ended September 30,
(in thousands)2025202420252024
Weighted-Average Common Shares Outstanding – Basic41,877 41,800 41,859 41,770 
Effect of Dilutive Securities:
Stock Performance Awards164 188 150 199 
Restricted Stock Awards94 91 95 97 
Employee Stock Purchase Plan
3 2 2 2 
Dilutive Effect of Potential Common Shares261 281 247 298 
Weighted-Average Common Shares Outstanding – Diluted42,138 42,081 42,106 42,068 
v3.25.3
Derivative Instruments (Tables)
 9 Months Ended
Sep. 30, 2025
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 September 30, 2025 and December 31, 2024:
(in thousands)
September 30,
2025		December 31,
2024
Megawatt hours of electricity
347167
Derivative Assets:
Other Current Assets
$1,285 $— 
Other Noncurrent Assets
119 — 
Total Derivative Assets
1,404 — 
Derivative Liabilities:
Other Current Liabilities
795 1,989 
Other Noncurrent Liabilities
512 — 
Total Derivative Liabilities
$1,307 $1,989 
v3.25.3
Fair Value Measurements (Tables)
 9 Months Ended
Sep. 30, 2025
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 September 30, 2025 and December 31, 2024 classified by the input method used to measure fair value:
(in thousands)Level 1Level 2Level 3
September 30, 2025
Assets:
Investments:
Money Market Funds$1,237 $— $— 
Mutual Funds16,454 — — 
Corporate Debt Securities— 1,290 — 
Government Debt Securities
— 62,504 — 
Derivative Instruments— 1,404 — 
Total Assets17,691 65,198 — 
Liabilities:
Derivative Instruments— 1,307 — 
Total Liabilities$— $1,307 $— 
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 $— 
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 September 30, 2025 and December 31, 2024:
 September 30, 2025December 31, 2024
(in thousands)Carrying
Amount		Fair ValueCarrying
Amount		Fair Value
Assets:
Cash and Cash Equivalents$325,786 $325,786 $294,651 $294,651 
Total325,786 325,786 294,651 294,651 
Liabilities:
Short-Term Debt— — 69,615 69,615 
Long-Term Debt1,043,437 930,550 943,734 806,826 
Total$1,043,437 $930,550 $1,013,349 $876,441 
v3.25.3
Summary of Significant Accounting Policies (Details)
$ in Thousands
 9 Months Ended
Sep. 30, 2025
USD ($)
segment
Dec. 31, 2024
USD ($)
Reclassification [Line Items]    
Number of reportable segments | segment 3  
Investments $ 53,877 $ 753
Other current assets $ (25,688) (29,826)
Revision of Prior Period, Reclassification, Adjustment    
Reclassification [Line Items]    
Investments   800
Other current assets   $ 800
v3.25.3
Segment Information - Narrative (Details)
 9 Months Ended
Sep. 30, 2025
segment
Segment Reporting [Abstract]  
Number of reportable segments 3
v3.25.3
Segment Information - Schedule of Segment Profit or Loss (Details) - USD ($)
$ in Thousands
 3 Months Ended 9 Months Ended
Sep. 30, 2025
Sep. 30, 2024
Sep. 30, 2025
Sep. 30, 2024
Segment Reporting Information [Line Items]        
Operating Revenue $ 325,563 $ 338,033 $ 995,959 $ 1,027,437
Operating and Maintenance Expenses 41,144 43,737 136,830 136,367
Depreciation and Amortization 29,554 27,051 88,376 79,579
Property Taxes 4,333 3,705 12,788 11,691
Interest Expense 11,790 11,173 35,063 31,225
Selling, General, and Administrative Expenses 17,792 18,829 56,434 55,896
Income Tax (Benefit) Expense 12,817 18,677 36,553 57,077
Net Income 78,292 85,479 224,119 246,812
Electric        
Segment Reporting Information [Line Items]        
Operating Revenue 138,597 130,380 417,048 384,696
Manufacturing        
Segment Reporting Information [Line Items]        
Operating Revenue 76,951 79,896 237,363 275,961
Plastics        
Segment Reporting Information [Line Items]        
Operating Revenue 110,015 127,757 341,548 366,780
Operating Segments        
Segment Reporting Information [Line Items]        
Interest Expense 11,670 11,084 34,791 30,843
Income Tax (Benefit) Expense 15,757 19,960 42,487 61,224
Net Income 74,719 85,183 220,179 245,586
Operating Segments | Electric        
Segment Reporting Information [Line Items]        
Operating Revenue 138,597 130,380 417,048 384,696
Production Fuel and Purchased Power 34,937 25,726 111,917 87,516
Operating and Maintenance Expenses 41,144 43,737 136,830 136,367
Depreciation and Amortization 22,433 20,741 67,087 61,014
Property Taxes 4,333 3,705 12,788 11,691
Interest Expense 10,891 10,274 32,370 28,534
Income Tax (Benefit) Expense (701) 948 (9,178) 723
Other Segment Items (1,748) (3,281) (5,977) (10,635)
Net Income 27,308 28,530 71,211 69,486
Operating Segments | Manufacturing        
Segment Reporting Information [Line Items]        
Operating Revenue 76,951 79,896 237,363 275,961
Depreciation and Amortization 5,411 5,199 16,357 15,199
Interest Expense 633 664 1,882 1,865
Cost of Goods Sold 61,283 68,360 193,108 224,595
Selling, General, and Administrative Expenses 9,978 8,853 31,071 31,667
Income Tax (Benefit) Expense 1,141 (154) 2,371 3,563
Other Segment Items 0 (1) 1 0
Net Income 3,916 2,174 8,930 14,271
Operating Segments | Plastics        
Segment Reporting Information [Line Items]        
Operating Revenue 110,015 127,757 341,548 366,780
Depreciation and Amortization 1,636 1,086 4,771 3,293
Interest Expense 146 146 539 444
Cost of Goods Sold 44,139 47,056 131,155 129,867
Selling, General, and Administrative Expenses 6,919 6,956 20,526 17,777
Income Tax (Benefit) Expense 15,317 19,166 49,294 56,938
Other Segment Items (1) (46) (4) (75)
Net Income $ 43,495 $ 54,479 $ 140,038 $ 161,829
v3.25.3
Segment Information - Schedule of Capital Expenditures and Identifiable Assets (Details) - USD ($)
$ in Thousands
 9 Months Ended
Sep. 30, 2025
Sep. 30, 2024
Dec. 31, 2024
Segment Reporting Information [Line Items]      
Capital Expenditures $ 213,329 $ 259,750  
Identifiable Assets 3,843,823   $ 3,652,082
Operating Segments | Electric      
Segment Reporting Information [Line Items]      
Capital Expenditures 199,618 215,056  
Identifiable Assets 2,924,509   2,785,522
Operating Segments | Manufacturing      
Segment Reporting Information [Line Items]      
Capital Expenditures 6,763 25,034  
Identifiable Assets 251,906   254,445
Operating Segments | Plastics      
Segment Reporting Information [Line Items]      
Capital Expenditures 6,336 19,580  
Identifiable Assets 210,180   186,043
Corporate      
Segment Reporting Information [Line Items]      
Capital Expenditures 612 $ 80  
Identifiable Assets $ 457,228   $ 426,072
v3.25.3
Segment Information - Schedule of Reconciliation to Consolidated Amounts (Details) - USD ($)
$ in Thousands
 3 Months Ended 9 Months Ended
Sep. 30, 2025
Sep. 30, 2024
Sep. 30, 2025
Sep. 30, 2024
Segment Reporting Information [Line Items]        
Depreciation and Amortization $ 29,554 $ 27,051 $ 88,376 $ 79,579
Interest Expense 11,790 11,173 35,063 31,225
Income Tax Expense (Benefit) 12,817 18,677 36,553 57,077
Net Income 78,292 85,479 224,119 246,812
Operating Segments        
Segment Reporting Information [Line Items]        
Interest Expense 11,670 11,084 34,791 30,843
Income Tax Expense (Benefit) 15,757 19,960 42,487 61,224
Net Income 74,719 85,183 220,179 245,586
Operating Segments | Electric        
Segment Reporting Information [Line Items]        
Depreciation and Amortization 22,433 20,741 67,087 61,014
Interest Expense 10,891 10,274 32,370 28,534
Income Tax Expense (Benefit) (701) 948 (9,178) 723
Net Income 27,308 28,530 71,211 69,486
Operating Segments | Manufacturing        
Segment Reporting Information [Line Items]        
Depreciation and Amortization 5,411 5,199 16,357 15,199
Interest Expense 633 664 1,882 1,865
Income Tax Expense (Benefit) 1,141 (154) 2,371 3,563
Net Income 3,916 2,174 8,930 14,271
Operating Segments | Plastics        
Segment Reporting Information [Line Items]        
Depreciation and Amortization 1,636 1,086 4,771 3,293
Interest Expense 146 146 539 444
Income Tax Expense (Benefit) 15,317 19,166 49,294 56,938
Net Income 43,495 54,479 140,038 161,829
Corporate        
Segment Reporting Information [Line Items]        
Depreciation and Amortization 74 25 161 73
Interest Expense 120 89 272 382
Income Tax Expense (Benefit) (2,940) (1,283) (5,934) (4,147)
Net Income $ 3,573 $ 296 $ 3,940 $ 1,226
v3.25.3
Revenue (Details) - USD ($)
$ in Thousands
 3 Months Ended 9 Months Ended
Sep. 30, 2025
Sep. 30, 2024
Sep. 30, 2025
Sep. 30, 2024
Disaggregation of Revenue [Line Items]        
Total Operating Revenues $ 325,563 $ 338,033 $ 995,959 $ 1,027,437
Total Operating Revenues from Contracts with Customers 325,886 337,386 995,654 1,027,024
Electric        
Disaggregation of Revenue [Line Items]        
Total Operating Revenues 138,597 130,380 417,048 384,696
Electric Segment - ARP Revenues (323) 647 305 413
Manufacturing        
Disaggregation of Revenue [Line Items]        
Total Operating Revenues 76,951 79,896 237,363 275,961
Plastics        
Disaggregation of Revenue [Line Items]        
Total Operating Revenues 110,015 127,757 341,548 366,780
Total Retail | Electric        
Disaggregation of Revenue [Line Items]        
Total Operating Revenues 111,642 110,802 355,144 331,673
Retail: Residential | Electric        
Disaggregation of Revenue [Line Items]        
Total Operating Revenues 32,106 34,150 104,939 99,971
Retail: Commercial and Industrial | Electric        
Disaggregation of Revenue [Line Items]        
Total Operating Revenues 77,507 74,602 244,105 225,827
Retail: Other | Electric        
Disaggregation of Revenue [Line Items]        
Total Operating Revenues 2,029 2,050 6,100 5,875
Transmission | Electric        
Disaggregation of Revenue [Line Items]        
Total Operating Revenues 14,934 15,152 40,297 39,805
Wholesale | Electric        
Disaggregation of Revenue [Line Items]        
Total Operating Revenues 10,379 3,139 16,418 8,273
Other | Electric        
Disaggregation of Revenue [Line Items]        
Total Operating Revenues 1,642 1,287 5,189 4,945
Metal Parts and Tooling | Manufacturing        
Disaggregation of Revenue [Line Items]        
Total Operating Revenues 67,649 69,904 207,868 245,972
Plastic Products and Tooling | Manufacturing        
Disaggregation of Revenue [Line Items]        
Total Operating Revenues 7,710 8,537 24,255 23,989
Scrap Metal | Manufacturing        
Disaggregation of Revenue [Line Items]        
Total Operating Revenues $ 1,592 $ 1,455 $ 5,240 $ 6,000
v3.25.3
Select Balance Sheet Information - Schedule of Receivables (Details) - USD ($)
$ in Thousands
Sep. 30, 2025
Dec. 31, 2024
Sep. 30, 2024
Dec. 31, 2023
Receivables        
Trade $ 140,890 $ 112,169    
Other 10,593 13,799    
Unbilled Receivables 16,968 21,916    
Total Receivables 168,451 147,884    
Less: Allowance for Credit Losses 1,757 1,920 $ 1,950 $ 2,522
Receivables, net of allowance for credit losses $ 166,694 $ 145,964    
v3.25.3
Select Balance Sheet Information - Schedule of Activity in Allowance for Credit Losses (Details) - USD ($)
$ in Thousands
 9 Months Ended
Sep. 30, 2025
Sep. 30, 2024
Accounts Receivable, Allowance for Credit Loss [Roll Forward]    
Beginning Balance, January 1 $ 1,920 $ 2,522
Additions Charged to Expense 1,134 904
Reductions for Amounts Written Off, Net of Recoveries (1,297) (1,476)
Ending Balance, September 30 $ 1,757 $ 1,950
v3.25.3
Select Balance Sheet Information - Schedule of Inventories (Details) - USD ($)
$ in Thousands
Sep. 30, 2025
Dec. 31, 2024
Organization, Consolidation and Presentation of Financial Statements [Abstract]    
Finished Goods $ 39,146 $ 43,345
Work in Process 24,174 22,637
Raw Material, Fuel and Supplies 92,445 82,903
Total Inventories $ 155,765 $ 148,885
v3.25.3
Select Balance Sheet Information - Schedule of Investments (Details) - USD ($)
$ in Thousands
Sep. 30, 2025
Dec. 31, 2024
Short-term Investments    
Total Short-term Investments $ 53,877 $ 753
Long-term Investments    
Corporate-Owned Life Insurance Policies 50,027 47,895
Mutual Funds 16,454 10,653
Money Market Funds 1,237 596
Other Investments 27 27
Total Long-term Investments 77,662 121,177
Total Investments 131,539 121,930
Government Debt Securities    
Short-term Investments    
Debt Securities 53,482 753
Long-term Investments    
Debt Securities 9,022 60,378
Corporate Debt Securities    
Short-term Investments    
Debt Securities 395 0
Long-term Investments    
Debt Securities $ 895 $ 1,628
v3.25.3
Select Balance Sheet Information - Schedule of Amortized Costs and Fair Value of Debt Securities, Available-for-Sale (Details) - USD ($)
$ in Thousands
Sep. 30, 2025
Dec. 31, 2024
Debt Securities, Available-for-Sale [Line Items]    
Amortized Cost $ 63,247 $ 62,520
Gross Unrealized Gains 598 433
Gross Unrealized (Losses) (51) (194)
Fair Value 63,794 62,759
Government Debt Securities    
Debt Securities, Available-for-Sale [Line Items]    
Amortized Cost 61,967 60,891
Gross Unrealized Gains 586 424
Gross Unrealized (Losses) (49) (184)
Fair Value 62,504 61,131
Corporate Debt Securities    
Debt Securities, Available-for-Sale [Line Items]    
Amortized Cost 1,280 1,629
Gross Unrealized Gains 12 9
Gross Unrealized (Losses) (2) (10)
Fair Value $ 1,290 $ 1,628
v3.25.3
Select Balance Sheet Information - Schedule of Investments Classified by Contractual Maturity Date (Details) - USD ($)
$ in Thousands
Sep. 30, 2025
Dec. 31, 2024
Investments, Debt and Equity Securities [Abstract]    
Due in one year or less $ 53,877  
Due in one to five years 9,917  
Total $ 63,794 $ 62,759
v3.25.3
Select Balance Sheet Information - Schedule of Property, Plant and Equipment (Details) - USD ($)
$ in Thousands
Sep. 30, 2025
Dec. 31, 2024
Property, Plant and Equipment [Line Items]    
Property, Plant and Equipment, net of accumulated depreciation $ 2,820,689 $ 2,692,460
Electric Plant    
Property, Plant and Equipment [Line Items]    
Property, plant and equipment, gross 3,569,042 3,412,833
Less Accumulated Depreciation and Amortization 921,038 899,049
Property, Plant and Equipment, net of accumulated depreciation 2,648,004 2,513,784
Nonelectric Property, Plant and Equipment    
Property, Plant and Equipment [Line Items]    
Property, plant and equipment, gross 413,416 403,101
Less Accumulated Depreciation and Amortization 240,731 224,425
Property, Plant and Equipment, net of accumulated depreciation 172,685 178,676
Electric Plant in Service | Electric Plant    
Property, Plant and Equipment [Line Items]    
Property, plant and equipment, gross 3,234,585 3,180,943
Construction Work in Progress | Electric Plant    
Property, Plant and Equipment [Line Items]    
Property, plant and equipment, gross 334,457 231,890
Construction Work in Progress | Nonelectric Property, Plant and Equipment    
Property, Plant and Equipment [Line Items]    
Property, plant and equipment, gross 12,223 40,536
Nonelectric Property, Plant and Equipment in Service | Nonelectric Property, Plant and Equipment    
Property, Plant and Equipment [Line Items]    
Property, plant and equipment, gross $ 401,193 $ 362,565
v3.25.3
Regulatory Matters (Details) - USD ($)
$ in Thousands
Sep. 30, 2025
Dec. 31, 2024
Schedule of Regulatory Assets and Liabilities [Line Items]    
Regulatory assets - current $ 9,433 $ 9,962
Regulatory assets, long-term 97,936 98,673
Regulatory liabilities, current 21,753 29,307
Regulatory liabilities, long -term 296,216 288,928
Deferred Income Taxes    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Regulatory liabilities, current 0 0
Regulatory liabilities, long -term 124,832 130,387
Plant Removal Obligations    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Regulatory liabilities, current 0 0
Regulatory liabilities, long -term $ 132,438 126,263
Fuel Clause Adjustments    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Regulatory liabilities - Remaining Recovery/Refund Period 1 year  
Regulatory liabilities, current $ 5,197 11,432
Regulatory liabilities, long -term $ 0 0
Alternative Revenue Program Riders    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Regulatory liabilities - Remaining Recovery/Refund Period 1 year  
Regulatory liabilities, current $ 10,888 14,255
Regulatory liabilities, long -term 0 0
North Dakota PTC Refunds    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Regulatory liabilities, current 0 0
Regulatory liabilities, long -term 26,348 20,099
Pension and Other Postretirement Benefit Plans    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Regulatory liabilities, current 2,547 2,547
Regulatory liabilities, long -term 10,350 10,758
Other    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Regulatory liabilities, current 3,121 1,073
Regulatory liabilities, long -term 2,248 1,421
Pension and Other Postretirement Benefit Plans    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Regulatory assets - current 0 0
Regulatory assets, long-term 87,182 88,161
Alternative Revenue Program Riders    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Regulatory assets - current 4,610 4,257
Regulatory assets, long-term $ 146 195
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 9,070 8,944
Fuel Clause Adjustments    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Regulatory assets - current 3,229 2,218
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 $ 795 1,989
Regulatory assets, long-term $ 512 0
Regulatory assets - Remaining Recovery/Refund Period 2 years  
Other    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Regulatory assets - current $ 799 1,498
Regulatory assets, long-term $ 1,026 $ 1,373
v3.25.3
Regulatory Matters - Narrative (Details) - USD ($)
$ in Millions
Oct. 31, 2025
Jun. 04, 2025
South Dakota Rate Case | SDPUC    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Requested rate increase, amount   $ 5.7
Requested rate increase, percent (as a percent)   12.50%
Requested return on base rate, percent (as a percent)   8.29%
Requested return on equity, percent (as a percent)   10.80%
Requested equity ratio, percent (as a percent)   53.54%
Requested rate increase rates on interim, term   180 days
Minnesota Rate Case | MPUC | Subsequent Event    
Schedule of Regulatory Assets and Liabilities [Line Items]    
Requested rate increase, amount $ 44.8  
Requested rate increase, percent (as a percent) 17.70%  
Requested return on base rate, percent (as a percent) 7.92%  
Requested return on equity, percent (as a percent) 10.65%  
Requested equity ratio, percent (as a percent) 53.50%  
Requested accelerated recovery of remaining investments $ 4.3  
Requested interim rate , increase amount $ 31.8  
Requested interim rate increase percentage (as a percent) 12.60%  
v3.25.3
Short-Term and Long-Term Borrowings - Lines of Credit (Details) - USD ($)
Sep. 30, 2025
Dec. 31, 2024
Line of Credit Facility [Line Items]    
Borrowing Limit $ 390,000,000  
Amount Outstanding 0 $ 69,615,000
Letters of Credit 10,461,000  
Amount Available 379,539,000 311,613,000
OTC Credit Agreement    
Line of Credit Facility [Line Items]    
Borrowing 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]    
Borrowing Limit 220,000,000  
Amount Outstanding 0  
Letters of Credit 10,461,000  
Amount Available $ 209,539,000 $ 141,613,000
v3.25.3
Short-Term and Long-Term Borrowings - Narrative (Details) - USD ($)
 9 Months Ended
Mar. 27, 2025
Sep. 30, 2025
Dec. 31, 2024
Debt Instrument [Line Items]      
Debt, weighted average interest rate (as a percent)     5.61%
Long-term debt   $ 1,047,000,000 $ 947,000,000
Debt instrument, redemption price, percentage of principal amount redeemed (as a percent) 10.00%    
Debt instrument, redemption price, percentage (as a percent) 100.00%    
Series 2025A Senior Unsecured Notes      
Debt Instrument [Line Items]      
Long-term debt   $ 50,000,000 0
Interest rate (as a percent) 5.49% 5.49%  
Series 2025B Senior Unsecured Notes      
Debt Instrument [Line Items]      
Long-term debt   $ 50,000,000 $ 0
Interest rate (as a percent)   5.98%  
Senior Notes      
Debt Instrument [Line Items]      
Long-term debt $ 100,000,000    
Senior Notes | Series 2025A Senior Unsecured Notes      
Debt Instrument [Line Items]      
Long-term debt 50,000,000    
Senior Notes | Series 2025B Senior Unsecured Notes      
Debt Instrument [Line Items]      
Long-term debt $ 50,000,000    
Interest rate (as a percent) 5.98%    
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  
v3.25.3
Short-Term and Long-Term Borrowings - Breakdown of Outstanding Long-term Debt (Details) - USD ($)
$ in Thousands
Sep. 30, 2025
Mar. 27, 2025
Dec. 31, 2024
Debt Instrument [Line Items]      
Long-term debt $ 1,047,000   $ 947,000
Unamortized Long-Term Debt Issuance Costs 3,563   3,266
Total Long-Term Debt, Net of Unamortized Debt Issuance Costs $ 1,043,437   943,734
Guaranteed Senior Notes      
Debt Instrument [Line Items]      
Rate 3.55%    
Long-term debt $ 80,000   80,000
Series 2007C Senior Unsecured Notes      
Debt Instrument [Line Items]      
Rate 6.37%    
Long-term debt $ 42,000   42,000
Series 2013A Senior Unsecured Notes      
Debt Instrument [Line Items]      
Rate 4.68%    
Long-term debt $ 60,000   60,000
Series 2019A Senior Unsecured Notes      
Debt Instrument [Line Items]      
Rate 3.07%    
Long-term debt $ 10,000   10,000
Series 2020A Senior Unsecured Notes      
Debt Instrument [Line Items]      
Rate 3.22%    
Long-term debt $ 10,000   10,000
Series 2020B Senior Unsecured Notes      
Debt Instrument [Line Items]      
Rate 3.22%    
Long-term debt $ 40,000   40,000
Series 2021A Senior Unsecured Notes      
Debt Instrument [Line Items]      
Rate 2.74%    
Long-term debt $ 40,000   40,000
Series 2024A Senior Unsecured Notes      
Debt Instrument [Line Items]      
Rate 5.48%    
Long-term debt $ 60,000   60,000
Series 2025A Senior Unsecured Notes      
Debt Instrument [Line Items]      
Rate 5.49% 5.49%  
Long-term debt $ 50,000   0
Series 2007D Senior Unsecured Notes      
Debt Instrument [Line Items]      
Rate 6.47%    
Long-term debt $ 50,000   50,000
Series 2019B Senior Unsecured Notes      
Debt Instrument [Line Items]      
Rate 3.52%    
Long-term debt $ 26,000   26,000
Series 2020C Senior Unsecured Notes      
Debt Instrument [Line Items]      
Rate 3.62%    
Long-term debt $ 10,000   10,000
Series 2013B Senior Unsecured Notes      
Debt Instrument [Line Items]      
Rate 5.47%    
Long-term debt $ 90,000   90,000
Series 2018A Senior Unsecured Notes      
Debt Instrument [Line Items]      
Rate 4.07%    
Long-term debt $ 100,000   100,000
Series 2019C Senior Unsecured Notes      
Debt Instrument [Line Items]      
Rate 3.82%    
Long-term debt $ 64,000   64,000
Series 2020D Senior Unsecured Notes      
Debt Instrument [Line Items]      
Rate 3.92%    
Long-term debt $ 15,000   15,000
Series 2021B Senior Unsecured Notes      
Debt Instrument [Line Items]      
Rate 3.69%    
Long-term debt $ 100,000   100,000
Series 2022A Senior Unsecured Notes      
Debt Instrument [Line Items]      
Rate 3.77%    
Long-term debt $ 90,000   90,000
Series 2024B Senior Unsecured Notes      
Debt Instrument [Line Items]      
Rate 5.77%    
Long-term debt $ 60,000   60,000
Series 2025B Senior Unsecured Notes      
Debt Instrument [Line Items]      
Rate 5.98%    
Long-term debt $ 50,000   $ 0
v3.25.3
Employee Postretirement Benefits - Components of Net Periodic Benefit Cost (Details) - USD ($)
$ in Thousands
 3 Months Ended 9 Months Ended
Sep. 30, 2025
Sep. 30, 2024
Sep. 30, 2025
Sep. 30, 2024
Defined Benefit Plan Disclosure [Line Items]        
Net Periodic Benefit Cost (Income) $ (603) $ (1,651) $ (1,812) $ (4,952)
Pension Benefits (Pension Plan)        
Defined Benefit Plan Disclosure [Line Items]        
Service Cost 876 972 2,627 2,915
Interest Cost 4,325 4,297 12,977 12,891
Expected Return on Assets (6,191) (6,380) (18,573) (19,139)
Amortization of Prior Service Cost 0 0 0 0
Amortization of Net Actuarial Loss 335 40 1,006 119
Net Periodic Benefit Cost (Income) (655) (1,071) (1,963) (3,214)
Pension Benefits (ESSRP)        
Defined Benefit Plan Disclosure [Line Items]        
Service Cost 0 0 0 0
Interest Cost 474 474 1,421 1,422
Expected Return on Assets 0 0 0 0
Amortization of Prior Service Cost 0 0 0 0
Amortization of Net Actuarial Loss 0 0 0 0
Net Periodic Benefit Cost (Income) 474 474 1,421 1,422
Postretirement Benefits        
Defined Benefit Plan Disclosure [Line Items]        
Service Cost 123 123 368 368
Interest Cost 403 399 1,208 1,199
Expected Return on Assets 0 0 0 0
Amortization of Prior Service Cost (948) (1,576) (2,846) (4,727)
Amortization of Net Actuarial Loss 0 0 0 0
Net Periodic Benefit Cost (Income) $ (422) $ (1,054) $ (1,270) $ (3,160)
v3.25.3
Employee Postretirement Benefits - Composition of Periodic Benefit Cost (Details) - USD ($)
$ in Thousands
 3 Months Ended 9 Months Ended
Sep. 30, 2025
Sep. 30, 2024
Sep. 30, 2025
Sep. 30, 2024
Retirement Benefits [Abstract]        
Net Periodic Benefit Cost (Income) $ (603) $ (1,651) $ (1,812) $ (4,952)
Net Amount Amortized Due to the Effect of Regulation 1,298 379 2,366 1,038
Net Periodic Benefit Cost (Income) Recognized $ 695 $ (1,272) $ 554 $ (3,914)
v3.25.3
Employee Postretirement Benefits - Narrative (Details) - USD ($)
$ in Millions
 9 Months Ended
Sep. 30, 2025
Sep. 30, 2024
Retirement Benefits [Abstract]    
Pension contribution $ 0.0 $ 0.0
v3.25.3
Income Taxes (Details) - USD ($)
$ in Millions
 3 Months Ended 9 Months Ended
Sep. 30, 2025
Sep. 30, 2024
Sep. 30, 2025
Sep. 30, 2024
Income Tax Disclosure [Abstract]        
Effective income tax rate (as a percent) 14.10% 17.90% 14.00% 18.80%
Reduction in current year income tax payable due to deduction of research and development costs     $ 7.7  
v3.25.3
Commitments and Contingencies (Details)
$ in Thousands
 1 Months Ended 9 Months Ended 24 Months Ended
Aug. 31, 2024
case
Sep. 30, 2025
USD ($)
Sep. 30, 2024
USD ($)
regionalTransmissionOrganization
Dec. 31, 2026
USD ($)
Other Commitments [Line Items]        
Payments to acquire productive assets   $ 213,329 $ 259,750  
Number of regional transmission organizations with show cause proceedings | regionalTransmissionOrganization     4  
Number of putative cases | case 3      
Solar | Forecast        
Other Commitments [Line Items]        
Payments to acquire productive assets       $ 23,600
Commitments, termination fee       $ 5,000
v3.25.3
Shareholders' Equity (Details) - USD ($)
$ in Thousands
 9 Months Ended
May 03, 2024
Sep. 30, 2025
Dec. 31, 2024
Class of Stock [Line Items]      
Total capitalization   $ 2,874,888 $ 2,612,233
Otter Tail Power Company      
Class of Stock [Line Items]      
Equity to total capitalization ratio   55.10%  
Net assets restricted from distribution   $ 861,000  
Minimum | Otter Tail Power Company | Minnesota Public Utilities Commission      
Class of Stock [Line Items]      
Public utilities, requested equity capital structure (as a percent)   47.20%  
Maximum | Otter Tail Power Company      
Class of Stock [Line Items]      
Total capitalization   $ 2,200,000  
Maximum | Otter Tail Power Company | Minnesota Public Utilities Commission      
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)   75,173  
Number of shares available for grant (in shares)   1,354,358  
v3.25.3
Accumulated Other Comprehensive Income (Loss) (Details) - USD ($)
$ in Thousands
 3 Months Ended 9 Months Ended
Sep. 30, 2025
Sep. 30, 2024
Sep. 30, 2025
Sep. 30, 2024
AOCI Attributable to Parent, Net of Tax [Roll Forward]        
Balance, Beginning of Period $ 1,774,462 $ 1,565,652 $ 1,668,499 $ 1,443,006
Other Comprehensive Income Before Reclassifications, net of tax 37 1,233 272 989
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss) (29) (17) (62) (158)
Total Other Comprehensive Income (Loss) 8 1,216 210 831
Balance, End of Period 1,831,451 1,633,331 1,831,451 1,633,331
Pension and Other Postretirement Benefits        
AOCI Attributable to Parent, Net of Tax [Roll Forward]        
Balance, Beginning of Period 350 1,271 373 1,375
Other Comprehensive Income Before Reclassifications, net of tax 0 0 0 0
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss) (12) (28) (35) (132)
Total Other Comprehensive Income (Loss) (12) (28) (35) (132)
Balance, End of Period 338 1,243 338 1,243
Net Unrealized Gains (Losses) on Available-for-Sale Securities        
AOCI Attributable to Parent, Net of Tax [Roll Forward]        
Balance, Beginning of Period 384 (508) 159 (227)
Other Comprehensive Income Before Reclassifications, net of tax 37 1,233 272 989
Amounts Reclassified from Accumulated Other Comprehensive Income (Loss) (17) 11 (27) (26)
Total Other Comprehensive Income (Loss) 20 1,244 245 963
Balance, End of Period 404 736 404 736
Accumulated Other Comprehensive Income        
AOCI Attributable to Parent, Net of Tax [Roll Forward]        
Balance, Beginning of Period 734 763 532 1,148
Total Other Comprehensive Income (Loss) 8 1,216 210 831
Balance, End of Period $ 742 $ 1,979 $ 742 $ 1,979
v3.25.3
Share-Based Payments - Narrative (Details)
$ in Millions
 3 Months Ended 9 Months Ended
Sep. 30, 2025
USD ($)
measure
Sep. 30, 2024
USD ($)
Sep. 30, 2025
USD ($)
measure
Sep. 30, 2024
USD ($)
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Share-based payment arrangement, expense $ 0.7 $ 1.3 $ 8.1  
Restricted Stock Awards        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Cliff awards vesting period (in years)     4 years  
Fair value of awards vested     $ 3.6 $ 5.1
Restricted Stock Awards | Minimum        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Award vesting period (in years)     3 years  
Restricted Stock Awards | Maximum        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Award vesting period (in years)     4 years  
Stock Performance Awards        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Award vesting period (in years)     3 years  
Fair value of awards vested     $ 5.5 $ 12.3
Number of performance measures | measure 2   2  
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%  
v3.25.3
Share-Based Payments - Summary of Restricted Stock Awards and Stock Performance Awards (Details)
 9 Months Ended
Sep. 30, 2025
$ / shares
shares
Restricted Stock Awards  
Shares  
Beginning of year (in shares) | shares 143,417
Granted (in shares) | shares 57,185
Vested (in shares) | shares (47,593)
Forfeited (in shares) | shares (6,600)
End of year (in shares) | shares 146,409
Weighted-Average Grant-Date Fair Value  
Beginning of year (in dollars per share) | $ / shares $ 68.47
Granted (in dollars per share) | $ / shares 74.22
Vested (in dollars per share) | $ / shares 59.74
Forfeited (in dollars per share) | $ / shares 68.72
End of year (in dollars per share) | $ / shares $ 73.55
Stock Performance Awards  
Shares  
Beginning of year (in shares) | shares 144,800
Granted (in shares) | shares 57,000
Vested (in shares) | shares (49,000)
Forfeited (in shares) | shares 0
End of year (in shares) | shares 152,800
Weighted-Average Grant-Date Fair Value  
Beginning of year (in dollars per share) | $ / shares $ 68.85
Granted (in dollars per share) | $ / shares 73.90
Vested (in dollars per share) | $ / shares 53.93
Forfeited (in dollars per share) | $ / shares 0
End of year (in dollars per share) | $ / shares $ 75.52
v3.25.3
Share-Based Payments - Weighted-Average Assumptions (Details) - Stock Performance Awards
 9 Months Ended
Sep. 30, 2025
Sep. 30, 2024
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Risk-free interest rate 4.28% 4.16%
Expected term (in years) 3 years 3 years
Expected volatility 30.30% 35.10%
Dividend yield 2.50% 2.40%
v3.25.3
Earnings Per Share (Details) - shares
shares in Thousands
 3 Months Ended 9 Months Ended
Sep. 30, 2025
Sep. 30, 2024
Sep. 30, 2025
Sep. 30, 2024
Earnings Per Share, Diluted, by Common Class, Including Two Class Method [Line Items]        
Weighted-Average Common Shares Outstanding – Basic (in shares) 41,877 41,800 41,859 41,770
Effect of Dilutive Securities:        
Dilutive Effect of Potential Common Shares (in shares) 261 281 247 298
Weighted-Average Common Shares Outstanding – Diluted (in shares) 42,138 42,081 42,106 42,068
Stock Performance Awards        
Effect of Dilutive Securities:        
Effect of Dilutive Securities (in shares) 164 188 150 199
Restricted Stock Awards        
Effect of Dilutive Securities:        
Effect of Dilutive Securities (in shares) 94 91 95 97
Employee Stock Purchase Plan        
Effect of Dilutive Securities:        
Effect of Dilutive Securities (in shares) 3 2 2 2
v3.25.3
Derivative Instruments - Schedule of Notional Amounts and Fair Value (Details)
$ in Thousands
 9 Months Ended 12 Months Ended
Sep. 30, 2025
USD ($)
MWh
Dec. 31, 2024
USD ($)
MWh
Derivative Assets:    
Other Current Assets $ 1,285 $ 0
Other Noncurrent Assets 119 0
Total Derivative Assets $ 1,404 $ 0
Swap    
Derivative Instruments and Hedging Activities Disclosures [Line Items]    
Megawatt hours of electricity | MWh 347,000 167,000
Derivative Liabilities:    
Other Current Liabilities $ 795 $ 1,989
Other Noncurrent Liabilities 512 0
Total Derivative Liabilities $ 1,307 $ 1,989
v3.25.3
Derivative Instruments - Narrative (Details) - USD ($)
$ in Millions
 9 Months Ended
Sep. 30, 2025
Sep. 30, 2024
Swap    
Derivative Instruments and Hedging Activities Disclosures [Line Items]    
Loss on derivatives, net $ 2.6 $ 2.7
v3.25.3
Fair Value Measurements - Schedule of Fair Value Measurements, Recurring and Nonrecurring (Details) - USD ($)
$ in Thousands
Sep. 30, 2025
Dec. 31, 2024
Investments:    
Fair Value $ 63,794 $ 62,759
Derivative Instruments 1,404 0
Corporate Debt Securities    
Investments:    
Fair Value 1,290 1,628
Government Debt Securities    
Investments:    
Fair Value 62,504 61,131
Level 1 | Fair Value, Recurring    
Investments:    
Money Market Funds 1,237 596
Mutual Funds 16,454 10,653
Derivative Instruments 0  
Total Assets 17,691 11,249
Liabilities:    
Derivative Instruments 0 0
Total 0 0
Level 1 | Fair Value, Recurring | Corporate Debt Securities    
Investments:    
Fair Value 0 0
Level 1 | Fair Value, Recurring | Government Debt Securities    
Investments:    
Fair Value 0 0
Level 2 | Fair Value, Recurring    
Investments:    
Money Market Funds 0 0
Mutual Funds 0 0
Derivative Instruments 1,404  
Total Assets 65,198 62,759
Liabilities:    
Derivative Instruments 1,307 1,989
Total 1,307 1,989
Level 2 | Fair Value, Recurring | Corporate Debt Securities    
Investments:    
Fair Value 1,290 1,628
Level 2 | Fair Value, Recurring | Government Debt Securities    
Investments:    
Fair Value 62,504 61,131
Level 3 | Fair Value, Recurring    
Investments:    
Money Market Funds 0 0
Mutual Funds 0 0
Derivative Instruments 0  
Total Assets 0 0
Liabilities:    
Derivative Instruments 0 0
Total 0 0
Level 3 | Fair Value, Recurring | Corporate Debt Securities    
Investments:    
Fair Value 0 0
Level 3 | Fair Value, Recurring | Government Debt Securities    
Investments:    
Fair Value $ 0 $ 0
v3.25.3
Fair Value Measurements - Schedule of Fair Value of Assets and Liabilities (Details) - USD ($)
$ in Thousands
Sep. 30, 2025
Dec. 31, 2024
Carrying Amount    
Assets:    
Cash and Cash Equivalents $ 325,786 $ 294,651
Total Assets 325,786 294,651
Liabilities:    
Short-Term Debt 0 69,615
Long-Term Debt 1,043,437 943,734
Total 1,043,437 1,013,349
Fair Value    
Assets:    
Cash and Cash Equivalents 325,786 294,651
Total Assets 325,786 294,651
Liabilities:    
Short-Term Debt 0 69,615
Long-Term Debt 930,550 806,826
Total $ 930,550 $ 876,441