NORTHERN STATES POWER CO, 10-Q filed on 10/26/2018
Quarterly Report
v3.10.0.1
Document and Entity Information - shares
 9 Months Ended
Sep. 30, 2018
Oct. 26, 2018
Document and Entity Information [Abstract]    
Entity Registrant Name NORTHERN STATES POWER CO  
Entity Central Index Key 0001123852  
Current Fiscal Year End Date --12-31  
Entity Current Reporting Status Yes  
Entity Filer Category Non-accelerated Filer  
Entity Emerging Growth Company false  
Entity Small Business false  
Entity Common Stock, Shares Outstanding   1,000,000
Document Fiscal Year Focus 2018  
Document Fiscal Period Focus Q3  
Document Type 10-Q  
Amendment Flag false  
Document Period End Date Sep. 30, 2018  
v3.10.0.1
CONSOLIDATED STATEMENTS OF INCOME (UNAUDITED) - USD ($)
$ in Thousands
 3 Months Ended 9 Months Ended
Sep. 30, 2018
Sep. 30, 2017
Sep. 30, 2018
Sep. 30, 2017
Operating revenues        
Total operating revenues $ 1,351,754 $ 1,355,779 $ 3,850,240 $ 3,827,859
Operating expenses        
Cost of natural gas sold and transported 19,595 20,723 211,679 198,968
Cost of sales — other 5,703 4,313 14,539 13,085
Operating and maintenance expenses 310,536 285,372 912,027 899,187
Conservation program expenses 29,434 31,674 87,762 92,238
Depreciation and amortization 187,201 176,739 547,333 522,070
Taxes (other than income taxes) 59,990 62,220 191,820 191,989
Total operating expenses 1,092,272 1,016,044 3,269,247 3,132,608
Operating income 259,482 339,735 580,993 695,251
Interest charges and financing costs        
Interest charges — includes other financing costs of $1,853, $1,845, $5,501 and $5,437, respectively 56,398 57,577 170,517 172,548
Allowance for funds used during construction — debt (3,500) (5,383) (10,287) (11,909)
Total interest charges and financing costs 52,898 52,194 160,230 160,639
Other expense, net (1,976) (1,278) (3,994) (6,444)
Allowance for funds used during construction — equity 5,899 10,683 19,550 23,391
Income before income taxes 210,507 296,946 436,319 551,559
Income taxes 9,331 67,943 31,016 140,728
Net income 201,176 229,003 405,303 410,831
Electric        
Operating expenses        
Electric fuel and purchased power 479,813 435,003 1,304,087 1,215,071
Natural gas        
Operating revenues        
Total operating revenues 54,625 57,442 379,169 356,631
Other        
Operating revenues        
Total operating revenues 7,631 7,366 22,902 21,448
Related Party Transaction, Electric Domestic Regulated Revenue [Member]        
Operating revenues        
Related Party Transaction, Electric Domestic Regulated Revenue 122,636 123,524 357,025 366,598
Non-Related Party Transaction, Electric Domestic Regulated Revenue [Member] | Electric        
Operating revenues        
Total operating revenues $ 1,166,862 $ 1,167,447 $ 3,091,144 $ 3,083,182
v3.10.0.1
CONSOLIDATED STATEMENTS OF INCOME (UNAUDITED) (Parenthetical) - USD ($)
$ in Thousands
 3 Months Ended 9 Months Ended
Sep. 30, 2018
Sep. 30, 2017
Sep. 30, 2018
Sep. 30, 2017
Interest charges and financing costs        
Other financing costs $ 1,853 $ 1,845 $ 5,501 $ 5,437
v3.10.0.1
CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME (UNAUDITED) - USD ($)
$ in Thousands
 3 Months Ended 9 Months Ended
Sep. 30, 2018
Sep. 30, 2017
Sep. 30, 2018
Sep. 30, 2017
Comprehensive income:        
Net income $ 201,176 $ 229,003 $ 405,303 $ 410,831
Pension and retiree medical benefits:        
Amortization of losses included in net periodic benefit cost, net of tax of $20, $21, $65 and $70, respectively 56 39 163 110
Derivative instruments:        
Net fair value decrease, net of tax of $2, $16, $13 and $33, respectively 5 22 32 48
Reclassification of losses to net income, net of tax of $56, $222, $185 and $502, respectively 172 379 504 786
Total derivative instruments, net of tax 177 401 536 834
Marketable securities:        
Reclassification of gains to net income, net of tax of $0, $0, $(51) and $0, respectively 0 0 (128) 0
Other comprehensive income 233 440 571 944
Comprehensive income $ 201,409 $ 229,443 $ 405,874 $ 411,775
v3.10.0.1
CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME (UNAUDITED) (Parenthetical) - USD ($)
$ in Thousands
 3 Months Ended 9 Months Ended
Sep. 30, 2018
Sep. 30, 2017
Sep. 30, 2018
Sep. 30, 2017
Pension and retiree medical benefits:        
Amortization of losses included in net periodic benefit cost, tax $ 20 $ 21 $ 65 $ 70
Derivative instruments:        
Net fair value (decrease) increase, tax 2 16 13 33
Reclassification of losses to net income, tax 56 222 185 502
Other Comprehensive Income (Loss), Securities, Available-for-sale, Tax [Abstract]        
Reclassification of gains to net income, tax $ 0 $ 0 $ (51) $ 0
v3.10.0.1
CONSOLIDATED STATEMENTS OF CASH FLOWS (UNAUDITED) - USD ($)
$ in Thousands
 9 Months Ended
Sep. 30, 2018
Sep. 30, 2017
Operating activities    
Net income $ 405,303 $ 410,831
Adjustments to reconcile net income to cash provided by operating activities:    
Depreciation and amortization 552,141 526,784
Nuclear fuel amortization 92,393 87,654
Deferred income taxes 32,672 98,125
Amortization of Investment Tax Credits (1,226) (1,241)
Allowance for equity funds used during construction (19,550) (23,391)
Net realized and unrealized hedging and derivative transactions (2,278) (2,703)
Other, net (1,038) (1,072)
Changes in operating assets and liabilities:    
Accounts receivable (29,320) (14,677)
Accrued unbilled revenues 28,610 33,465
Inventories 17,168 (4,265)
Other current assets 84,801 35,591
Accounts payable (21,377) (34,275)
Net regulatory assets and liabilities 115,043 (15,467)
Other current liabilities (78,895) (73,898)
Pension and other employee benefit obligations (57,593) (55,548)
Change in other noncurrent assets 880 (3,585)
Change in other noncurrent liabilities (32,596) (30,704)
Net cash provided by operating activities 1,085,138 931,624
Investing activities    
Utility capital/construction expenditures (697,705) (696,857)
Allowance for equity funds used during construction 19,550 23,391
Purchases of investment securities (493,893) (965,086)
Proceeds from the sale of investment securities 478,614 948,558
Investments in utility money pool arrangement (748,000) (122,000)
Repayments from utility money pool arrangement 748,000 122,000
Other, net (2,775) (3,463)
Net cash used in investing activities (696,209) (693,457)
Financing activities    
Repayments of short-term borrowings, net 4,000 (85,000)
Borrowings under utility money pool arrangement 345,000 516,000
Repayments under utility money pool arrangement (430,000) (466,000)
Proceeds from Issuance of Long-term Debt 0 586,264
Repayments of long-term debt (1) (507,865)
Capital contributions from parent 49,622 123,247
Dividends paid to parent (371,960) (418,133)
Other (93) 0
Net cash (used in) provided by financing activities (403,432) (251,487)
Net change in cash and cash equivalents (14,503) (13,320)
Cash and cash equivalents at beginning of period 43,781 47,595
Cash and cash equivalents at end of period 29,278 34,275
Supplemental disclosure of cash flow information:    
Cash paid for interest (net of amounts capitalized) (167,301) (177,336)
Cash received (paid) for income taxes, net 59,461 (60,911)
Supplemental disclosure of non-cash investing transactions:    
Property, plant and equipment additions in accounts payable $ 79,346 $ 47,261
v3.10.0.1
CONSOLIDATED BALANCE SHEETS (UNAUDITED) - USD ($)
$ in Thousands
Sep. 30, 2018
Dec. 31, 2017
Current assets    
Cash and cash equivalents $ 29,278 $ 43,781
Accounts receivable, net 387,855 345,110
Accounts receivable from affiliates 17,758 48,494
Accrued unbilled revenues 249,106 277,716
Inventories 288,678 337,712
Regulatory assets 278,288 276,392
Derivative instruments 30,450 25,230
Prepaid Taxes 20,789 79,145
Prepayments and other 32,591 43,682
Total current assets 1,334,793 1,477,262
Property, plant and equipment, net 13,215,718 13,033,612
Other assets    
Nuclear decommissioning fund and other investments 2,262,751 2,192,344
Regulatory assets 1,326,785 1,190,429
Derivative instruments 22,083 28,102
Other 10,032 4,142
Total other assets 3,621,651 3,415,017
Total assets 18,172,162 17,925,891
Current liabilities    
Current portion of long-term debt 16 7
Short-term debt 24,000 20,000
Borrowings under utility money pool arrangement 0 85,000
Accounts payable 391,695 368,342
Accounts payable to affiliates 61,347 80,070
Regulatory liabilities 219,169 83,403
Taxes accrued 246,397 229,335
Accrued interest 52,530 65,896
Dividends payable to parent 84,177 98,687
Derivative instruments 18,664 17,697
Customer deposits 58,949 95,369
Other 146,862 152,965
Total current liabilities 1,303,806 1,296,771
Deferred credits and other liabilities    
Deferred income taxes 1,700,494 1,612,341
Deferred investment tax credits 21,301 22,528
Regulatory liabilities 1,984,514 1,978,527
Asset retirement obligations 2,167,201 2,083,874
Derivative instruments 86,748 102,742
Pension and employee benefit obligations 278,614 331,087
Other 160,242 89,440
Total deferred credits and other liabilities 6,399,114 6,220,539
Commitments and contingencies
Capitalization    
Long-term debt 4,936,119 4,933,011
Common stock — authorized 5,000,000 shares of $0.01 par value; 1,000,000 shares outstanding at Sept. 30, 2018 and Dec. 31, 2017, respectively 10 10
Additional paid in capital 3,589,363 3,580,234
Retained earnings 1,967,716 1,919,863
Accumulated other comprehensive loss (23,966) (24,537)
Total common stockholder’s equity 5,533,123 5,475,570
Total liabilities and equity $ 18,172,162 $ 17,925,891
v3.10.0.1
CONSOLIDATED BALANCE SHEETS (UNAUDITED) (Parenthetical) - $ / shares
Sep. 30, 2018
Dec. 31, 2017
Capitalization    
Common stock, shares authorized (in shares) 5,000,000 5,000,000
Common stock, par value (in dollars per share) $ 0.01 $ 0.01
Common stock, shares outstanding (in shares) 1,000,000 1,000,000
v3.10.0.1
Management's Opinion
 9 Months Ended
Sep. 30, 2018
Organization, Consolidation and Presentation of Financial Statements [Abstract]  
Management's Opinion
In the opinion of management, the accompanying unaudited consolidated financial statements contain all adjustments necessary to present fairly, in accordance with accounting principles generally accepted in the United States of America (GAAP), the financial position of NSP-Minnesota and its subsidiaries as of Sept. 30, 2018 and Dec. 31, 2017; the results of its operations, including the components of net income and comprehensive income, for the three and nine months ended Sept. 30, 2018 and 2017; and its cash flows for the nine months ended Sept. 30, 2018 and 2017. All adjustments are of a normal, recurring nature, except as otherwise disclosed. Management has also evaluated the impact of events occurring after Sept. 30, 2018 up to the date of issuance of these consolidated financial statements. These statements contain all necessary adjustments and disclosures resulting from that evaluation. The Dec. 31, 2017 balance sheet information has been derived from the audited 2017 consolidated financial statements included in the NSP-Minnesota Annual Report on Form 10-K for the year ended Dec. 31, 2017. These notes to the consolidated financial statements have been prepared pursuant to the rules and regulations of the SEC for Quarterly Reports on Form 10-Q. Certain information and note disclosures normally included in financial statements prepared in accordance with GAAP on an annual basis have been condensed or omitted pursuant to such rules and regulations. For further information, refer to the consolidated financial statements and notes thereto, included in the NSP-Minnesota Annual Report on Form 10-K for the year ended Dec. 31, 2017, filed with the SEC on Feb. 26, 2018. Due to the seasonality of NSP-Minnesota’s electric and natural gas sales, interim results are not necessarily an appropriate base from which to project annual results.
v3.10.0.1
Summary of Significant Accounting Policies
 9 Months Ended
Sep. 30, 2018
Accounting Policies [Abstract]  
Summary of Significant Accounting Policies
Summary of Significant Accounting Policies

The significant accounting policies set forth in Note 1 to the consolidated financial statements in the NSP-Minnesota Annual Report on Form 10-K for the year ended Dec. 31, 2017, appropriately represent, in all material respects, the current status of accounting policies and are incorporated herein by reference.
v3.10.0.1
Accounting Pronouncements
 9 Months Ended
Sep. 30, 2018
New Accounting Pronouncements and Changes in Accounting Principles [Abstract]  
Accounting Pronouncements
Accounting Pronouncements

Recently Issued

Leases — In February 2016, the Financial Accounting Standards Board (FASB) issued Leases, Topic 842 (Accounting Standards Update (ASU) No. 2016-02), which for lessees requires balance sheet recognition of right-of-use assets and lease liabilities for most leases. This guidance will be effective for interim and annual reporting periods beginning after Dec. 15, 2018. Adoption will occur on Jan. 1, 2019 utilizing the practical expedients provided by the standard and included in Targeted Improvements, Topic 842 (ASU No. 2018-11). On Jan. 1, 2019, agreements historically disclosed as operating leases for the use of real estate, equipment and certain fossil-fueled generating facilities operated under purchased power agreements (PPAs) are expected to be recognized on the consolidated balance sheet. Other than first-time recognition of these types of operating leases on the consolidated balance sheet, the implementation is not expected to have a significant impact on NSP-Minnesota’s consolidated financial statements.

Recently Adopted

Revenue Recognition — In May 2014, the FASB issued Revenue from Contracts with Customers, Topic 606 (ASU No. 2014-09), which provides a new framework for the recognition of revenue. NSP-Minnesota implemented the guidance on a modified retrospective basis on Jan. 1, 2018. Results for reporting periods beginning after Dec. 31, 2017 are presented in accordance with Topic 606, while prior period results have not been adjusted and continue to be reported in accordance with prior accounting guidance. Other than increased disclosures regarding revenues related to contracts with customers, the implementation did not have a material impact on NSP-Minnesota’s consolidated financial statements. For related disclosures, see Note 13 to the consolidated financial statements.

Classification and Measurement of Financial Instruments — In January 2016, the FASB issued Recognition and Measurement of Financial Assets and Financial Liabilities, Subtopic 825-10 (ASU No. 2016-01), which eliminated the available-for-sale classification for marketable equity securities and also replaced the cost method of accounting for non-marketable equity securities with a model for recognizing impairments and observable price changes. Under the new standard, other than when the consolidation or equity method of accounting is utilized, changes in the fair value of equity securities are recognized in earnings. NSP-Minnesota implemented the guidance on Jan. 1, 2018. As a result of application of accounting principles for rate regulated entities, changes in the fair value of the securities in the nuclear decommissioning fund, historically classified as available-for-sale, continue to be deferred to a regulatory asset, and the overall adoption impacts were not material.

Presentation of Net Periodic Benefit Cost — In March 2017, the FASB issued Improving the Presentation of Net Periodic Pension Cost and Net Periodic Postretirement Benefit Cost, Topic 715 (ASU No. 2017-07), which establishes that only the service cost element of pension cost may be presented as a component of operating income in the income statement. Also under the guidance, only the service cost component of pension cost is eligible for capitalization. As a result of the application of accounting principles for rate regulated entities, a similar amount of pension cost, including non-service components, will be recognized consistent with the historical ratemaking treatment, and the impacts of adoption will be limited to changes in classification of non-service costs in the consolidated statement of income. NSP-Minnesota implemented the new guidance on Jan. 1, 2018, and as a result, $3.4 million and $10.2 million of pension costs were retrospectively reclassified from operating and maintenance expenses to other income, net on the consolidated income statement for the three and nine months ended Sept. 30, 2017, respectively. Under a practical expedient permitted by the standard, NSP-Minnesota used benefit cost amounts disclosed for prior periods as the basis for retrospective application.
v3.10.0.1
Selected Balance Sheet Data
 9 Months Ended
Sep. 30, 2018
Balance Sheet Related Disclosures [Abstract]  
Selected Balance Sheet Data
Selected Balance Sheet Data
(Thousands of Dollars)
 
Sept. 30, 2018
 
Dec. 31, 2017
Accounts receivable, net
 
 
 
 
Accounts receivable
 
$
409,740

 
$
366,388

Less allowance for bad debts
 
(21,885
)
 
(21,278
)
 
 
$
387,855

 
$
345,110



(Thousands of Dollars)
 
Sept. 30, 2018
 
Dec. 31, 2017
Inventories
 
 
 
 
Materials and supplies
 
$
176,183

 
$
209,236

Fuel
 
76,962

 
94,483

Natural gas
 
35,533

 
33,993

 
 
$
288,678

 
$
337,712


(Thousands of Dollars)
 
Sept. 30, 2018
 
Dec. 31, 2017
Property, plant and equipment, net
 
 
 
 
Electric plant
 
$
17,363,519

 
$
17,024,925

Natural gas plant
 
1,426,646

 
1,370,330

Common and other property
 
726,127

 
724,066

Construction work in progress
 
685,047

 
530,126

Total property, plant and equipment
 
20,201,339

 
19,649,447

Less accumulated depreciation
 
(7,314,826
)
 
(7,018,249
)
Nuclear fuel
 
2,716,595

 
2,697,412

Less accumulated amortization
 
(2,387,390
)
 
(2,294,998
)
 
 
$
13,215,718

 
$
13,033,612
v3.10.0.1
Income Taxes
 9 Months Ended
Sep. 30, 2018
Income Tax Disclosure [Abstract]  
Income Taxes
Income Taxes

Except to the extent noted below, Note 6 to the consolidated financial statements included in NSP-Minnesota’s Annual Report on Form 10-K for the year ended Dec. 31, 2017 appropriately represents, in all material respects, the current status of other income tax matters, and are incorporated herein by reference.

Total income tax expense from operations differs from the amount computed by applying the statutory federal income tax rate to income before income tax expense. The following reconciles such differences:
 
 
Nine Months Ended Sept. 30
 
 
2018
 
2017
Federal statutory rate
 
21.0
 %
 
35.0
 %
State tax (net of federal tax effect)
 
7.1

 
5.8

Increases (decreases) in tax from:
 

 

Wind production tax credits (PTCs) (a)
 
(12.1
)
 
(11.3
)
Regulatory differences - ARAM (b)
 
(9.5
)
 
(0.1
)
Regulatory differences - ARAM deferral (c)
 
7.7

 
—

Regulatory differences - reversal of prior quarters’ ARAM deferral (c)
 
(7.1
)
 
—

Regulatory differences - other utility plant items
 
0.2

 
(0.3
)
Other tax credits (net of federal income tax expense)
 
(1.4
)
 
(1.1
)
Other (net)
 
1.2

 
(2.5
)
Effective income tax rate
 
7.1
 %
 
25.5
 %

(a)  
Quarterly PTCs may vary due to production and timing differences. Annual 2018 PTCs are forecasted to exceed 2017.
(b)
The average rate assumption method (ARAM); a method to flow back excess deferred taxes to customers.
(c) 
ARAM has been deferred when regulatory treatment has not been established. As NSP-Minnesota received direction from its regulatory commissions regarding the return of excess deferred taxes to customers, the ARAM deferral was reversed. This resulted in a reduction to tax expense with a corresponding reduction to revenue.

Federal Audits — NSP-Minnesota is a member of the Xcel Energy affiliated group that files a consolidated federal income tax return. The statute of limitations applicable to Xcel Energy’s federal income tax returns expire as follows:

Tax Year(s)
 
Expiration
2009 - 2014
 
October 2019
2015
 
September 2019
2016
 
September 2020


In 2012, the Internal Revenue Service (IRS) commenced an examination of tax years 2010 and 2011, including the 2009 carryback claim. In 2017 Xcel Energy and the Office of Appeals (Appeals) reached an agreement and the benefit related to the agreed upon portions was recognized. In the second quarter of 2018, the Joint Committee on Taxation completed its review and took no exception to the agreement. As a result, the remaining unrecognized tax benefit was released and recorded as a payable to the IRS.

In the third quarter of 2015, the IRS commenced an examination of tax years 2012 and 2013. In the third quarter of 2017, the IRS concluded the audit of tax years 2012 and 2013 and proposed an adjustment that would impact Xcel Energy’s net operating loss (NOL) and effective tax rate (ETR). Xcel Energy filed a protest with the IRS. As of Sept. 30, 2018 the case has been forwarded to Appeals and Xcel Energy has recognized its best estimate of income tax expense that will result from a final resolution of this issue; however, the outcome and timing of a resolution is unknown.

State Audits — NSP-Minnesota is a member of the Xcel Energy affiliated group that files consolidated state income tax returns. As of Sept. 30, 2018, NSP-Minnesota’s earliest open tax year that is subject to examination by state taxing authorities under applicable statutes of limitations is 2009. In 2016, the state of Minnesota began an audit of years 2010 through 2014. As of Sept. 30, 2018, Minnesota had not proposed any material adjustments.

Unrecognized Benefits — The unrecognized tax benefit balance includes permanent tax positions, which if recognized would affect the annual ETR. In addition, the unrecognized tax benefit balance includes temporary tax positions for which the ultimate deductibility is highly certain but for which there is uncertainty about the timing of such deductibility. A change in the period of deductibility would not affect the ETR but would accelerate the payment of cash to the taxing authority to an earlier period.

A reconciliation of the amount of unrecognized tax benefit is as follows:
(Millions of Dollars)
 
Sept. 30, 2018
 
Dec. 31, 2017
Unrecognized tax benefit — Permanent tax positions
 
$
11.4

 
$
10.2

Unrecognized tax benefit — Temporary tax positions
 
5.4

 
7.9

Total unrecognized tax benefit
 
$
16.8

 
$
18.1



The unrecognized tax benefit amounts were reduced by the tax benefits associated with NOL and tax credit carryforwards. The amounts of tax benefits associated with NOL and tax credit carryforwards are as follows:
(Millions of Dollars)
 
Sept. 30, 2018
 
Dec. 31, 2017
NOL and tax credit carryforwards
 
$
(14.0
)
 
$
(12.8
)


It is reasonably possible that NSP-Minnesota’s amount of unrecognized tax benefits could significantly change in the next 12 months as the IRS Appeals progresses and audit resumes, the Minnesota audit progresses, and other state audits resume. As the IRS Appeals and Minnesota audit progress and the IRS audit resumes, it is reasonably possible that the amount of unrecognized tax benefit could decrease up to approximately $13 million.

The payable for interest related to unrecognized tax benefits is partially offset by the interest benefit associated with NOL and tax credit carryforwards. A reconciliation of the beginning and ending amount of the payable for interest related to unrecognized tax benefits are as follows:
(Millions of Dollars)
 
Sept. 30, 2018
 
Dec. 31, 2017
Payable for interest related to unrecognized tax benefits at beginning of period
 
$
(0.9
)
 
$
(2.0
)
Interest (expense) income related to unrecognized tax benefits recorded during the period
 
(1.1
)
 
1.1

Payable for interest related to unrecognized tax benefits at end of period
 
$
(2.0
)
 
$
(0.9
)


No amounts were accrued for penalties related to unrecognized tax benefits as of Sept. 30, 2018 or Dec. 31, 2017.
v3.10.0.1
Rate Matters
 9 Months Ended
Sep. 30, 2018
Public Utilities, General Disclosures [Abstract]  
Rate Matters
Rate Matters

Except to the extent noted below, the circumstances set forth in Note 10 to the consolidated financial statements included in NSP-Minnesota’s Annual Report on Form 10-K for the year ended Dec. 31, 2017 and in Note 5 to the consolidated financial statements to NSP-Minnesota’s Quarterly Reports on Form 10-Q for the quarterly periods ended March 31, 2018 and June 30, 2018, appropriately represent, in all material respects, the current status of other rate matters, and are incorporated herein by reference.

Tax Reform — Regulatory Proceedings

The specific impacts of the TCJA on customer rates are subject to regulatory approval. The following details the status of regulatory decisions in each state where NSP-Minnesota operates.

Minnesota — In August 2018, the Minnesota Public Utilities Commission (MPUC) ordered NSP-Minnesota to refund the 2018 impacts of TCJA, including $5 million to natural gas customers and $131 million to electric customers, including low income program funding of $2 million.

South Dakota — In July 2018, the South Dakota Public Utilities Commission approved a settlement providing a one-time customer refund of $11 million for the 2018 impact of the TCJA, while NSP-Minnesota would retain the benefits of the TCJA in 2019 and 2020 in exchange for a two-year rate case moratorium.

North Dakota — Natural Gas — In August 2018, NSP-Minnesota and the North Dakota Public Service Commission (NDPSC) Staff reached a TCJA settlement, in which NSP-Minnesota would amortize $1 million annually of the regulatory asset for the remediation of the manufactured gas plant (MGP) site in Fargo, N.D. beginning in 2018, and retain the TCJA savings to approximately offset the MGP amortization expense. The TCJA benefits would be incorporated into a future rate case and the MGP amortization would then be recoverable through the cost of gas rider until fully amortized. A NDPSC decision related to the settlement is expected to be received by the end of 2018. See Note 6 for further discussion of the Fargo, N.D. MGP Site.

North Dakota — Electric — In October 2018, NSP-Minnesota and the NDPSC Staff reached a settlement which included a one-time customer refund of $10 million for 2018, while NSP-Minnesota would retain the benefits of the TCJA in 2019 and 2020 in exchange for a two-year rate case moratorium. The settlement also includes an earnings sharing provision in which annual weather normalized earnings exceeding an ROE of 9.85 percent are returned to customers.
A NDPSC decision related to the settlement is expected to be received by the end of 2018 or during the first quarter of 2019.

Recently Concluded Regulatory Proceedings — MPUC and the NDPSC

PPA Terminations and Amendments — In June 2018, NSP-Minnesota terminated the Benson and Laurentian PPAs, and purchased the Benson biomass facility. As a result, a $103 million regulatory asset was recognized for the costs of the Benson transaction, including payments to Benson of $93 million, as well as other transaction costs and future estimated facility removal costs. For Laurentian, a regulatory asset of $109 million was recognized for annual termination payments over six years. The regulatory approvals provide for recovery of the Benson regulatory asset over approximately 10 years, and for recovery of the Laurentian termination payments as they occur, through fuel and purchased energy recovery mechanisms. The termination of the PPAs is expected to save customers over $600 million over the next 10 years.

Pending Regulatory Proceeding — Federal Energy Regulatory Commission (FERC)

Midcontinent Independent System Operator, Inc. (MISO) Return on Equity (ROE) Complaints — In November 2013, a group of customers filed a complaint at the FERC against MISO transmission owners (TOs), including NSP-Minnesota and NSP-Wisconsin. The complaint argued for a reduction in the ROE in transmission formula rates in the MISO region from 12.38 percent to 9.15 percent, and the removal of ROE adders (including those for Regional Transmission Organization (RTO) membership), effective Nov. 12, 2013.

In September 2016, the FERC approved an Administrative Law Judge (ALJ) recommendation that MISO TOs be granted a 10.32 percent base ROE using the methodology adopted by FERC in June 2014 (Opinion 531). This ROE would be applicable for the 15-month refund period from Nov. 12, 2013 to Feb. 11, 2015, and prospectively from the date of the FERC order. The total prospective ROE would be 10.82 percent, including a 50 basis point adder for RTO membership. The requests are pending FERC action.

In February 2015, a second complaint seeking to reduce the MISO ROE from 12.38 percent to 8.67 percent prior to any RTO adder was filed, resulting in a second period of potential refunds from Feb. 12, 2015 to May 11, 2016. In June 2016, an ALJ recommended a base ROE of 9.7 percent, applying the FERC Opinion 531 methodology. FERC action is pending. In April 2017, the United States Court of Appeals for the District of Columbia Circuit (D.C. Circuit) vacated and remanded Opinion 531. It is unclear how the D.C. Circuit’s opinion to vacate and remand Opinion 531 will affect the September 2016 FERC order or the timing and outcome of the second ROE complaint.

NSP-Minnesota has recognized a current refund liability consistent with the best estimate of the final ROE.
v3.10.0.1
Commitments and Contingencies
 9 Months Ended
Sep. 30, 2018
Commitments and Contingencies Disclosure [Abstract]  
Commitments and Contingencies
6.
Commitments and Contingencies

Except to the extent noted below and in Note 5 to the consolidated financial statements, the circumstances set forth in Notes 10, 11 and 12 to the consolidated financial statements included in the NSP-Minnesota Annual Report on Form 10-K for the year ended Dec. 31, 2017 and in Notes 5 and 6 to NSP-Minnesota’s Quarterly Reports on Form 10-Q for the quarterly periods ended March 31, 2018 and June 30, 2018, appropriately represent, in all material respects, the current status of commitments and contingent liabilities and are incorporated herein by reference. The following include commitments, contingencies and unresolved contingencies that are material to NSP-Minnesota’s financial position.

PPAs

NSP-Minnesota purchases power from independent power producing entities for which NSP-Minnesota is required to reimburse natural gas or biomass fuel costs, or to participate in tolling arrangements under which NSP-Minnesota procures the natural gas required to produce the energy that it purchases. These specific PPAs create a variable interest in the associated independent power producing entity.

NSP-Minnesota had approximately 1,002 Megawatts (MW) of capacity under long-term PPAs as of Sept. 30, 2018 and 1,069 MW as of Dec. 31, 2017, with entities that have been determined to be variable interest entities. NSP-Minnesota has concluded that these entities are not required to be consolidated in its consolidated financial statements because it does not have the power to direct the activities that most significantly impact the entities’ economic performance. These agreements have various expiration dates through 2027.

Guarantees

Under NSP-Minnesota’s railcar lease agreement, accounted for as an operating lease, NSP-Minnesota guarantees the lessor proceeds from sale of the leased assets at the end of the lease term will at least equal the guaranteed residual value. The guarantee issued by NSP-Minnesota has a stated maximum amount; however, NSP-Minnesota expects sale proceeds to exceed the guaranteed amount. This agreement expires in 2019.

The following table presents the guarantee issued and outstanding for NSP-Minnesota:
(Millions of Dollars)
 
Sept. 30, 2018
 
Dec. 31, 2017
Guarantee issued and outstanding
 
$
4.8

 
$
4.8
Environmental Loss Contingency Disclosure [Text Block]
Environmental Contingencies

Fargo, N.D. Manufactured Gas Plant (MGP) Site — NSP-Minnesota is remediating a former MGP site in Fargo, N.D. Remediation is expected to be completed in early November 2018, and several years of groundwater monitoring is expected to follow. NSP-Minnesota has also initiated insurance recovery litigation in North Dakota. The U.S. District Court for the District of North Dakota has set a trial date for Spring of 2020.

NSP-Minnesota recorded an estimated liability of $6 million as of Sept. 30, 2018 and $16 million as of Dec. 31, 2017, for the Fargo MGP Site. The current cost estimate for the remediation of the site is approximately $25 million, of which approximately $19 million has been spent. NSP-Minnesota has deferred Fargo MGP Site costs allocable to the North Dakota jurisdiction, or approximately 88 percent of all remediation costs, as approved by the NDPSC. In October 2018, the MPUC denied NSP-Minnesota’s request to defer post-2017 MGP remediation expenditures allocable to the Minnesota jurisdiction, including the Fargo MGP Site.

Other MGP, Landfill or Disposal Sites — NSP-Minnesota is currently involved in investigating and/or remediating several MGP, landfill or other disposal sites. NSP-Minnesota has identified five sites, in addition to the Fargo MGP Site, where investigation and/or remediation activities are currently underway. Other parties may have responsibility for some portion of the investigation and/or remediation activities. NSP-Minnesota anticipates that these investigation or remediation activities will continue through at least 2019. NSP-Minnesota accrued $3 million as of Sept. 30, 2018 and Dec. 31, 2017 for all of these sites. There may be insurance recovery and/or recovery from other responsible parties that will offset any costs incurred.

Environmental Requirements

Water and Waste
Coal Ash Regulation — NSP-Minnesota’s operations are subject to federal and state laws that impose requirements for handling, storage, treatment and disposal of solid waste. In 2015, the United States Environmental Protection Agency published a final rule regulating the management, storage, and disposal of coal combustion residuals (CCRs) as a nonhazardous waste (CCR Rule).

Under the CCR Rule, utilities are required to complete certain groundwater sampling around their CCR landfills and surface impoundments. NSP-Minnesota is currently conducting additional groundwater sampling and will evaluate whether corrective action is required at any CCR landfills or surface impoundments. Until NSP-Minnesota completes additional groundwater sampling, it is uncertain what impact, if any, there will be on the operations, financial position or cash flows. NSP-Minnesota believes that any associated costs would be recoverable through regulatory mechanisms.
Legal Matters and Contingencies
Legal Contingencies

NSP-Minnesota is involved in various litigation matters that are being defended and handled in the ordinary course of business. The assessment of whether a loss is probable or is a reasonable possibility, and whether the loss or a range of loss is estimable, often involves a series of complex judgments about future events. Management maintains accruals for such losses that are probable of being incurred and subject to reasonable estimation. Management is sometimes unable to estimate an amount or range of a reasonably possible loss in certain situations, including but not limited to when (1) the damages sought are indeterminate, (2) the proceedings are in the early stages, or (3) the matters involve novel or unsettled legal theories. In such cases, there is considerable uncertainty regarding the timing or ultimate resolution of such matters, including a possible eventual loss. For current proceedings not specifically reported herein, management does not anticipate that the ultimate liabilities, if any, arising from such current proceedings would have a material effect on NSP-Minnesota’s financial statements. Unless otherwise required by GAAP, legal fees are expensed as incurred.
v3.10.0.1
Borrowings and Other Financing Instruments
 9 Months Ended
Sep. 30, 2018
Debt Disclosure [Abstract]  
Borrowings and Other Financing Instruments
Borrowings and Other Financing Instruments

Short-Term Borrowings

Money Pool — Xcel Energy Inc. and its utility subsidiaries have established a money pool arrangement that allows for short-term investments in and borrowings between the utility subsidiaries. Xcel Energy Inc. may make investments in the utility subsidiaries at market-based interest rates; however, the money pool arrangement does not allow the utility subsidiaries to make investments in Xcel Energy Inc. Money pool borrowings for NSP-Minnesota were as follows:
(Amounts in Millions, Except Interest Rates)
 
Three Months Ended Sept. 30, 2018
 
Year Ended Dec. 31, 2017
Borrowing limit
 
$
250

 
$
250

Amount outstanding at period end
 
—

 
85

Average amount outstanding
 
34

 
25

Maximum amount outstanding
 
143

 
142

Weighted average interest rate, computed on a daily basis
 
1.97
%
 
1.14
%
Weighted average interest rate at period end
 
N/A

 
1.18



Commercial Paper — NSP-Minnesota meets its short-term liquidity requirements primarily through the issuance of commercial paper and borrowings under its credit facility and the money pool. Commercial paper outstanding for NSP-Minnesota was as follows:
(Amounts in Millions, Except Interest Rates)
 
Three Months Ended Sept. 30, 2018
 
Year Ended Dec. 31, 2017
Borrowing limit
 
$
500

 
$
500

Amount outstanding at period end
 
24

 
20

Average amount outstanding
 
6

 
62

Maximum amount outstanding
 
73

 
237

Weighted average interest rate, computed on a daily basis
 
2.12
%
 
1.10
%
Weighted average interest rate at period end
 
2.30

 
1.93



Letters of Credit — NSP-Minnesota uses letters of credit, generally with terms of one year, to provide financial guarantees for certain operating obligations. At Sept. 30, 2018 and Dec. 31, 2017, there were $37 million and $24 million, respectively, of letters of credit outstanding under the credit facility. The contract amounts of these letters of credit approximate their fair value and are subject to fees.

Credit Facility — In order to use its commercial paper program to fulfill short-term funding needs, NSP-Minnesota must have a revolving credit facility in place at least equal to the amount of its commercial paper borrowing limit and cannot issue commercial paper in an amount exceeding available capacity under this credit facility. The line of credit provides short-term financing in the form of notes payable to banks, letters of credit and back-up support for commercial paper borrowings.

At Sept. 30, 2018, NSP-Minnesota had the following committed credit facility available (in millions of dollars):
Credit Facility (a)
 
Drawn (b)
 
Available
$
500

 
$
61

 
$
439

(a) 
This credit facility expires in June 2021.
(b) 
Includes outstanding letters of credit.

All credit facility bank borrowings, outstanding letters of credit and outstanding commercial paper reduce the available capacity under the credit facility. NSP-Minnesota had no direct advances on the credit facility outstanding at Sept. 30, 2018 and Dec. 31, 2017.
v3.10.0.1
Fair Value of Financial Assets and Liabilities
 9 Months Ended
Sep. 30, 2018
Fair Value Disclosures [Abstract]  
Fair Value of Financial Assets and Liabilities
Fair Value of Financial Assets and Liabilities

Fair Value Measurements

The accounting guidance for fair value measurements and disclosures provides a single definition of fair value and requires certain disclosures about assets and liabilities measured at fair value. A hierarchical framework for disclosing the observability of the inputs utilized in measuring assets and liabilities at fair value is established by this guidance. The three levels in the hierarchy are as follows:

Level 1 — Quoted prices are available in active markets for identical assets or liabilities as of the reporting date. The types of assets and liabilities included in Level 1 are highly liquid and actively traded instruments with quoted prices.

Level 2 — Pricing inputs are other than quoted prices in active markets, but are either directly or indirectly observable as of the reporting date. The types of assets and liabilities included in Level 2 are typically either comparable to actively traded securities or contracts, or priced with models using highly observable inputs.

Level 3 — Significant inputs to pricing have little or no observability as of the reporting date. The types of assets and liabilities included in Level 3 are those valued with models requiring significant management judgment or estimation.

Specific valuation methods include the following:

Cash equivalents — The fair values of cash equivalents are generally based on cost plus accrued interest; money market funds are measured using quoted net asset value (NAV).

Investments in equity securities and other funds — Equity securities are valued using quoted prices in active markets. The fair values for commingled funds are measured using NAVs, which take into consideration the value of underlying fund investments, as well as the other accrued assets and liabilities of a fund, in order to determine a per-share market value. The investments in commingled funds may be redeemed for NAV with proper notice. Proper notice varies by fund and can range from daily with one or two days notice to annually with 90 days notice. Private equity investments require approval of the fund for any unscheduled redemption, and such redemptions may be approved or denied by the fund at its sole discretion. Unscheduled distributions from real estate investments may be redeemed with proper notice, which is typically quarterly with 45-90 days notice; however, withdrawals from real estate investments may be delayed or discounted as a result of fund illiquidity.

Investments in debt securities — Fair values for debt securities are determined by a third party pricing service using recent trades and observable spreads from benchmark interest rates for similar securities.

Interest rate derivatives — The fair values of interest rate derivatives are based on broker quotes that utilize current market interest rate forecasts.

Commodity derivatives — The methods used to measure the fair value of commodity derivative forwards and options utilize forward prices and volatilities, as well as pricing adjustments for specific delivery locations, and are generally assigned a Level 2 classification. When contractual settlements extend to periods beyond those readily observable on active exchanges or quoted by brokers, the significance of the use of less observable forecasts of long-term forward prices and volatilities on a valuation is evaluated, and may result in Level 3 classification.

Electric commodity derivatives held by NSP-Minnesota include transmission congestion instruments, generally referred to as financial transmission rights (FTRs), purchased from MISO. FTRs purchased from a RTO are financial instruments that entitle or obligate the holder to monthly revenues or charges based on transmission congestion across a given transmission path. The value of an FTR is derived from, and designed to offset, the cost of transmission congestion. In addition to overall transmission load, congestion is also influenced by the operating schedules of power plants and the consumption of electricity pertinent to a given transmission path. Unplanned plant outages, scheduled plant maintenance, changes in the relative costs of fuels used in generation, weather and overall changes in demand for electricity can each impact the operating schedules of the power plants on the transmission grid and the value of an FTR. NSP-Minnesota’s valuation process for FTRs utilizes the cleared prices for each FTR for the most recent auction.

If forecasted costs of electric transmission congestion increase or decrease for a given FTR path, the value of that particular FTR instrument will likewise increase or decrease. Given the limited transparency in the auction process, fair value measurements for FTRs have been assigned a Level 3. Non-trading monthly FTR settlements are included in fuel and purchased energy cost recovery mechanisms, and therefore changes in the fair value of the yet to be settled portions of most FTRs are deferred as a regulatory asset or liability. Given this regulatory treatment and the limited magnitude of NSP-Minnesota’s FTRs, the limited transparency associated with the valuation of FTRs are insignificant to the consolidated financial statements of NSP-Minnesota.

Non-Derivative Instruments Fair Value Measurements

Nuclear Decommissioning Fund

The Nuclear Regulatory Commission requires NSP-Minnesota to maintain a portfolio of investments to fund the costs of decommissioning its nuclear generating plants. Together with all accumulated earnings or losses, the assets of the nuclear decommissioning fund are legally restricted for the decommissioning the Monticello and Prairie Island (PI) nuclear generating plants. The fund contains cash equivalents, debt securities, equity securities and other investments. NSP-Minnesota plans to reinvest matured securities until decommissioning begins. NSP-Minnesota uses the asset class target allocations approved by the MPUC for the qualified trust.

NSP-Minnesota recognizes the costs of funding the decommissioning of its nuclear generating plants over the lives of the plants, assuming rate recovery of all costs. Given the purpose and legal restrictions on the use of nuclear decommissioning fund assets, realized and unrealized gains on fund investments over the life of the fund are deferred as an offset of NSP-Minnesota’s regulatory asset for nuclear decommissioning costs. Consequently, any realized and unrealized gains and losses on securities in the nuclear decommissioning fund, including any impairments, are deferred as a component of the regulatory asset for nuclear decommissioning.

Unrealized gains for the nuclear decommissioning fund were $600 million and $560 million as of Sept. 30, 2018 and Dec. 31, 2017, respectively, and unrealized losses and amounts recorded as other-than-temporary impairments were $22 million and $7 million as of Sept. 30, 2018 and Dec. 31, 2017, respectively.

The following tables present the cost and fair value of NSP-Minnesota’s non-derivative instruments with recurring fair value measurements in the nuclear decommissioning fund as of Sept. 30, 2018 and Dec. 31, 2017:
 
 
Sept. 30, 2018
 
 
 
 
Fair Value
(Thousands of Dollars)
 
Cost
 
Level 1
 
Level 2
 
Level 3
 
Investments Measured at NAV (b)
 
Total
Nuclear decommissioning fund (a)
 
 
 
 
 
 
 
 
 
 
 
 
Cash equivalents
 
$
33,040

 
$
33,040

 
$
—

 
$
—

 
$
—

 
$
33,040

Commingled funds:
 
 
 
 
 
 
 
 
 
 
 
 
Non U.S. equities
 
262,345

 
195,959

 
—

 
—

 
91,426

 
287,385

Emerging market debt funds
 
162,835

 
—

 
—

 
—

 
164,772

 
164,772

Private equity investments
 
169,676

 
—

 
—

 
—

 
249,881

 
249,881

Real estate
 
125,422

 
—

 
—

 
—

 
198,422

 
198,422

Debt securities:
 
 
 
 
 
 
 
 
 
 
 
 
Government securities
 
75,799

 
—

 
73,317

 
—

 
—

 
73,317

U.S. corporate bonds
 
334,219

 
—

 
329,727

 
—

 
—

 
329,727

Non U.S. corporate bonds
 
55,856

 
—

 
54,813

 
—

 
—

 
54,813

Equity securities:
 
 
 
 
 
 
 
 
 
 
 
 
U.S. equities
 
257,932

 
590,310

 
—

 
—

 
—

 
590,310

Non U.S. equities
 
156,225

 
229,245

 
—

 
—

 
—

 
229,245

Total
 
$
1,633,349

 
$
1,048,554

 
$
457,857

 
$
—

 
$
704,501

 
$
2,210,912


(a) 
Reported in nuclear decommissioning fund and other investments on the consolidated balance sheet, which also includes $52 million of rabbi trust assets and miscellaneous investments.
(b) 
Due to limited availability of published pricing and a lack of immediate redeemability, certain fund investments measured at NAV are not required to be categorized within the fair value hierarchy.
 
 
Dec. 31, 2017
 
 
 
 
Fair Value
(Thousands of Dollars)
 
Cost
 
Level 1
 
Level 2
 
Level 3
 
Investments Measured at NAV (b)
 
Total
Nuclear decommissioning fund (a)
 
 
 
 
 
 
 
 
 
 
 
 
Cash equivalents
 
$
28,741

 
$
28,741

 
$
—

 
$
—

 
$
—

 
$
28,741

Commingled funds:
 
 
 
 
 
 
 
 
 
 
 
 
Non U.S. equities
 
263,694

 
216,551

 
—

 
—

 
89,857

 
306,408

Emerging market debt funds
 
156,057

 
—

 
—

 
—

 
166,375

 
166,375

Private equity investments
 
141,413

 
—

 
—

 
—

 
198,037

 
198,037

Real estate
 
130,787

 
—

 
—

 
—

 
201,842

 
201,842

Other commingled funds
 
9,340

 
6,286

 
—

 
—

 
2,975

 
9,261

Debt securities:
 
 
 
 
 
 
 
 
 
 
 
 
Government securities
 
67,760

 
—

 
69,413

 
—

 
—

 
69,413

U.S. corporate bonds
 
319,809

 
—

 
322,129

 
—

 
—

 
322,129

Non U.S. corporate bonds
 
50,121

 
—

 
50,102

 
—

 
—

 
50,102

Equity securities:
 
 
 
 
 
 
 
 
 
 
 
 
U.S. equities
 
271,166

 
556,974

 
—

 
—

 
—

 
556,974

Non U.S. equities
 
151,961

 
233,999

 
—

 
—

 
—

 
233,999

Total
 
$
1,590,849

 
$
1,042,551

 
$
441,644

 
$
—

 
$
659,086

 
$
2,143,281


(a) 
Reported in nuclear decommissioning fund and other investments on the consolidated balance sheet, which also includes $49 million of rabbi trust assets and miscellaneous investments.
(b) 
Due to limited availability of published pricing and a lack of immediate redeemability, certain fund investments measured at NAV are not required to be categorized within the fair value hierarchy.

For the three and nine months ended Sept. 30, 2018 and 2017 there were no Level 3 nuclear decommissioning fund investments and no transfers of amounts between levels.

The following table summarizes the final contractual maturity dates of the debt securities in the nuclear decommissioning fund, by asset class, as of Sept. 30, 2018:
 
 
Final Contractual Maturity
(Thousands of Dollars)
 
Due in 1 Year
or Less
 
Due in 1 to 5
Years
 
Due in 5 to 10
Years
 
Due after 10
Years
 
Total
Government securities
 
$
—

 
$
—

 
$
1,750

 
$
71,567

 
$
73,317

U.S. corporate bonds
 
12,759

 
90,982

 
175,662

 
50,324

 
329,727

Non U.S. corporate bonds
 
1,998

 
20,498

 
28,151

 
4,166

 
54,813

Debt securities
 
$
14,757

 
$
111,480

 
$
205,563

 
$
126,057

 
$
457,857


Rabbi Trusts

In 2016, NSP-Minnesota established a rabbi trust to provide partial funding for future deferred compensation plan distributions. The following tables present the cost and fair value of the assets held in rabbi trust at Sept. 30, 2018 and Dec. 31, 2017:
 
 
Sept. 30, 2018
 
 
 
 
Fair Value
(Thousands of Dollars)
 
Cost
 
Level 1
 
Level 2
 
Level 3
 
Total
Rabbi Trust (a)
 
 
 
 
 
 
 
 
 
 
Cash equivalents
 
$
2,321

 
$
2,321

 
$
—

 
$
—

 
$
2,321

Mutual funds
 
10,474

 
11,748

 
—

 
—

 
11,748

Total
 
$
12,795

 
$
14,069

 
$
—

 
$
—

 
$
14,069


 
 
Dec. 31, 2017
 
 
 
 
Fair Value
(Thousands of Dollars)
 
Cost
 
Level 1
 
Level 2
 
Level 3
 
Total
Rabbi Trusts (a)
 
 
 
 
 
 
 
 
 
 
Cash equivalents
 
$
783

 
$
783

 
$
—

 
$
—

 
$
783

Mutual funds
 
10,332

 
11,283

 
—

 
—

 
11,283

Total
 
$
11,115

 
$
12,066

 
$
—

 
$
—

 
$
12,066

(a) 
Reported in nuclear decommissioning fund and other investments on the consolidated balance sheet.

Derivative Instruments Fair Value Measurements

NSP-Minnesota enters into derivative instruments, including forward contracts, futures, swaps and options, for trading purposes and to manage risk in connection with changes in interest rates, utility commodity prices and vehicle fuel prices.

Interest Rate Derivatives — NSP-Minnesota enters into various instruments that effectively fix the interest payments on certain floating rate debt obligations or effectively fix the yield or price on a specified benchmark interest rate for an anticipated debt issuance for a specific period. These derivative instruments are generally designated as cash flow hedges for accounting purposes.

At Sept. 30, 2018, accumulated other comprehensive losses related to interest rate derivatives included $0.8 million of net losses expected to be reclassified into earnings during the next 12 months as the related hedged interest rate transactions impact earnings, including forecasted amounts for unsettled hedges, as applicable.

Wholesale and Commodity Trading Risk — NSP-Minnesota conducts various wholesale and commodity trading activities, including the purchase and sale of electric capacity, energy, energy-related instruments and natural gas related instruments, including derivatives. NSP-Minnesota’s risk management policy allows management to conduct these activities within guidelines and limitations as approved by its risk management committee, which is made up of management personnel not directly involved in the activities governed by this policy.

Commodity Derivatives — NSP-Minnesota enters into derivative instruments to manage variability of future cash flows from changes in commodity prices in its electric and natural gas operations, as well as for trading purposes. This could include the purchase or sale of energy or energy-related products, natural gas to generate electric energy, natural gas for resale, FTRs, vehicle fuel, and weather derivatives.

At Sept. 30, 2018, NSP-Minnesota had various vehicle fuel contracts designated as cash flow hedges extending through December 2018. NSP-Minnesota enters into derivative instruments that mitigate commodity price risk on behalf of electric and natural gas customers, but may not be designated as qualifying hedging transactions. Changes in the fair value of non-trading commodity derivative instruments are recorded in other comprehensive income or deferred as a regulatory asset or liability. The classification as a regulatory asset or liability is based on commission approved regulatory recovery mechanisms. NSP-Minnesota recorded immaterial amounts to income related to the ineffectiveness of cash flow hedges for the three and nine months ended Sept. 30, 2018 and 2017.

At Sept. 30, 2018, net gains related to commodity derivative cash flow hedges recorded as a component of accumulated other comprehensive losses included immaterial net gains expected to be reclassified into earnings during the next 12 months as the hedged transactions occur.

Additionally, NSP-Minnesota enters into commodity derivative instruments for trading purposes not directly related to commodity price risks associated with serving its electric and natural gas customers. Changes in the fair value of these commodity derivatives are recorded in electric operating revenues, net of amounts credited to customers under margin-sharing mechanisms.

The following table details the gross notional amounts of commodity forwards, options and FTRs at Sept. 30, 2018 and Dec. 31, 2017:
(Amounts in Thousands) (a)(b)
 
Sept. 30, 2018
 
Dec. 31, 2017
Megawatt hours of electricity
 
61,746

 
41,711

Million British thermal units of natural gas
 
11,131

 
23,829

Gallons of vehicle fuel
 
60

 
240


(a) 
Amounts are not reflective of net positions in the underlying commodities.
(b) 
Notional amounts for options are included on a gross basis, but are weighted for the probability of exercise.

The following tables detail the impact of derivative activity during the three and nine months ended Sept. 30, 2018 and 2017 on accumulated other comprehensive loss, regulatory assets and liabilities and income:
 
 
Three Months Ended Sept. 30, 2018
 
 
 
Pre-Tax Fair Value
Gains (Losses) Recognized
During the Period in:
 
Pre-Tax (Gains) Losses
Reclassified into Income
During the Period from:
 
Pre-Tax Gains Recognized
During the Period in Income
 
(Thousands of Dollars)
 
Accumulated
Other
Comprehensive Loss
 
Regulatory
(Assets) and Liabilities
 
Accumulated
Other
Comprehensive Loss
 
Regulatory
Assets and (Liabilities)
 
 
Derivatives designated as cash flow hedges
 
 
 
 
 
 
 
 
 
 
 
Interest rate
 
$
—

 
$
—

 
$
267

(a) 
$
—

 
$
—

 
Vehicle fuel and other commodity
 
7

 
—

 
(39
)
(b) 
—

 
—

 
Total
 
$
7

 
$
—

 
$
228

 
$
—

 
$
—

 
Other derivative instruments
 
 
 
 
 
 
 
 
 
 
 
Commodity trading
 
$
—

 
$
—

 
$
—

 
$
—

 
$
3,447

(c) 
Electric commodity
 
—

 
1,127

 
—

 
447

(d) 
—

 
Natural gas commodity
 
—

 
(370
)
 
—

 
—

(e) 
—

(e) 
Total
 
$
—

 
$
757

 
$
—

 
$
447

 
$
3,447

 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Nine Months Ended Sept. 30, 2018
 
 
 
Pre-Tax Fair Value Gains (Losses) Recognized
During the Period in:
 
Pre-Tax (Gains) Losses
Reclassified into Income
During the Period from:
 
Pre-Tax Gains (Losses)
Recognized
During the Period in Income
 
(Thousands of Dollars)
 
Accumulated
Other
Comprehensive Loss
 
Regulatory
(Assets) and Liabilities
 
Accumulated
Other
Comprehensive Loss
 
Regulatory
Assets and (Liabilities)
 
 
Derivatives designated as cash flow hedges
 
 
 
 
 
 
 
 
 
 
 
Interest rate
 
$
—

 
$
—

 
$
792

(a) 
$
—

 
$
—

 
Vehicle fuel and other commodity
 
45

 
—

 
(103
)
(b) 
—

 
—

 
Total
 
$
45

 
$
—

 
$
689

 
$
—

 
$
—

 
Other derivative instruments
 
 
 
 
 
 
 
 
 
 
 
Commodity trading
 
$
—

 
$
—

 
$
—

 
$
—

 
$
11,114

(c) 
Electric commodity
 
—

 
(4,487
)
 
—

 
3,758

(d) 
—

 
Natural gas commodity
 
—

 
460

 
—

 
(521
)
(e) 
(404
)
(e) 
Total
 
$
—

 
$
(4,027
)
 
$
—

 
$
3,237

 
$
10,710

 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Three Months Ended Sept. 30, 2017
 
 
 
Pre-Tax Fair Value
Gains (Losses) Recognized
During the Period in:
 
Pre-Tax (Gains) Losses
Reclassified into Income
During the Period from:
 
Pre-Tax Gains
Recognized
During the Period in Income
 
(Thousands of Dollars)
 
Accumulated
Other
Comprehensive Loss
 
Regulatory
(Assets) and Liabilities
 
Accumulated
Other
Comprehensive Loss
 
Regulatory
Assets and (Liabilities)
 
 
Derivatives designated as cash flow hedges
 
 
 
 
 
 
 
 
 
 
 
Interest rate
 
$
—

 
$
—

 
$
612

(a) 
$
—

 
$
—

 
Vehicle fuel and other commodity
 
38

 
—

 
(11
)
(b) 
—

 
—

 
Total
 
$
38

 
$
—

 
$
601

 
$
—

 
$
—

 
Other derivative instruments
 
 
 
 
 
 
 
 
 
 
 
Commodity trading
 
$
—

 
$
—

 
$
—

 
$
—

 
$
1,493

(c) 
Electric commodity
 
—

 
20,216

 
—

 
(5,356
)
(d) 
—

 
Natural gas commodity
 
—

 
(383
)
 
—

 
—

(e) 
—

(e) 
Total
 
$
—

 
$
19,833

 
$
—

 
$
(5,356
)
 
$
1,493

 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Nine Months Ended Sept. 30, 2017
 
 
 
Pre-Tax Fair Value
Gains (Losses) Recognized
During the Period in:
 
Pre-Tax (Gains) Losses
Reclassified into Income
During the Period from:
 
Pre-Tax Gains (Losses)
Recognized
During the Period in Income
 
(Thousands of Dollars)
 
Accumulated
Other
Comprehensive Loss
 
Regulatory
(Assets) and Liabilities
 
Accumulated
Other
Comprehensive Loss
 
Regulatory
Assets and (Liabilities)
 
 
Derivatives designated as cash flow hedges
 
 
 
 
 
 
 
 
 
 
 
Interest rate
 
$
—

 
$
—

 
$
1,304

(a) 
$
—

 
$
—

 
Vehicle fuel and other commodity
 
81

 
—

 
(16
)
(b) 
—

 
—

 
Total
 
$
81

 
$
—

 
$
1,288

 
$
—

 
$
—

 
Other derivative instruments
 
 
 
 
 
 
 
 
 
 
 
Commodity trading
 
$
—

 
$
—

 
$
—

 
$
—

 
$
8,092

(c) 
Electric commodity
 
—

 
17,444

 
—

 
(9,293
)
(d) 
—

 
Natural gas commodity
 
—

 
(1,010
)
 
—

 
698

(e) 
(945
)
(e) 
Total
 
$
—

 
$
16,434

 
$
—

 
$
(8,595
)
 
$
7,147

 

(a) 
Amounts are recorded to interest charges.
(b) 
Amounts are recorded to operating and maintenance (O&M) expenses.
(c) 
Amounts are recorded to electric operating revenues. Portions of these gains and losses are subject to sharing with electric customers through margin-sharing mechanisms and deducted from gross revenue, as appropriate.
(d) 
Amounts are recorded to electric fuel and purchased power. These derivative settlement gains and losses are shared with electric customers through fuel and purchased energy cost-recovery mechanisms, and reclassified out of income as regulatory assets or liabilities, as appropriate.
(e)
Amounts are recorded to cost of natural gas sold and transported. These derivative settlement gains and losses are shared with natural gas customers through purchased natural gas cost-recovery mechanisms, and reclassified out of income as regulatory assets or liabilities, as appropriate.

NSP-Minnesota had no derivative instruments designated as fair value hedges during the three and nine months ended Sept. 30, 2018 and 2017. Therefore, no gains or losses from fair value hedges or related hedged transactions were recognized for these periods.

Consideration of Credit Risk and Concentrations — NSP-Minnesota continuously monitors the creditworthiness of the counterparties to its interest rate derivatives and commodity derivative contracts prior to settlement, and assesses each counterparty’s ability to perform on the transactions set forth in the contracts. Given this assessment, as well as an assessment of the impact of NSP-Minnesota’s own credit risk when determining the fair value of derivative liabilities, the impact of credit risk was immaterial to the fair value of unsettled commodity derivatives presented in the consolidated balance sheets.

NSP-Minnesota employs additional credit risk control mechanisms when appropriate, such as letters of credit, parental guarantees, standardized master netting agreements and termination provisions that allow for offsetting of positive and negative exposures. Credit exposure is monitored and, when necessary, the activity with a specific counterparty is limited until credit enhancement is provided.

NSP-Minnesota’s most significant concentrations of credit risk with particular entities or industries are contracts with counterparties to its wholesale, trading and non-trading commodity activities. At Sept. 30, 2018, eight of NSP-Minnesota’s 10 most significant counterparties for these activities, comprising $41.8 million or 51 percent of this credit exposure, had investment grade credit ratings from Standard & Poor’s, Moody’s or Fitch Ratings. Two of the 10 most significant counterparties, comprising $14.0 million or 17 percent of this credit exposure, were not rated by these external agencies, but based on NSP-Minnesota’s internal analysis, had credit quality consistent with investment grade. All ten of these significant counterparties are municipal or cooperative electric entities, or other utilities.

Credit Related Contingent Features — Contract provisions for derivative instruments that NSP-Minnesota enters into, including those accounted for as normal purchase-normal sale contracts and therefore not reflected on the balance sheet, may require the posting of collateral or settlement of the contracts for various reasons, including if NSP-Minnesota’s credit ratings are downgraded below its investment grade credit rating by any of the major credit rating agencies or for cross-default contractual provisions that could result in the settlement of such contracts if there was a failure under other financing arrangements related to payment terms or other covenants. At Sept. 30, 2018 and Dec. 31, 2017, there were no derivative instruments in a material liability position with such underlying contract provisions.

Certain derivative instruments are also subject to contract provisions that contain adequate assurance clauses. These provisions allow counterparties to seek performance assurance, including cash collateral, in the event that NSP-Minnesota’s ability to fulfill its contractual obligations is reasonably expected to be impaired. NSP-Minnesota had no collateral posted related to adequate assurance clauses in derivative contracts as of Sept. 30, 2018 and Dec. 31, 2017.

Recurring Fair Value Measurements — The following table presents for each of the fair value hierarchy levels, NSP-Minnesota’s derivative assets and liabilities measured at fair value on a recurring basis at Sept. 30, 2018:
 
 
Sept. 30, 2018
 
 
Fair Value
 
Fair Value Total
 
Counterparty Netting (b)
 
 
(Thousands of Dollars)
 
Level 1
 
Level 2
 
Level 3
 
 
 
Total
Current derivative assets
 
 
 
 
 
 
 
 
 
 
 
 
Derivatives designated as cash flow hedges:
 
 
 
 
 
 
 
 
 
 
 
 
Vehicle fuel and other commodity
 
$
—

 
$
48

 
$
—

 
$
48

 
$
—

 
$
48

Other derivative instruments:
 
 
 
 
 
 
 
 
 
 
 
 
Commodity trading
 
422

 
17,565

 
1,951

 
19,938

 
(8,162
)
 
11,776

Electric commodity
 
—

 
—

 
18,002

 
18,002

 
(172
)
 
17,830

Natural gas commodity
 
—

 
771

 
—

 
771

 
—

 
771

Total current derivative assets
 
$
422

 
$
18,384

 
$
19,953

 
$
38,759

 
$
(8,334
)
 
30,425

PPAs (a)
 
 
 
 
 
 
 
 
 
 
 
25

Current derivative instruments
 
 
 
 
 
 
 
 
 
 
 
$
30,450

Noncurrent derivative assets
 
 
 
 
 
 
 
 
 
 
 
 
Other derivative instruments:
 
 
 
 
 
 
 
 
 
 
 
 
Commodity trading
 
$
19

 
$
28,125

 
$
5,399

 
$
33,543

 
$
(11,561
)
 
$
21,982

Total noncurrent derivative assets
 
$
19

 
$
28,125

 
$
5,399

 
$
33,543

 
$
(11,561
)
 
21,982

PPAs (a)
 
 
 
 
 
 
 
 
 
 
 
101

Noncurrent derivative instruments
 
 
 
 
 
 
 
 
 
 
 
$
22,083


 
 
Sept. 30, 2018
 
 
Fair Value
 
Fair Value Total
 
Counterparty Netting (b)
 
 
(Thousands of Dollars)
 
Level 1
 
Level 2
 
Level 3
 
 
 
Total
Current derivative liabilities
 
 
 
 
 
 
 
 
 
 
 
 
Other derivative instruments:
 
 
 
 
 
 
 
 
 
 
 
 
Commodity trading
 
$
135

 
$
14,792

 
$
1,754

 
$
16,681

 
$
(11,868
)
 
$
4,813

Electric commodity
 
—

 
—

 
172

 
172

 
(172
)
 
—

Total current derivative liabilities
 
$
135

 
$
14,792

 
$
1,926

 
$
16,853

 
$
(12,040
)
 
4,813

PPAs (a)
 
 
 
 
 
 
 
 
 
 
 
13,851

Current derivative instruments
 
 
 
 
 
 
 
 
 
 
 
$
18,664

Noncurrent derivative liabilities
 
 
 
 
 
 
 
 
 
 
 
 
Other derivative instruments:
 
 
 
 
 
 
 
 
 
 
 
 
Commodity trading
 
$
2

 
$
19,507

 
$
411

 
$
19,920

 
$
(12,696
)
 
$
7,224

Total noncurrent derivative liabilities
 
$
2

 
$
19,507

 
$
411

 
$
19,920

 
$
(12,696
)
 
7,224

PPAs (a)
 
 
 
 
 
 
 
 
 
 
 
79,524

Noncurrent derivative instruments
 
 
 
 
 
 
 
 
 
 
 
$
86,748



(a) 
During 2006, NSP-Minnesota qualified these contracts under the normal purchase exception. Based on this qualification, the contracts are no longer adjusted to fair value and the previous carrying value of these contracts is being amortized over the remaining contract lives along with the offsetting regulatory assets and liabilities.
(b) 
NSP-Minnesota nets derivative instruments and related collateral in its consolidated balance sheet when supported by a legally enforceable master netting agreement, and all derivative instruments and related collateral amounts were subject to master netting agreements at Sept. 30, 2018. At Sept. 30, 2018, derivative assets and liabilities include no obligations to return cash collateral and rights to reclaim cash collateral of $4.9 million. The counterparty netting amounts presented exclude settlement receivables and payables and non-derivative amounts that may be subject to the same master netting agreements.

The following table presents for each of the fair value hierarchy levels, NSP-Minnesota’s derivative assets and liabilities measured at fair value on a recurring basis at Dec. 31, 2017:
 
 
Dec. 31, 2017
 
 
Fair Value
 
Fair Value Total
 
Counterparty Netting (b)
 
 
(Thousands of Dollars)
 
Level 1
 
Level 2
 
Level 3
 
 
 
Total
Current derivative assets
 
 
 
 
 
 
 
 
 
 
 
 
Derivatives designated as cash flow hedges:
 
 
 
 
 
 
 
 
 
 
 
 
Vehicle fuel and other commodity
 
$
—

 
$
107

 
$
—

 
$
107

 
$
—

 
$
107

Other derivative instruments:
 
 
 
 
 
 
 
 
 
 
 
 
Commodity trading
 
1,691

 
17,144

 
142

 
18,977

 
(11,744
)
 
7,233

Electric commodity
 
—

 
—

 
17,581

 
17,581

 
(425
)
 
17,156

Natural gas commodity
 
—

 
77

 
—

 
77

 
—

 
77

Total current derivative assets
 
$
1,691

 
$
17,328

 
$
17,723

 
$
36,742

 
$
(12,169
)
 
24,573

PPAs (a)
 
 
 
 
 
 
 
 
 
 
 
657

Current derivative instruments
 
 
 
 
 
 
 
 
 
 
 
$
25,230

Noncurrent derivative assets
 
 
 
 
 
 
 
 
 
 
 
 
Other derivative instruments:
 
 
 
 
 
 
 
 
 
 
 
 
Commodity trading
 
$
—

 
$
29,121

 
$
5,363

 
$
34,484

 
$
(6,502
)
 
$
27,982

Total noncurrent derivative assets
 
$
—

 
$
29,121

 
$
5,363

 
$
34,484

 
$
(6,502
)
 
27,982

PPAs (a)
 
 
 
 
 
 
 
 
 
 
 
120

Noncurrent derivative instruments
 
 
 
 
 
 
 
 
 
 
 
$
28,102


 
 
Dec. 31, 2017
 
 
Fair Value
 
Fair Value Total
 
Counterparty Netting (b)
 
 
(Thousands of Dollars)
 
Level 1
 
Level 2
 
Level 3
 
 
 
Total
Current derivative liabilities
 
 
 
 
 
 
 
 
 
 
 
 
Other derivative instruments:
 
 
 
 
 
 
 
 
 
 
 
 
Commodity trading
 
$
1,713

 
$
13,853

 
$
—

 
$
15,566

 
$
(11,974
)
 
$
3,592

Electric commodity
 
—

 
—

 
425

 
425

 
(425
)
 
—

Total current derivative liabilities
 
$
1,713

 
$
13,853

 
$
425

 
$
15,991

 
$
(12,399
)
 
3,592

PPAs (a)
 
 
 
 
 
 
 
 
 
 
 
14,105

Current derivative instruments
 
 
 
 
 
 
 
 
 
 
 
$
17,697

Noncurrent derivative liabilities
 
 
 
 
 
 
 
 
 
 
 
 
Other derivative instruments:
 
 
 
 
 
 
 
 
 
 
 
 
Commodity trading
 
$
—

 
$
22,163

 
$
—

 
$
22,163

 
$
(9,334
)
 
$
12,829

Total noncurrent derivative liabilities
 
$
—

 
$
22,163

 
$
—

 
$
22,163

 
$
(9,334
)
 
12,829

PPAs (a)
 
 
 
 
 
 
 
 
 
 
 
89,913

Noncurrent derivative instruments
 
 
 
 
 
 
 
 
 
 
 
$
102,742



(a) 
During 2006, NSP-Minnesota qualified these contracts under the normal purchase exception. Based on this qualification, the contracts are no longer adjusted to fair value and the previous carrying value of these contracts is being amortized over the remaining contract lives along with the offsetting regulatory assets and liabilities.
(b) 
NSP-Minnesota nets derivative instruments and related collateral in its consolidated balance sheet when supported by a legally enforceable master netting agreement, and all derivative instruments and related collateral amounts were subject to master netting agreements at Dec. 31, 2017. At Dec. 31, 2017, derivative assets and liabilities include no obligations to return cash collateral and rights to reclaim cash collateral of $3.1 million. The counterparty netting amounts presented exclude settlement receivables and payables and non-derivative amounts that may be subject to the same master netting agreements.

The following table presents the changes in Level 3 commodity derivatives for the three and nine months ended Sept. 30, 2018 and 2017:
 
 
Three Months Ended Sept. 30
(Thousands of Dollars)
 
2018
 
2017
Balance at July 1
 
$
28,029

 
$
40,572

Settlements
 
(8,694
)
 
(23,186
)
Net transactions recorded during the period:
 
 
 
 
Gains recognized in earnings (a)
 
303

 
527

Net gains recognized as regulatory assets and liabilities
 
3,377

 
27,500

Balance at Sept. 30
 
$
23,015

 
$
45,413

 
 
 
 
 
 
 
Nine Months Ended Sept. 30
(Thousands of Dollars)
 
2018
 
2017
Balance at Jan. 1
 
$
22,661

 
$
15,320

Purchases
 
26,397

 
40,740

Settlements
 
(15,798
)
 
(34,681
)
Net transactions recorded during the period:
 
 
 
 
(Losses) gains recognized in earnings (a)
 
(70
)
 
5,742

Net (losses) gains recognized as regulatory assets and liabilities
 
(10,175
)
 
18,292

Balance at Sept. 30
 
$
23,015

 
$
45,413


(a) 
These amounts relate to commodity derivatives held at the end of the period.

NSP-Minnesota recognizes transfers between levels as of the beginning of each period. There were no transfers of amounts between levels for derivative instruments for the three and nine months ended Sept. 30, 2018 and 2017.

Fair Value of Long-Term Debt

As of Sept. 30, 2018 and Dec. 31, 2017, other financial instruments for which the carrying amount did not equal fair value were as follows:
 
 
Sept. 30, 2018
 
Dec. 31, 2017
(Thousands of Dollars)
 
Carrying
Amount
 
Fair Value
 
Carrying
Amount
 
Fair Value
Long-term debt, including current portion
 
$
4,936,135

 
$
5,159,080

 
$
4,933,018

 
$
5,601,919


The fair value of NSP-Minnesota’s long-term debt is estimated based on recent trades and observable spreads from benchmark interest rates for similar securities. The fair value estimates are based on information available to management as of Sept. 30, 2018 and Dec. 31, 2017, and given the observability of the inputs to these estimates, the fair values presented for long-term debt have been assigned a Level 2.
v3.10.0.1
Other (Expense) Income, Net
 9 Months Ended
Sep. 30, 2018
Other Income and Expenses [Abstract]  
Other (Expense) Income, Net
Other Expense, Net

Other expense, net consisted of the following:
 
 
Three Months Ended Sept. 30
 
Nine Months Ended Sept. 30
(Thousands of Dollars)
 
2018
 
2017
 
2018
 
2017
Interest income
 
$
1,596

 
$
2,936

 
$
5,053

 
$
6,250

Other nonoperating expense
 
(722
)
 
(429
)
 
(24
)
 
(402
)
Insurance policy income (expense)
 
157

 
(387
)
 
(1,038
)
 
(2,098
)
Benefits non-service cost
 
(3,007
)
 
(3,398
)
 
(7,985
)
 
(10,194
)
Other expense, net
 
$
(1,976
)
 
$
(1,278
)
 
$
(3,994
)
 
$
(6,444
)
v3.10.0.1
Segment Information
 9 Months Ended
Sep. 30, 2018
Segment Reporting [Abstract]  
Segment Information
Segment Information

Operating results from the regulated electric utility and regulated natural gas utility are each separately and regularly reviewed by NSP-Minnesota’s chief operating decision maker. NSP-Minnesota evaluates performance based on profit or loss generated from the product or service provided. These segments are managed separately because the revenue streams are dependent upon regulated rate recovery, which is separately determined for each segment.

NSP-Minnesota has the following reportable segments: regulated electric utility, regulated natural gas utility and all other.

•
NSP-Minnesota’s regulated electric utility segment generates, transmits and distributes electricity primarily in portions of Minnesota, North Dakota and South Dakota. In addition, this segment includes sales for resale and provides wholesale transmission service to various entities in the United States. Regulated electric utility also includes NSP-Minnesota’s commodity trading operations.
•
NSP-Minnesota’s regulated natural gas utility segment transports, stores and distributes natural gas primarily in portions of Minnesota and North Dakota.
•
Revenues from operating segments not included above are below the necessary quantitative thresholds and are therefore included in the all other category. Those primarily include appliance repair services, nonutility real estate activities and revenues associated with processing solid waste into refuse-derived fuel.

Asset and capital expenditure information is not provided for NSP-Minnesota’s reportable segments because as an integrated electric and natural gas utility, NSP-Minnesota operates significant assets that are not dedicated to a specific business segment, and reporting assets and capital expenditures by business segment would require arbitrary and potentially misleading allocations which may not necessarily reflect the assets that would be required for the operation of the business segments on a stand-alone basis.

To report income from operations for regulated electric and regulated natural gas utility segments, the majority of costs are directly assigned to each segment. However, some costs, such as common depreciation, common operating and maintenance (O&M) expenses and interest expense are allocated based on cost causation allocators. A general allocator is used for certain general and administrative expenses, including office supplies, rent, property insurance and general advertising.
(Thousands of Dollars)
 
Regulated Electric
 
Regulated Natural Gas
 
All Other
 
Reconciling Eliminations
 
Consolidated Total
Three Months Ended Sept. 30, 2018
 
 
 
 
 
 
 
 
 
 
Operating revenues (a)(b)
 
$
1,289,498

 
$
54,625

 
$
7,631

 
$
—

 
$
1,351,754

Intersegment revenues
 
219

 
138

 
—

 
(357
)
 
—

Total revenues
 
$
1,289,717

 
$
54,763

 
$
7,631

 
$
(357
)
 
$
1,351,754

Net income (loss)
 
$
207,316

 
$
(8,416
)
 
$
2,276

 
$
—

 
$
201,176

(Thousands of Dollars)
 
Regulated Electric
 
Regulated Natural Gas
 
All Other
 
Reconciling Eliminations
 
Consolidated Total
Three Months Ended Sept. 30, 2017
 
 
 
 
 
 
 
 
 
 
Operating revenues (a)(b)
 
$
1,290,971

 
$
57,442

 
$
7,366

 
$
—

 
$
1,355,779

Intersegment revenues
 
160

 
100

 
—

 
(260
)
 
—

Total revenues
 
$
1,291,131

 
$
57,542

 
$
7,366

 
$
(260
)
 
$
1,355,779

Net income (loss)
 
$
232,078

 
$
(6,242
)
 
$
3,167

 
$
—

 
$
229,003


(a) 
Operating revenues include $123 million and $124 million of affiliate electric revenue for the three months ended Sept. 30, 2018 and 2017.
(b) 
Operating revenues include an immaterial amount of affiliate gas revenue for the three months ended Sept. 30, 2018 and 2017.

(Thousands of Dollars)
 
Regulated Electric
 
Regulated Natural Gas
 
All Other
 
Reconciling Eliminations
 
Consolidated Total
Nine Months Ended Sept. 30, 2018
 
 
 
 
 
 
 
 
 
 
Operating revenues (a)(b)
 
$
3,448,169

 
$
379,169

 
$
22,902

 
$
—

 
$
3,850,240

Intersegment revenues
 
512

 
398

 
—

 
(910
)
 
—

Total revenues
 
$
3,448,681

 
$
379,567

 
$
22,902

 
$
(910
)
 
$
3,850,240

Net income
 
$
381,913

 
$
17,428

 
$
5,962

 
$
—

 
$
405,303


(Thousands of Dollars)
 
Regulated Electric
 
Regulated Natural Gas
 
All Other
 
Reconciling Eliminations
 
Consolidated Total
Nine Months Ended Sept. 30, 2017
 
 
 
 
 
 
 
 
 
 
Operating revenues (a)(b)
 
$
3,449,780

 
$
356,631

 
$
21,448

 
$
—

 
$
3,827,859

Intersegment revenues
 
512

 
371

 
—

 
(883
)
 
—

Total revenues
 
$
3,450,292

 
$
357,002

 
$
21,448

 
$
(883
)
 
$
3,827,859

Net income
 
$
399,637

 
$
7,903

 
$
3,291

 
$
—

 
$
410,831

(a) 
Operating revenues include $357 million and $367 million of affiliate electric revenue for the nine months ended Sept. 30, 2018 and 2017.
(b) 
Operating revenues include an immaterial amount of affiliate gas revenue for the nine months ended Sept. 30, 2018 and 2017.
v3.10.0.1
Benefit Plans and Other Postretirement Benefits
 9 Months Ended
Sep. 30, 2018
Retirement Benefits [Abstract]  
Benefit Plans and Other Postretirement Benefits
Benefit Plans and Other Postretirement Benefits

Components of Net Periodic Benefit Cost
 
 
Three Months Ended Sept. 30
 
 
2018
 
2017
 
2018
 
2017
(Thousands of Dollars)
 
Pension Benefits
 
Postretirement Health
Care Benefits
Service cost
 
$
6,982

 
$
6,958

 
$
42

 
$
36

Interest cost (a)
 
8,805

 
10,177

 
768

 
854

Expected return on plan assets (a)
 
(14,542
)
 
(15,016
)
 
(95
)
 
(53
)
Amortization of prior service (credit) cost (a)
 
(29
)
 
265

 
(759
)
 
(759
)
Amortization of net loss (a)
 
9,614

 
9,902

 
596

 
506

Settlement charge (b)
 
34,907

 
—

 
—

 
—

Net periodic benefit cost
 
45,737

 
12,286

 
552

 
584

Costs not recognized due to the effects of regulation
 
(42,037
)
 
(4,899
)
 
—

 
—

Net benefit cost recognized for financial reporting
 
$
3,700

 
$
7,387

 
$
552

 
$
584


 
 
Nine Months Ended Sept. 30
 
 
2018
 
2017
 
2018
 
2017
(Thousands of Dollars)
 
Pension Benefits
 
Postretirement Health Care Benefits
Service cost
 
$
20,946

 
$
20,874

 
$
128

 
$
108

Interest cost (a)
 
26,413

 
30,531

 
2,306

 
2,562

Expected return on plan assets (a)
 
(43,625
)
 
(45,050
)
 
(287
)
 
(161
)
Amortization of prior service (credit) cost (a)
 
(86
)
 
795

 
(2,277
)
 
(2,277
)
Amortization of net loss (a)
 
28,843

 
29,706

 
1,786

 
1,520

Settlement charge (b)
 
34,907

 
—

 
—

 
—

Net periodic benefit cost
 
67,398

 
36,856

 
1,656

 
1,752

Costs not recognized due to the effects of regulation
 
(47,869
)
 
(14,696
)
 
—

 
—

Net benefit cost recognized for financial reporting
 
$
19,529

 
$
22,160

 
$
1,656

 
$
1,752


(a)
The components of net periodic cost other than the service cost component are included in the line item “other expense, net” in the income statement or capitalized on the balance sheet as a regulatory asset.
(b)
A settlement charge is required when the amount of lump-sum distributions during the year is greater than the sum of the service and interest cost components of the annual net periodic pension cost. In the third quarter of 2018 as a result of lump-sum distributions during the 2018 plan year, NSP-Minnesota recorded a total pension settlement charge of $35 million, which was not recognized due to the effects of rate making. In the fourth quarter of 2017 as a result of lump-sum distributions during the 2017 plan year, NSP-Minnesota recorded a total pension settlement charge of $48 million, which was not recognized due to the effects of rate making.

In January 2018, contributions of $150 million were made across four of Xcel Energy’s pension plans, of which $63.0 million was attributable to NSP-Minnesota. Xcel Energy does not expect additional pension contributions during 2018.
v3.10.0.1
Other Comprehensive Income
 9 Months Ended
Sep. 30, 2018
Stockholders' Equity Note [Abstract]  
Other Comprehensive Income
Other Comprehensive Income (Loss)

Changes in accumulated other comprehensive loss, net of tax, for the three and nine months ended Sept. 30, 2018 and 2017 were as follows:
 
 
Three Months Ended Sept. 30, 2018
(Thousands of Dollars)
 
Gains and
Losses on Cash Flow
Hedges
 
Unrealized
Gains on Marketable
Securities
 
Defined Benefit
Pension and
Postretirement Items
 
Total
Accumulated other comprehensive loss at July 1
 
$
(20,536
)
 
$
—

 
$
(3,663
)
 
$
(24,199
)
Other comprehensive income before reclassifications
 
5

 
—

 
—

 
5

Losses reclassified from net accumulated other comprehensive loss
 
172

 
—

 
56

 
228

Net current period other comprehensive income
 
177

 
—

 
56

 
233

Accumulated other comprehensive loss at Sept. 30
 
$
(20,359
)
 
$
—

 
$
(3,607
)
 
$
(23,966
)

 
 
Three Months Ended Sept. 30, 2017
(Thousands of Dollars)
 
Gains and
Losses on Cash Flow
Hedges
 
Unrealized
Gains on
Marketable
Securities
 
Defined Benefit
Pension and
Postretirement Items
 
Total
Accumulated other comprehensive loss at July 1
 
$
(17,775
)
 
$
105

 
$
(2,609
)
 
$
(20,279
)
Other comprehensive income before reclassifications
 
22

 
—

 
—

 
22

Losses reclassified from net accumulated other comprehensive loss
 
379

 
—

 
39

 
418

Net current period other comprehensive income
 
401

 
—

 
39

 
440

Accumulated other comprehensive loss at Sept. 30
 
$
(17,374
)
 
$
105

 
$
(2,570
)
 
$
(19,839
)
 
 
Nine Months Ended Sept. 30, 2018
(Thousands of Dollars)
 
Gains and
Losses on Cash Flow Hedges
 
Unrealized
Gains on Marketable
Securities
 
Defined Benefit
Pension and
Postretirement Items
 
Total
Accumulated other comprehensive loss at Jan. 1
 
$
(20,895
)
 
$
128

 
$
(3,770
)
 
$
(24,537
)
Other comprehensive income before reclassifications
 
32

 
—

 
—

 
32

Losses (gains) reclassified from net accumulated other comprehensive loss
 
504

 
(128
)
 
163

 
539

Net current period other comprehensive income
 
536

 
(128
)
 
163

 
571

Accumulated other comprehensive loss at Sept. 30
 
$
(20,359
)
 
$
—

 
$
(3,607
)
 
$
(23,966
)
 
 
Nine Months Ended Sept. 30, 2017
(Thousands of Dollars)
 
Gains and
Losses on Cash Flow Hedges
 
Unrealized
Gains on Marketable
Securities
 
Defined Benefit
Pension and
Postretirement Items
 
Total
Accumulated other comprehensive loss at Jan. 1
 
$
(18,208
)
 
$
105

 
$
(2,680
)
 
$
(20,783
)
Other comprehensive income before reclassifications
 
48

 
—

 
—

 
48

Losses reclassified from net accumulated other comprehensive loss
 
786

 
—

 
110

 
896

Net current period other comprehensive income
 
834

 
—

 
110

 
944

Accumulated other comprehensive loss at Sept. 30
 
$
(17,374
)
 
$
105

 
$
(2,570
)
 
$
(19,839
)


Reclassifications from accumulated other comprehensive loss for the three and nine months ended Sept. 30, 2018 and 2017 were as follows:
 
 
Amounts Reclassified from
Accumulated Other
Comprehensive Loss
 
(Thousands of Dollars)
 
Three Months Ended Sept. 30, 2018
 
Three Months Ended Sept. 30, 2017
 
Losses (gains) on cash flow hedges:
 
 
 
 
 
Interest rate derivatives
 
$
267

(a) 
$
612

(a) 
Vehicle fuel derivatives
 
(39
)
(b) 
(11
)
(b) 
Total, pre-tax
 
228

 
601

 
Tax benefit
 
(56
)
 
(222
)
 
Total, net of tax
 
172

 
379

 
Defined benefit pension and postretirement losses:
 
 
 
 
 
Amortization of net loss
 
125

(c) 
109

(c) 
Prior service credit
 
(49
)
(c) 
(49
)
(c) 
Total, pre-tax
 
76

 
60

 
Tax benefit
 
(20
)
 
(21
)
 
Total, net of tax
 
56

 
39

 
Marketable securities:
 
 
 
 
 
Realization of gains
 
—

 
—

 
Total, pre-tax
 
—

 
—

 
Tax expense
 
—

 
—

 
Total, net of tax
 
—

 
—

 
Total amounts reclassified, net of tax
 
$
228

 
$
418

 
 
 
Amounts Reclassified from
Accumulated Other
Comprehensive Loss
 
(Thousands of Dollars)
 
Nine Months Ended Sept. 30, 2018
 
Nine Months Ended Sept. 30, 2017
 
Losses (gains) on cash flow hedges:
 
 
 
 
 
Interest rate derivatives
 
$
792

(a) 
$
1,304

(a) 
Vehicle fuel derivatives
 
(103
)
(b) 
(16
)
(b) 
Total, pre-tax
 
689

 
1,288

 
Tax benefit
 
(185
)
 
(502
)
 
Total, net of tax
 
504

 
786

 
Defined benefit pension and postretirement losses:
 
 
 
 
 
Amortization of net loss
 
375

(c) 
327

(c) 
Prior service credit
 
(147
)
(c) 
(147
)
(c) 
Total, pre-tax
 
228

 
180

 
Tax benefit
 
(65
)
 
(70
)
 
Total, net of tax
 
163

 
110

 
Marketable securities:
 
 
 
 
 
Realization of gains
 
(179
)
 
—

 
Total, pre-tax
 
(179
)
 
—

 
Tax expense
 
51

 
—

 
Total, net of tax
 
(128
)
 
—

 
Total amounts reclassified, net of tax
 
$
539

 
$
896

 

(a) 
Included in interest charges.
(b) 
Included in O&M expenses.
(c) 
Included in the computation of net periodic pension and postretirement benefit costs. See Note 11 to the consolidated financial statements for details regarding these benefit plans.
v3.10.0.1
Revenues
 9 Months Ended
Sep. 30, 2018
Revenue from Contract with Customer [Abstract]  
Revenues
Revenues

NSP-Minnesota principally generates revenue from the generation, transmission, distribution and sale of electricity and the transportation, distribution and sale of natural gas to wholesale and retail customers. Performance obligations related to the sale of energy are satisfied as energy is delivered to customers. NSP-Minnesota recognizes revenue in an amount that corresponds directly to the price of the energy delivered to the customer. The measurement of energy sales to customers is generally based on the reading of their meters, which occurs on a systematic basis throughout the month. At the end of each month, amounts of energy delivered to customers since the date of the last meter reading are estimated, and the corresponding unbilled revenue is recognized. Contract terms are generally short-term in nature, and as such NSP-Minnesota does not recognize a separate financing component of its collections from customers. NSP-Minnesota presents its revenues net of any excise or other fiduciary-type taxes or fees.

NSP-Minnesota participates in MISO. NSP-Minnesota recognizes sales to both native load and other end use customers on a gross basis in electric revenues and cost of sales. Revenues and charges for short term wholesale sales of excess energy transacted through RTOs are also recorded on a gross basis. Other revenues and charges related to participating and transacting in RTOs are recorded on a net basis in cost of sales.

NSP-Minnesota has various rate-adjustment mechanisms in place that provide for the recovery of natural gas, electric fuel and purchased energy costs. These cost-adjustment tariffs may increase or decrease the level of revenue collected from customers and are revised periodically for differences between the total amount collected under the clauses and the costs incurred.

When applicable, under governing regulatory commission rate orders, fuel cost over-recoveries (the excess of fuel revenue billed to customers over fuel costs incurred) are deferred as regulatory liabilities and under-recoveries (the excess of fuel costs incurred over fuel revenues billed to customers) are deferred as regulatory assets.

Certain rate rider mechanisms qualify as alternative revenue programs under GAAP. These mechanisms arise from costs imposed upon the utility by action of a regulator or legislative body related to an environmental, public safety or other mandate. When certain criteria are met (including collection within 24 months), revenue is recognized equal to the revenue requirement, which may include return on rate base items and incentives. The mechanisms are revised periodically for differences between the total amount collected and the revenue recognized, which may increase or decrease the level of revenue collected from customers. Alternative revenue is recorded on a gross basis and is disclosed separate from revenue from contracts with customers in the period earned.

In the following tables, revenue is classified by the type of goods/services rendered and market/customer type. The tables also reconcile revenue to the reportable segments.
 
 
Three Months Ended Sept. 30, 2018
(Thousands of Dollars)
 
Electric
 
Natural Gas
 
All Other
 
Total
Major revenue types
 
 
 
 
 
 
 
 
Revenue from contracts with customers:
 
 
 
 
 
 
 
 
Residential
 
$
392,334

 
$
24,863

 
$
6,999

 
$
424,196

Commercial and industrial (C&I)
 
604,301

 
22,337

 
42

 
626,680

Other
 
8,694

 
—

 
590

 
9,284

Total retail
 
1,005,329

 
47,200

 
7,631

 
1,060,160

Wholesale
 
47,708

 
—

 
—

 
47,708

Transmission
 
66,166

 
—

 
—

 
66,166

Interchange
 
122,636

 
—

 
—

 
122,636

Other
 
4,586

 
5,280

 
—

 
9,866

Total revenue from contracts with customers
 
1,246,425

 
52,480

 
7,631

 
1,306,536

Alternative revenue and other
 
43,073

 
2,145

 
—

 
45,218

Total revenues
 
$
1,289,498

 
$
54,625

 
$
7,631

 
$
1,351,754


 
 
Three Months Ended Sept. 30, 2017
(Thousands of Dollars)
 
Electric
 
Natural Gas
 
All Other
 
Total
Major revenue types
 
 
 
 
 
 
 
 
Revenue from contracts with customers:
 
 
 
 
 
 
 
 
Residential
 
$
357,209

 
$
24,475

 
$
6,791

 
$
388,475

C&I
 
626,590

 
22,389

 
39

 
649,018

Other
 
9,327

 
—

 
536

 
9,863

Total retail
 
993,126

 
46,864

 
7,366

 
1,047,356

Wholesale
 
36,598

 
—

 
—

 
36,598

Transmission
 
65,667

 
—

 
—

 
65,667

Interchange
 
123,523

 
—

 
—

 
123,523

Other
 
8,632

 
1,589

 
—

 
10,221

Total revenue from contracts with customers
 
1,227,546

 
48,453

 
7,366

 
1,283,365

Alternative revenue and other
 
63,425

 
8,989

 
—

 
72,414

Total revenues
 
$
1,290,971

 
$
57,442

 
$
7,366

 
$
1,355,779


 
 
Nine Months Ended Sept. 30, 2018
(Thousands of Dollars)
 
Electric
 
Natural Gas
 
All Other
 
Total
Major revenue types
 
 
 
 
 
 
 
 
Revenue from contracts with customers:
 
 
 
 
 
 
 
 
Residential
 
$
1,014,549

 
$
198,920

 
$
20,032

 
$
1,233,501

Commercial and industrial (C&I)
 
1,587,034

 
152,662

 
127

 
1,739,823

Other
 
27,052

 
—

 
2,743

 
29,795

Total retail
 
2,628,635

 
351,582

 
22,902

 
3,003,119

Wholesale
 
136,106

 
—

 
—

 
136,106

Transmission
 
180,915

 
—

 
—

 
180,915

Interchange
 
357,025

 
—

 
—

 
357,025

Other
 
22,164

 
10,808

 
—

 
32,972

Total revenue from contracts with customers
 
3,324,845

 
362,390

 
22,902

 
3,710,137

Alternative revenue and other
 
123,324

 
16,779

 
—

 
140,103

Total revenues
 
$
3,448,169

 
$
379,169

 
$
22,902

 
$
3,850,240


 
 
Nine Months Ended Sept. 30, 2017
(Thousands of Dollars)
 
Electric
 
Natural Gas
 
All Other
 
Total
Major revenue types
 
 
 
 
 
 
 
 
Revenue from contracts with customers:
 
 
 
 
 
 
 
 
Residential
 
$
950,355

 
$
182,748

 
$
19,417

 
$
1,152,520

C&I
 
1,638,560

 
142,262

 
169

 
1,780,991

Other
 
25,384

 
—

 
1,862

 
27,246

Total retail
 
2,614,299

 
325,010

 
21,448

 
2,960,757

Wholesale
 
111,358

 
—

 
—

 
111,358

Transmission
 
175,803

 
—

 
—

 
175,803

Interchange
 
366,596

 
—

 
—

 
366,596

Other
 
22,040

 
4,286

 
—

 
26,326

Total revenue from contracts with customers
 
3,290,096

 
329,296

 
21,448

 
3,640,840

Alternative revenue and other
 
159,684

 
27,335

 
—

 
187,019

Total revenues
 
$
3,449,780

 
$
356,631

 
$
21,448

 
$
3,827,859
v3.10.0.1
Selected Balance Sheet Data (Tables)
 9 Months Ended
Sep. 30, 2018
Balance Sheet Related Disclosures [Abstract]  
Accounts receivable, net
(Thousands of Dollars)
 
Sept. 30, 2018
 
Dec. 31, 2017
Accounts receivable, net
 
 
 
 
Accounts receivable
 
$
409,740

 
$
366,388

Less allowance for bad debts
 
(21,885
)
 
(21,278
)
 
 
$
387,855

 
$
345,110
Inventories
(Thousands of Dollars)
 
Sept. 30, 2018
 
Dec. 31, 2017
Inventories
 
 
 
 
Materials and supplies
 
$
176,183

 
$
209,236

Fuel
 
76,962

 
94,483

Natural gas
 
35,533

 
33,993

 
 
$
288,678

 
$
337,712
Property, plant and equipment, net
(Thousands of Dollars)
 
Sept. 30, 2018
 
Dec. 31, 2017
Property, plant and equipment, net
 
 
 
 
Electric plant
 
$
17,363,519

 
$
17,024,925

Natural gas plant
 
1,426,646

 
1,370,330

Common and other property
 
726,127

 
724,066

Construction work in progress
 
685,047

 
530,126

Total property, plant and equipment
 
20,201,339

 
19,649,447

Less accumulated depreciation
 
(7,314,826
)
 
(7,018,249
)
Nuclear fuel
 
2,716,595

 
2,697,412

Less accumulated amortization
 
(2,387,390
)
 
(2,294,998
)
 
 
$
13,215,718

 
$
13,033,612
v3.10.0.1
Income Taxes (Tables)
 9 Months Ended
Sep. 30, 2018
Income Tax Disclosure [Abstract]  
Schedule of Effective Income Tax Rate Reconciliation [Table Text Block]
Total income tax expense from operations differs from the amount computed by applying the statutory federal income tax rate to income before income tax expense. The following reconciles such differences:
 
 
Nine Months Ended Sept. 30
 
 
2018
 
2017
Federal statutory rate
 
21.0
 %
 
35.0
 %
State tax (net of federal tax effect)
 
7.1

 
5.8

Increases (decreases) in tax from:
 

 

Wind production tax credits (PTCs) (a)
 
(12.1
)
 
(11.3
)
Regulatory differences - ARAM (b)
 
(9.5
)
 
(0.1
)
Regulatory differences - ARAM deferral (c)
 
7.7

 
—

Regulatory differences - reversal of prior quarters’ ARAM deferral (c)
 
(7.1
)
 
—

Regulatory differences - other utility plant items
 
0.2

 
(0.3
)
Other tax credits (net of federal income tax expense)
 
(1.4
)
 
(1.1
)
Other (net)
 
1.2

 
(2.5
)
Effective income tax rate
 
7.1
 %
 
25.5
 %

(a)  
Quarterly PTCs may vary due to production and timing differences. Annual 2018 PTCs are forecasted to exceed 2017.
(b)
The average rate assumption method (ARAM); a method to flow back excess deferred taxes to customers.
(c) 
ARAM has been deferred when regulatory treatment has not been established. As NSP-Minnesota received direction from its regulatory commissions regarding the return of excess deferred taxes to customers, the ARAM deferral was reversed. This resulted in a reduction to tax expense with a corresponding reduction to revenue.
Summary of Statute of Limitations Applicable to Open Tax Years [Table Text Block]
NSP-Minnesota is a member of the Xcel Energy affiliated group that files a consolidated federal income tax return. The statute of limitations applicable to Xcel Energy’s federal income tax returns expire as follows:

Tax Year(s)
 
Expiration
2009 - 2014
 
October 2019
2015
 
September 2019
2016
 
September 2020
Reconciliation of Unrecognized Tax Benefits
A reconciliation of the amount of unrecognized tax benefit is as follows:
(Millions of Dollars)
 
Sept. 30, 2018
 
Dec. 31, 2017
Unrecognized tax benefit — Permanent tax positions
 
$
11.4

 
$
10.2

Unrecognized tax benefit — Temporary tax positions
 
5.4

 
7.9

Total unrecognized tax benefit
 
$
16.8

 
$
18.1
Tax Benefits Associated with NOL and Tax Credit Carryforwards
The unrecognized tax benefit amounts were reduced by the tax benefits associated with NOL and tax credit carryforwards. The amounts of tax benefits associated with NOL and tax credit carryforwards are as follows:
(Millions of Dollars)
 
Sept. 30, 2018
 
Dec. 31, 2017
NOL and tax credit carryforwards
 
$
(14.0
)
 
$
(12.8
)
Interest Payable related to Unrecognized Tax Benefits [Table Text Block]
(Millions of Dollars)
 
Sept. 30, 2018
 
Dec. 31, 2017
Payable for interest related to unrecognized tax benefits at beginning of period
 
$
(0.9
)
 
$
(2.0
)
Interest (expense) income related to unrecognized tax benefits recorded during the period
 
(1.1
)
 
1.1

Payable for interest related to unrecognized tax benefits at end of period
 
$
(2.0
)
 
$
(0.9
)
v3.10.0.1
Commitments and Contingencies Schedule of Guarantor Obligations (Tables)
 9 Months Ended
Sep. 30, 2018
Commitments and Contingencies Disclosure [Abstract]  
Schedule of Guarantor Obligations [Table Text Block]
The following table presents the guarantee issued and outstanding for NSP-Minnesota:
(Millions of Dollars)
 
Sept. 30, 2018
 
Dec. 31, 2017
Guarantee issued and outstanding
 
$
4.8

 
$
4.8
v3.10.0.1
Borrowings and Other Financing Instruments (Tables)
 9 Months Ended
Sep. 30, 2018
Borrowings and Other Financing Instruments [Abstract]  
Credit Facilities
At Sept. 30, 2018, NSP-Minnesota had the following committed credit facility available (in millions of dollars):
Credit Facility (a)
 
Drawn (b)
 
Available
$
500

 
$
61

 
$
439

(a) 
This credit facility expires in June 2021.
(b) 
Includes outstanding letters of credit.
Money Pool  
Borrowings and Other Financing Instruments [Abstract]  
Short-Term Borrowings
Money pool borrowings for NSP-Minnesota were as follows:
(Amounts in Millions, Except Interest Rates)
 
Three Months Ended Sept. 30, 2018
 
Year Ended Dec. 31, 2017
Borrowing limit
 
$
250

 
$
250

Amount outstanding at period end
 
—

 
85

Average amount outstanding
 
34

 
25

Maximum amount outstanding
 
143

 
142

Weighted average interest rate, computed on a daily basis
 
1.97
%
 
1.14
%
Weighted average interest rate at period end
 
N/A

 
1.18
Commercial Paper  
Borrowings and Other Financing Instruments [Abstract]  
Short-Term Borrowings
Commercial paper outstanding for NSP-Minnesota was as follows:
(Amounts in Millions, Except Interest Rates)
 
Three Months Ended Sept. 30, 2018
 
Year Ended Dec. 31, 2017
Borrowing limit
 
$
500

 
$
500

Amount outstanding at period end
 
24

 
20

Average amount outstanding
 
6

 
62

Maximum amount outstanding
 
73

 
237

Weighted average interest rate, computed on a daily basis
 
2.12
%
 
1.10
%
Weighted average interest rate at period end
 
2.30

 
1.93
v3.10.0.1
Fair Value of Financial Assets and Liabilities (Tables)
 9 Months Ended
Sep. 30, 2018
Fair Value Disclosures [Abstract]  
Cost and Fair Value of Nuclear Decommissioning Fund Investments
The following tables present the cost and fair value of NSP-Minnesota’s non-derivative instruments with recurring fair value measurements in the nuclear decommissioning fund as of Sept. 30, 2018 and Dec. 31, 2017:
 
 
Sept. 30, 2018
 
 
 
 
Fair Value
(Thousands of Dollars)
 
Cost
 
Level 1
 
Level 2
 
Level 3
 
Investments Measured at NAV (b)
 
Total
Nuclear decommissioning fund (a)
 
 
 
 
 
 
 
 
 
 
 
 
Cash equivalents
 
$
33,040

 
$
33,040

 
$
—

 
$
—

 
$
—

 
$
33,040

Commingled funds:
 
 
 
 
 
 
 
 
 
 
 
 
Non U.S. equities
 
262,345

 
195,959

 
—

 
—

 
91,426

 
287,385

Emerging market debt funds
 
162,835

 
—

 
—

 
—

 
164,772

 
164,772

Private equity investments
 
169,676

 
—

 
—

 
—

 
249,881

 
249,881

Real estate
 
125,422

 
—

 
—

 
—

 
198,422

 
198,422

Debt securities:
 
 
 
 
 
 
 
 
 
 
 
 
Government securities
 
75,799

 
—

 
73,317

 
—

 
—

 
73,317

U.S. corporate bonds
 
334,219

 
—

 
329,727

 
—

 
—

 
329,727

Non U.S. corporate bonds
 
55,856

 
—

 
54,813

 
—

 
—

 
54,813

Equity securities:
 
 
 
 
 
 
 
 
 
 
 
 
U.S. equities
 
257,932

 
590,310

 
—

 
—

 
—

 
590,310

Non U.S. equities
 
156,225

 
229,245

 
—

 
—

 
—

 
229,245

Total
 
$
1,633,349

 
$
1,048,554

 
$
457,857

 
$
—

 
$
704,501

 
$
2,210,912


(a) 
Reported in nuclear decommissioning fund and other investments on the consolidated balance sheet, which also includes $52 million of rabbi trust assets and miscellaneous investments.
(b) 
Due to limited availability of published pricing and a lack of immediate redeemability, certain fund investments measured at NAV are not required to be categorized within the fair value hierarchy.
 
 
Dec. 31, 2017
 
 
 
 
Fair Value
(Thousands of Dollars)
 
Cost
 
Level 1
 
Level 2
 
Level 3
 
Investments Measured at NAV (b)
 
Total
Nuclear decommissioning fund (a)
 
 
 
 
 
 
 
 
 
 
 
 
Cash equivalents
 
$
28,741

 
$
28,741

 
$
—

 
$
—

 
$
—

 
$
28,741

Commingled funds:
 
 
 
 
 
 
 
 
 
 
 
 
Non U.S. equities
 
263,694

 
216,551

 
—

 
—

 
89,857

 
306,408

Emerging market debt funds
 
156,057

 
—

 
—

 
—

 
166,375

 
166,375

Private equity investments
 
141,413

 
—

 
—

 
—

 
198,037

 
198,037

Real estate
 
130,787

 
—

 
—

 
—

 
201,842

 
201,842

Other commingled funds
 
9,340

 
6,286

 
—

 
—

 
2,975

 
9,261

Debt securities:
 
 
 
 
 
 
 
 
 
 
 
 
Government securities
 
67,760

 
—

 
69,413

 
—

 
—

 
69,413

U.S. corporate bonds
 
319,809

 
—

 
322,129

 
—

 
—

 
322,129

Non U.S. corporate bonds
 
50,121

 
—

 
50,102

 
—

 
—

 
50,102

Equity securities:
 
 
 
 
 
 
 
 
 
 
 
 
U.S. equities
 
271,166

 
556,974

 
—

 
—

 
—

 
556,974

Non U.S. equities
 
151,961

 
233,999

 
—

 
—

 
—

 
233,999

Total
 
$
1,590,849

 
$
1,042,551

 
$
441,644

 
$
—

 
$
659,086

 
$
2,143,281


(a) 
Reported in nuclear decommissioning fund and other investments on the consolidated balance sheet, which also includes $49 million of rabbi trust assets and miscellaneous investments.
(b) 
Due to limited availability of published pricing and a lack of immediate redeemability, certain fund investments measured at NAV are not required to be categorized within the fair value hierarchy.
Final Contractual Maturity Dates of Debt Securities in the Nuclear Decommissioning Fund by Asset Class
The following table summarizes the final contractual maturity dates of the debt securities in the nuclear decommissioning fund, by asset class, as of Sept. 30, 2018:
 
 
Final Contractual Maturity
(Thousands of Dollars)
 
Due in 1 Year
or Less
 
Due in 1 to 5
Years
 
Due in 5 to 10
Years
 
Due after 10
Years
 
Total
Government securities
 
$
—

 
$
—

 
$
1,750

 
$
71,567

 
$
73,317

U.S. corporate bonds
 
12,759

 
90,982

 
175,662

 
50,324

 
329,727

Non U.S. corporate bonds
 
1,998

 
20,498

 
28,151

 
4,166

 
54,813

Debt securities
 
$
14,757

 
$
111,480

 
$
205,563

 
$
126,057

 
$
457,857
Rabbi Trust Securities Amortized Cost and Fair Value Measured on Recurrring Basis [Table Text Block]
In 2016, NSP-Minnesota established a rabbi trust to provide partial funding for future deferred compensation plan distributions. The following tables present the cost and fair value of the assets held in rabbi trust at Sept. 30, 2018 and Dec. 31, 2017:
 
 
Sept. 30, 2018
 
 
 
 
Fair Value
(Thousands of Dollars)
 
Cost
 
Level 1
 
Level 2
 
Level 3
 
Total
Rabbi Trust (a)
 
 
 
 
 
 
 
 
 
 
Cash equivalents
 
$
2,321

 
$
2,321

 
$
—

 
$
—

 
$
2,321

Mutual funds
 
10,474

 
11,748

 
—

 
—

 
11,748

Total
 
$
12,795

 
$
14,069

 
$
—

 
$
—

 
$
14,069


 
 
Dec. 31, 2017
 
 
 
 
Fair Value
(Thousands of Dollars)
 
Cost
 
Level 1
 
Level 2
 
Level 3
 
Total
Rabbi Trusts (a)
 
 
 
 
 
 
 
 
 
 
Cash equivalents
 
$
783

 
$
783

 
$
—

 
$
—

 
$
783

Mutual funds
 
10,332

 
11,283

 
—

 
—

 
11,283

Total
 
$
11,115

 
$
12,066

 
$
—

 
$
—

 
$
12,066

(a) 
Reported in nuclear decommissioning fund and other investments on the consolidated balance sheet.
Gross Notional Amounts of Commodity Forwards, Options, and FTRs
The following table details the gross notional amounts of commodity forwards, options and FTRs at Sept. 30, 2018 and Dec. 31, 2017:
(Amounts in Thousands) (a)(b)
 
Sept. 30, 2018
 
Dec. 31, 2017
Megawatt hours of electricity
 
61,746

 
41,711

Million British thermal units of natural gas
 
11,131

 
23,829

Gallons of vehicle fuel
 
60

 
240


(a) 
Amounts are not reflective of net positions in the underlying commodities.
(b) 
Notional amounts for options are included on a gross basis, but are weighted for the probability of exercise.
Impact of Derivative Activity on Accumulated Other Comprehensive Loss, Regulatory Assets and Liabilities, and Income
The following tables detail the impact of derivative activity during the three and nine months ended Sept. 30, 2018 and 2017 on accumulated other comprehensive loss, regulatory assets and liabilities and income:
 
 
Three Months Ended Sept. 30, 2018
 
 
 
Pre-Tax Fair Value
Gains (Losses) Recognized
During the Period in:
 
Pre-Tax (Gains) Losses
Reclassified into Income
During the Period from:
 
Pre-Tax Gains Recognized
During the Period in Income
 
(Thousands of Dollars)
 
Accumulated
Other
Comprehensive Loss
 
Regulatory
(Assets) and Liabilities
 
Accumulated
Other
Comprehensive Loss
 
Regulatory
Assets and (Liabilities)
 
 
Derivatives designated as cash flow hedges
 
 
 
 
 
 
 
 
 
 
 
Interest rate
 
$
—

 
$
—

 
$
267

(a) 
$
—

 
$
—

 
Vehicle fuel and other commodity
 
7

 
—

 
(39
)
(b) 
—

 
—

 
Total
 
$
7

 
$
—

 
$
228

 
$
—

 
$
—

 
Other derivative instruments
 
 
 
 
 
 
 
 
 
 
 
Commodity trading
 
$
—

 
$
—

 
$
—

 
$
—

 
$
3,447

(c) 
Electric commodity
 
—

 
1,127

 
—

 
447

(d) 
—

 
Natural gas commodity
 
—

 
(370
)
 
—

 
—

(e) 
—

(e) 
Total
 
$
—

 
$
757

 
$
—

 
$
447

 
$
3,447

 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Nine Months Ended Sept. 30, 2018
 
 
 
Pre-Tax Fair Value Gains (Losses) Recognized
During the Period in:
 
Pre-Tax (Gains) Losses
Reclassified into Income
During the Period from:
 
Pre-Tax Gains (Losses)
Recognized
During the Period in Income
 
(Thousands of Dollars)
 
Accumulated
Other
Comprehensive Loss
 
Regulatory
(Assets) and Liabilities
 
Accumulated
Other
Comprehensive Loss
 
Regulatory
Assets and (Liabilities)
 
 
Derivatives designated as cash flow hedges
 
 
 
 
 
 
 
 
 
 
 
Interest rate
 
$
—

 
$
—

 
$
792

(a) 
$
—

 
$
—

 
Vehicle fuel and other commodity
 
45

 
—

 
(103
)
(b) 
—

 
—

 
Total
 
$
45

 
$
—

 
$
689

 
$
—

 
$
—

 
Other derivative instruments
 
 
 
 
 
 
 
 
 
 
 
Commodity trading
 
$
—

 
$
—

 
$
—

 
$
—

 
$
11,114

(c) 
Electric commodity
 
—

 
(4,487
)
 
—

 
3,758

(d) 
—

 
Natural gas commodity
 
—

 
460

 
—

 
(521
)
(e) 
(404
)
(e) 
Total
 
$
—

 
$
(4,027
)
 
$
—

 
$
3,237

 
$
10,710

 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Three Months Ended Sept. 30, 2017
 
 
 
Pre-Tax Fair Value
Gains (Losses) Recognized
During the Period in:
 
Pre-Tax (Gains) Losses
Reclassified into Income
During the Period from:
 
Pre-Tax Gains
Recognized
During the Period in Income
 
(Thousands of Dollars)
 
Accumulated
Other
Comprehensive Loss
 
Regulatory
(Assets) and Liabilities
 
Accumulated
Other
Comprehensive Loss
 
Regulatory
Assets and (Liabilities)
 
 
Derivatives designated as cash flow hedges
 
 
 
 
 
 
 
 
 
 
 
Interest rate
 
$
—

 
$
—

 
$
612

(a) 
$
—

 
$
—

 
Vehicle fuel and other commodity
 
38

 
—

 
(11
)
(b) 
—

 
—

 
Total
 
$
38

 
$
—

 
$
601

 
$
—

 
$
—

 
Other derivative instruments
 
 
 
 
 
 
 
 
 
 
 
Commodity trading
 
$
—

 
$
—

 
$
—

 
$
—

 
$
1,493

(c) 
Electric commodity
 
—

 
20,216

 
—

 
(5,356
)
(d) 
—

 
Natural gas commodity
 
—

 
(383
)
 
—

 
—

(e) 
—

(e) 
Total
 
$
—

 
$
19,833

 
$
—

 
$
(5,356
)
 
$
1,493

 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Nine Months Ended Sept. 30, 2017
 
 
 
Pre-Tax Fair Value
Gains (Losses) Recognized
During the Period in:
 
Pre-Tax (Gains) Losses
Reclassified into Income
During the Period from:
 
Pre-Tax Gains (Losses)
Recognized
During the Period in Income
 
(Thousands of Dollars)
 
Accumulated
Other
Comprehensive Loss
 
Regulatory
(Assets) and Liabilities
 
Accumulated
Other
Comprehensive Loss
 
Regulatory
Assets and (Liabilities)
 
 
Derivatives designated as cash flow hedges
 
 
 
 
 
 
 
 
 
 
 
Interest rate
 
$
—

 
$
—

 
$
1,304

(a) 
$
—

 
$
—

 
Vehicle fuel and other commodity
 
81

 
—

 
(16
)
(b) 
—

 
—

 
Total
 
$
81

 
$
—

 
$
1,288

 
$
—

 
$
—

 
Other derivative instruments
 
 
 
 
 
 
 
 
 
 
 
Commodity trading
 
$
—

 
$
—

 
$
—

 
$
—

 
$
8,092

(c) 
Electric commodity
 
—

 
17,444

 
—

 
(9,293
)
(d) 
—

 
Natural gas commodity
 
—

 
(1,010
)
 
—

 
698

(e) 
(945
)
(e) 
Total
 
$
—

 
$
16,434

 
$
—

 
$
(8,595
)
 
$
7,147

 

(a) 
Amounts are recorded to interest charges.
(b) 
Amounts are recorded to operating and maintenance (O&M) expenses.
(c) 
Amounts are recorded to electric operating revenues. Portions of these gains and losses are subject to sharing with electric customers through margin-sharing mechanisms and deducted from gross revenue, as appropriate.
(d) 
Amounts are recorded to electric fuel and purchased power. These derivative settlement gains and losses are shared with electric customers through fuel and purchased energy cost-recovery mechanisms, and reclassified out of income as regulatory assets or liabilities, as appropriate.
(e)
Amounts are recorded to cost of natural gas sold and transported. These derivative settlement gains and losses are shared with natural gas customers through purchased natural gas cost-recovery mechanisms, and reclassified out of income as regulatory assets or liabilities, as appropriate.
Derivative Assets and Liabilities Measured at Fair Value on a Recurring Basis by Hierarchy Level
Recurring Fair Value Measurements — The following table presents for each of the fair value hierarchy levels, NSP-Minnesota’s derivative assets and liabilities measured at fair value on a recurring basis at Sept. 30, 2018:
 
 
Sept. 30, 2018
 
 
Fair Value
 
Fair Value Total
 
Counterparty Netting (b)
 
 
(Thousands of Dollars)
 
Level 1
 
Level 2
 
Level 3
 
 
 
Total
Current derivative assets
 
 
 
 
 
 
 
 
 
 
 
 
Derivatives designated as cash flow hedges:
 
 
 
 
 
 
 
 
 
 
 
 
Vehicle fuel and other commodity
 
$
—

 
$
48

 
$
—

 
$
48

 
$
—

 
$
48

Other derivative instruments:
 
 
 
 
 
 
 
 
 
 
 
 
Commodity trading
 
422

 
17,565

 
1,951

 
19,938

 
(8,162
)
 
11,776

Electric commodity
 
—

 
—

 
18,002

 
18,002

 
(172
)
 
17,830

Natural gas commodity
 
—

 
771

 
—

 
771

 
—

 
771

Total current derivative assets
 
$
422

 
$
18,384

 
$
19,953

 
$
38,759

 
$
(8,334
)
 
30,425

PPAs (a)
 
 
 
 
 
 
 
 
 
 
 
25

Current derivative instruments
 
 
 
 
 
 
 
 
 
 
 
$
30,450

Noncurrent derivative assets
 
 
 
 
 
 
 
 
 
 
 
 
Other derivative instruments:
 
 
 
 
 
 
 
 
 
 
 
 
Commodity trading
 
$
19

 
$
28,125

 
$
5,399

 
$
33,543

 
$
(11,561
)
 
$
21,982

Total noncurrent derivative assets
 
$
19

 
$
28,125

 
$
5,399

 
$
33,543

 
$
(11,561
)
 
21,982

PPAs (a)
 
 
 
 
 
 
 
 
 
 
 
101

Noncurrent derivative instruments
 
 
 
 
 
 
 
 
 
 
 
$
22,083


 
 
Sept. 30, 2018
 
 
Fair Value
 
Fair Value Total
 
Counterparty Netting (b)
 
 
(Thousands of Dollars)
 
Level 1
 
Level 2
 
Level 3
 
 
 
Total
Current derivative liabilities
 
 
 
 
 
 
 
 
 
 
 
 
Other derivative instruments:
 
 
 
 
 
 
 
 
 
 
 
 
Commodity trading
 
$
135

 
$
14,792

 
$
1,754

 
$
16,681

 
$
(11,868
)
 
$
4,813

Electric commodity
 
—

 
—

 
172

 
172

 
(172
)
 
—

Total current derivative liabilities
 
$
135

 
$
14,792

 
$
1,926

 
$
16,853

 
$
(12,040
)
 
4,813

PPAs (a)
 
 
 
 
 
 
 
 
 
 
 
13,851

Current derivative instruments
 
 
 
 
 
 
 
 
 
 
 
$
18,664

Noncurrent derivative liabilities
 
 
 
 
 
 
 
 
 
 
 
 
Other derivative instruments:
 
 
 
 
 
 
 
 
 
 
 
 
Commodity trading
 
$
2

 
$
19,507

 
$
411

 
$
19,920

 
$
(12,696
)
 
$
7,224

Total noncurrent derivative liabilities
 
$
2

 
$
19,507

 
$
411

 
$
19,920

 
$
(12,696
)
 
7,224

PPAs (a)
 
 
 
 
 
 
 
 
 
 
 
79,524

Noncurrent derivative instruments
 
 
 
 
 
 
 
 
 
 
 
$
86,748



(a) 
During 2006, NSP-Minnesota qualified these contracts under the normal purchase exception. Based on this qualification, the contracts are no longer adjusted to fair value and the previous carrying value of these contracts is being amortized over the remaining contract lives along with the offsetting regulatory assets and liabilities.
(b) 
NSP-Minnesota nets derivative instruments and related collateral in its consolidated balance sheet when supported by a legally enforceable master netting agreement, and all derivative instruments and related collateral amounts were subject to master netting agreements at Sept. 30, 2018. At Sept. 30, 2018, derivative assets and liabilities include no obligations to return cash collateral and rights to reclaim cash collateral of $4.9 million. The counterparty netting amounts presented exclude settlement receivables and payables and non-derivative amounts that may be subject to the same master netting agreements.

The following table presents for each of the fair value hierarchy levels, NSP-Minnesota’s derivative assets and liabilities measured at fair value on a recurring basis at Dec. 31, 2017:
 
 
Dec. 31, 2017
 
 
Fair Value
 
Fair Value Total
 
Counterparty Netting (b)
 
 
(Thousands of Dollars)
 
Level 1
 
Level 2
 
Level 3
 
 
 
Total
Current derivative assets
 
 
 
 
 
 
 
 
 
 
 
 
Derivatives designated as cash flow hedges:
 
 
 
 
 
 
 
 
 
 
 
 
Vehicle fuel and other commodity
 
$
—

 
$
107

 
$
—

 
$
107

 
$
—

 
$
107

Other derivative instruments:
 
 
 
 
 
 
 
 
 
 
 
 
Commodity trading
 
1,691

 
17,144

 
142

 
18,977

 
(11,744
)
 
7,233

Electric commodity
 
—

 
—

 
17,581

 
17,581

 
(425
)
 
17,156

Natural gas commodity
 
—

 
77

 
—

 
77

 
—

 
77

Total current derivative assets
 
$
1,691

 
$
17,328

 
$
17,723

 
$
36,742

 
$
(12,169
)
 
24,573

PPAs (a)
 
 
 
 
 
 
 
 
 
 
 
657

Current derivative instruments
 
 
 
 
 
 
 
 
 
 
 
$
25,230

Noncurrent derivative assets
 
 
 
 
 
 
 
 
 
 
 
 
Other derivative instruments:
 
 
 
 
 
 
 
 
 
 
 
 
Commodity trading
 
$
—

 
$
29,121

 
$
5,363

 
$
34,484

 
$
(6,502
)
 
$
27,982

Total noncurrent derivative assets
 
$
—

 
$
29,121

 
$
5,363

 
$
34,484

 
$
(6,502
)
 
27,982

PPAs (a)
 
 
 
 
 
 
 
 
 
 
 
120

Noncurrent derivative instruments
 
 
 
 
 
 
 
 
 
 
 
$
28,102


 
 
Dec. 31, 2017
 
 
Fair Value
 
Fair Value Total
 
Counterparty Netting (b)
 
 
(Thousands of Dollars)
 
Level 1
 
Level 2
 
Level 3
 
 
 
Total
Current derivative liabilities
 
 
 
 
 
 
 
 
 
 
 
 
Other derivative instruments:
 
 
 
 
 
 
 
 
 
 
 
 
Commodity trading
 
$
1,713

 
$
13,853

 
$
—

 
$
15,566

 
$
(11,974
)
 
$
3,592

Electric commodity
 
—

 
—

 
425

 
425

 
(425
)
 
—

Total current derivative liabilities
 
$
1,713

 
$
13,853

 
$
425

 
$
15,991

 
$
(12,399
)
 
3,592

PPAs (a)
 
 
 
 
 
 
 
 
 
 
 
14,105

Current derivative instruments
 
 
 
 
 
 
 
 
 
 
 
$
17,697

Noncurrent derivative liabilities
 
 
 
 
 
 
 
 
 
 
 
 
Other derivative instruments:
 
 
 
 
 
 
 
 
 
 
 
 
Commodity trading
 
$
—

 
$
22,163

 
$
—

 
$
22,163

 
$
(9,334
)
 
$
12,829

Total noncurrent derivative liabilities
 
$
—

 
$
22,163

 
$
—

 
$
22,163

 
$
(9,334
)
 
12,829

PPAs (a)
 
 
 
 
 
 
 
 
 
 
 
89,913

Noncurrent derivative instruments
 
 
 
 
 
 
 
 
 
 
 
$
102,742



(a) 
During 2006, NSP-Minnesota qualified these contracts under the normal purchase exception. Based on this qualification, the contracts are no longer adjusted to fair value and the previous carrying value of these contracts is being amortized over the remaining contract lives along with the offsetting regulatory assets and liabilities.
(b) 
NSP-Minnesota nets derivative instruments and related collateral in its consolidated balance sheet when supported by a legally enforceable master netting agreement, and all derivative instruments and related collateral amounts were subject to master netting agreements at Dec. 31, 2017. At Dec. 31, 2017, derivative assets and liabilities include no obligations to return cash collateral and rights to reclaim cash collateral of $3.1 million. The counterparty netting amounts presented exclude settlement receivables and payables and non-derivative amounts that may be subject to the same master netting agreements.
Changes in Level 3 Commodity Derivatives

The following table presents the changes in Level 3 commodity derivatives for the three and nine months ended Sept. 30, 2018 and 2017:
 
 
Three Months Ended Sept. 30
(Thousands of Dollars)
 
2018
 
2017
Balance at July 1
 
$
28,029

 
$
40,572

Settlements
 
(8,694
)
 
(23,186
)
Net transactions recorded during the period:
 
 
 
 
Gains recognized in earnings (a)
 
303

 
527

Net gains recognized as regulatory assets and liabilities
 
3,377

 
27,500

Balance at Sept. 30
 
$
23,015

 
$
45,413

 
 
 
 
 
 
 
Nine Months Ended Sept. 30
(Thousands of Dollars)
 
2018
 
2017
Balance at Jan. 1
 
$
22,661

 
$
15,320

Purchases
 
26,397

 
40,740

Settlements
 
(15,798
)
 
(34,681
)
Net transactions recorded during the period:
 
 
 
 
(Losses) gains recognized in earnings (a)
 
(70
)
 
5,742

Net (losses) gains recognized as regulatory assets and liabilities
 
(10,175
)
 
18,292

Balance at Sept. 30
 
$
23,015

 
$
45,413


(a) 
These amounts relate to commodity derivatives held at the end of the period.
Carrying Amount and Fair Value of Long-term Debt
As of Sept. 30, 2018 and Dec. 31, 2017, other financial instruments for which the carrying amount did not equal fair value were as follows:
 
 
Sept. 30, 2018
 
Dec. 31, 2017
(Thousands of Dollars)
 
Carrying
Amount
 
Fair Value
 
Carrying
Amount
 
Fair Value
Long-term debt, including current portion
 
$
4,936,135

 
$
5,159,080

 
$
4,933,018

 
$
5,601,919
v3.10.0.1
Other (Expense) Income, Net (Tables)
 9 Months Ended
Sep. 30, 2018
Other Income and Expenses [Abstract]  
Other (Expense) Income, Net
Other expense, net consisted of the following:
 
 
Three Months Ended Sept. 30
 
Nine Months Ended Sept. 30
(Thousands of Dollars)
 
2018
 
2017
 
2018
 
2017
Interest income
 
$
1,596

 
$
2,936

 
$
5,053

 
$
6,250

Other nonoperating expense
 
(722
)
 
(429
)
 
(24
)
 
(402
)
Insurance policy income (expense)
 
157

 
(387
)
 
(1,038
)
 
(2,098
)
Benefits non-service cost
 
(3,007
)
 
(3,398
)
 
(7,985
)
 
(10,194
)
Other expense, net
 
$
(1,976
)
 
$
(1,278
)
 
$
(3,994
)
 
$
(6,444
)
v3.10.0.1
Segment Information (Tables)
 9 Months Ended
Sep. 30, 2018
Segment Reporting [Abstract]  
Results from Operations by Reportable Segment
(Thousands of Dollars)
 
Regulated Electric
 
Regulated Natural Gas
 
All Other
 
Reconciling Eliminations
 
Consolidated Total
Three Months Ended Sept. 30, 2018
 
 
 
 
 
 
 
 
 
 
Operating revenues (a)(b)
 
$
1,289,498

 
$
54,625

 
$
7,631

 
$
—

 
$
1,351,754

Intersegment revenues
 
219

 
138

 
—

 
(357
)
 
—

Total revenues
 
$
1,289,717

 
$
54,763

 
$
7,631

 
$
(357
)
 
$
1,351,754

Net income (loss)
 
$
207,316

 
$
(8,416
)
 
$
2,276

 
$
—

 
$
201,176

(Thousands of Dollars)
 
Regulated Electric
 
Regulated Natural Gas
 
All Other
 
Reconciling Eliminations
 
Consolidated Total
Three Months Ended Sept. 30, 2017
 
 
 
 
 
 
 
 
 
 
Operating revenues (a)(b)
 
$
1,290,971

 
$
57,442

 
$
7,366

 
$
—

 
$
1,355,779

Intersegment revenues
 
160

 
100

 
—

 
(260
)
 
—

Total revenues
 
$
1,291,131

 
$
57,542

 
$
7,366

 
$
(260
)
 
$
1,355,779

Net income (loss)
 
$
232,078

 
$
(6,242
)
 
$
3,167

 
$
—

 
$
229,003


(a) 
Operating revenues include $123 million and $124 million of affiliate electric revenue for the three months ended Sept. 30, 2018 and 2017.
(b) 
Operating revenues include an immaterial amount of affiliate gas revenue for the three months ended Sept. 30, 2018 and 2017.

(Thousands of Dollars)
 
Regulated Electric
 
Regulated Natural Gas
 
All Other
 
Reconciling Eliminations
 
Consolidated Total
Nine Months Ended Sept. 30, 2018
 
 
 
 
 
 
 
 
 
 
Operating revenues (a)(b)
 
$
3,448,169

 
$
379,169

 
$
22,902

 
$
—

 
$
3,850,240

Intersegment revenues
 
512

 
398

 
—

 
(910
)
 
—

Total revenues
 
$
3,448,681

 
$
379,567

 
$
22,902

 
$
(910
)
 
$
3,850,240

Net income
 
$
381,913

 
$
17,428

 
$
5,962

 
$
—

 
$
405,303


(Thousands of Dollars)
 
Regulated Electric
 
Regulated Natural Gas
 
All Other
 
Reconciling Eliminations
 
Consolidated Total
Nine Months Ended Sept. 30, 2017
 
 
 
 
 
 
 
 
 
 
Operating revenues (a)(b)
 
$
3,449,780

 
$
356,631

 
$
21,448

 
$
—

 
$
3,827,859

Intersegment revenues
 
512

 
371

 
—

 
(883
)
 
—

Total revenues
 
$
3,450,292

 
$
357,002

 
$
21,448

 
$
(883
)
 
$
3,827,859

Net income
 
$
399,637

 
$
7,903

 
$
3,291

 
$
—

 
$
410,831

(a) 
Operating revenues include $357 million and $367 million of affiliate electric revenue for the nine months ended Sept. 30, 2018 and 2017.
(b) 
Operating revenues include an immaterial amount of affiliate gas revenue for the nine months ended Sept. 30, 2018 and 2017.
v3.10.0.1
Benefit Plans and Other Postretirement Benefits (Tables)
 9 Months Ended
Sep. 30, 2018
Retirement Benefits [Abstract]  
Components of Net Periodic Benefit Cost (Credit)
Components of Net Periodic Benefit Cost
 
 
Three Months Ended Sept. 30
 
 
2018
 
2017
 
2018
 
2017
(Thousands of Dollars)
 
Pension Benefits
 
Postretirement Health
Care Benefits
Service cost
 
$
6,982

 
$
6,958

 
$
42

 
$
36

Interest cost (a)
 
8,805

 
10,177

 
768

 
854

Expected return on plan assets (a)
 
(14,542
)
 
(15,016
)
 
(95
)
 
(53
)
Amortization of prior service (credit) cost (a)
 
(29
)
 
265

 
(759
)
 
(759
)
Amortization of net loss (a)
 
9,614

 
9,902

 
596

 
506

Settlement charge (b)
 
34,907

 
—

 
—

 
—

Net periodic benefit cost
 
45,737

 
12,286

 
552

 
584

Costs not recognized due to the effects of regulation
 
(42,037
)
 
(4,899
)
 
—

 
—

Net benefit cost recognized for financial reporting
 
$
3,700

 
$
7,387

 
$
552

 
$
584


 
 
Nine Months Ended Sept. 30
 
 
2018
 
2017
 
2018
 
2017
(Thousands of Dollars)
 
Pension Benefits
 
Postretirement Health Care Benefits
Service cost
 
$
20,946

 
$
20,874

 
$
128

 
$
108

Interest cost (a)
 
26,413

 
30,531

 
2,306

 
2,562

Expected return on plan assets (a)
 
(43,625
)
 
(45,050
)
 
(287
)
 
(161
)
Amortization of prior service (credit) cost (a)
 
(86
)
 
795

 
(2,277
)
 
(2,277
)
Amortization of net loss (a)
 
28,843

 
29,706

 
1,786

 
1,520

Settlement charge (b)
 
34,907

 
—

 
—

 
—

Net periodic benefit cost
 
67,398

 
36,856

 
1,656

 
1,752

Costs not recognized due to the effects of regulation
 
(47,869
)
 
(14,696
)
 
—

 
—

Net benefit cost recognized for financial reporting
 
$
19,529

 
$
22,160

 
$
1,656

 
$
1,752


(a)
The components of net periodic cost other than the service cost component are included in the line item “other expense, net” in the income statement or capitalized on the balance sheet as a regulatory asset.
(b)
A settlement charge is required when the amount of lump-sum distributions during the year is greater than the sum of the service and interest cost components of the annual net periodic pension cost. In the third quarter of 2018 as a result of lump-sum distributions during the 2018 plan year, NSP-Minnesota recorded a total pension settlement charge of $35 million, which was not recognized due to the effects of rate making. In the fourth quarter of 2017 as a result of lump-sum distributions during the 2017 plan year, NSP-Minnesota recorded a total pension settlement charge of $48 million, which was not recognized due to the effects of rate making.

v3.10.0.1
Other Comprehensive Income (Tables)
 9 Months Ended
Sep. 30, 2018
Stockholders' Equity Note [Abstract]  
Changes in Accumulated Other Comprehensive Income (Loss), Net of Tax
Changes in accumulated other comprehensive loss, net of tax, for the three and nine months ended Sept. 30, 2018 and 2017 were as follows:
 
 
Three Months Ended Sept. 30, 2018
(Thousands of Dollars)
 
Gains and
Losses on Cash Flow
Hedges
 
Unrealized
Gains on Marketable
Securities
 
Defined Benefit
Pension and
Postretirement Items
 
Total
Accumulated other comprehensive loss at July 1
 
$
(20,536
)
 
$
—

 
$
(3,663
)
 
$
(24,199
)
Other comprehensive income before reclassifications
 
5

 
—

 
—

 
5

Losses reclassified from net accumulated other comprehensive loss
 
172

 
—

 
56

 
228

Net current period other comprehensive income
 
177

 
—

 
56

 
233

Accumulated other comprehensive loss at Sept. 30
 
$
(20,359
)
 
$
—

 
$
(3,607
)
 
$
(23,966
)

 
 
Three Months Ended Sept. 30, 2017
(Thousands of Dollars)
 
Gains and
Losses on Cash Flow
Hedges
 
Unrealized
Gains on
Marketable
Securities
 
Defined Benefit
Pension and
Postretirement Items
 
Total
Accumulated other comprehensive loss at July 1
 
$
(17,775
)
 
$
105

 
$
(2,609
)
 
$
(20,279
)
Other comprehensive income before reclassifications
 
22

 
—

 
—

 
22

Losses reclassified from net accumulated other comprehensive loss
 
379

 
—

 
39

 
418

Net current period other comprehensive income
 
401

 
—

 
39

 
440

Accumulated other comprehensive loss at Sept. 30
 
$
(17,374
)
 
$
105

 
$
(2,570
)
 
$
(19,839
)
 
 
Nine Months Ended Sept. 30, 2018
(Thousands of Dollars)
 
Gains and
Losses on Cash Flow Hedges
 
Unrealized
Gains on Marketable
Securities
 
Defined Benefit
Pension and
Postretirement Items
 
Total
Accumulated other comprehensive loss at Jan. 1
 
$
(20,895
)
 
$
128

 
$
(3,770
)
 
$
(24,537
)
Other comprehensive income before reclassifications
 
32

 
—

 
—

 
32

Losses (gains) reclassified from net accumulated other comprehensive loss
 
504

 
(128
)
 
163

 
539

Net current period other comprehensive income
 
536

 
(128
)
 
163

 
571

Accumulated other comprehensive loss at Sept. 30
 
$
(20,359
)
 
$
—

 
$
(3,607
)
 
$
(23,966
)
 
 
Nine Months Ended Sept. 30, 2017
(Thousands of Dollars)
 
Gains and
Losses on Cash Flow Hedges
 
Unrealized
Gains on Marketable
Securities
 
Defined Benefit
Pension and
Postretirement Items
 
Total
Accumulated other comprehensive loss at Jan. 1
 
$
(18,208
)
 
$
105

 
$
(2,680
)
 
$
(20,783
)
Other comprehensive income before reclassifications
 
48

 
—

 
—

 
48

Losses reclassified from net accumulated other comprehensive loss
 
786

 
—

 
110

 
896

Net current period other comprehensive income
 
834

 
—

 
110

 
944

Accumulated other comprehensive loss at Sept. 30
 
$
(17,374
)
 
$
105

 
$
(2,570
)
 
$
(19,839
)
Reclassifications out of Accumulated Other Comprehensive Loss
Reclassifications from accumulated other comprehensive loss for the three and nine months ended Sept. 30, 2018 and 2017 were as follows:
 
 
Amounts Reclassified from
Accumulated Other
Comprehensive Loss
 
(Thousands of Dollars)
 
Three Months Ended Sept. 30, 2018
 
Three Months Ended Sept. 30, 2017
 
Losses (gains) on cash flow hedges:
 
 
 
 
 
Interest rate derivatives
 
$
267

(a) 
$
612

(a) 
Vehicle fuel derivatives
 
(39
)
(b) 
(11
)
(b) 
Total, pre-tax
 
228

 
601

 
Tax benefit
 
(56
)
 
(222
)
 
Total, net of tax
 
172

 
379

 
Defined benefit pension and postretirement losses:
 
 
 
 
 
Amortization of net loss
 
125

(c) 
109

(c) 
Prior service credit
 
(49
)
(c) 
(49
)
(c) 
Total, pre-tax
 
76

 
60

 
Tax benefit
 
(20
)
 
(21
)
 
Total, net of tax
 
56

 
39

 
Marketable securities:
 
 
 
 
 
Realization of gains
 
—

 
—

 
Total, pre-tax
 
—

 
—

 
Tax expense
 
—

 
—

 
Total, net of tax
 
—

 
—

 
Total amounts reclassified, net of tax
 
$
228

 
$
418

 
 
 
Amounts Reclassified from
Accumulated Other
Comprehensive Loss
 
(Thousands of Dollars)
 
Nine Months Ended Sept. 30, 2018
 
Nine Months Ended Sept. 30, 2017
 
Losses (gains) on cash flow hedges:
 
 
 
 
 
Interest rate derivatives
 
$
792

(a) 
$
1,304

(a) 
Vehicle fuel derivatives
 
(103
)
(b) 
(16
)
(b) 
Total, pre-tax
 
689

 
1,288

 
Tax benefit
 
(185
)
 
(502
)
 
Total, net of tax
 
504

 
786

 
Defined benefit pension and postretirement losses:
 
 
 
 
 
Amortization of net loss
 
375

(c) 
327

(c) 
Prior service credit
 
(147
)
(c) 
(147
)
(c) 
Total, pre-tax
 
228

 
180

 
Tax benefit
 
(65
)
 
(70
)
 
Total, net of tax
 
163

 
110

 
Marketable securities:
 
 
 
 
 
Realization of gains
 
(179
)
 
—

 
Total, pre-tax
 
(179
)
 
—

 
Tax expense
 
51

 
—

 
Total, net of tax
 
(128
)
 
—

 
Total amounts reclassified, net of tax
 
$
539

 
$
896

 

(a) 
Included in interest charges.
(b) 
Included in O&M expenses.
(c) 
Included in the computation of net periodic pension and postretirement benefit costs. See Note 11 to the consolidated financial statements for details regarding these benefit plans.
v3.10.0.1
Revenues (Tables)
 9 Months Ended
Sep. 30, 2018
Revenue from Contract with Customer [Abstract]  
Disaggregation of Revenue
In the following tables, revenue is classified by the type of goods/services rendered and market/customer type. The tables also reconcile revenue to the reportable segments.
 
 
Three Months Ended Sept. 30, 2018
(Thousands of Dollars)
 
Electric
 
Natural Gas
 
All Other
 
Total
Major revenue types
 
 
 
 
 
 
 
 
Revenue from contracts with customers:
 
 
 
 
 
 
 
 
Residential
 
$
392,334

 
$
24,863

 
$
6,999

 
$
424,196

Commercial and industrial (C&I)
 
604,301

 
22,337

 
42

 
626,680

Other
 
8,694

 
—

 
590

 
9,284

Total retail
 
1,005,329

 
47,200

 
7,631

 
1,060,160

Wholesale
 
47,708

 
—

 
—

 
47,708

Transmission
 
66,166

 
—

 
—

 
66,166

Interchange
 
122,636

 
—

 
—

 
122,636

Other
 
4,586

 
5,280

 
—

 
9,866

Total revenue from contracts with customers
 
1,246,425

 
52,480

 
7,631

 
1,306,536

Alternative revenue and other
 
43,073

 
2,145

 
—

 
45,218

Total revenues
 
$
1,289,498

 
$
54,625

 
$
7,631

 
$
1,351,754


 
 
Three Months Ended Sept. 30, 2017
(Thousands of Dollars)
 
Electric
 
Natural Gas
 
All Other
 
Total
Major revenue types
 
 
 
 
 
 
 
 
Revenue from contracts with customers:
 
 
 
 
 
 
 
 
Residential
 
$
357,209

 
$
24,475

 
$
6,791

 
$
388,475

C&I
 
626,590

 
22,389

 
39

 
649,018

Other
 
9,327

 
—

 
536

 
9,863

Total retail
 
993,126

 
46,864

 
7,366

 
1,047,356

Wholesale
 
36,598

 
—

 
—

 
36,598

Transmission
 
65,667

 
—

 
—

 
65,667

Interchange
 
123,523

 
—

 
—

 
123,523

Other
 
8,632

 
1,589

 
—

 
10,221

Total revenue from contracts with customers
 
1,227,546

 
48,453

 
7,366

 
1,283,365

Alternative revenue and other
 
63,425

 
8,989

 
—

 
72,414

Total revenues
 
$
1,290,971

 
$
57,442

 
$
7,366

 
$
1,355,779


 
 
Nine Months Ended Sept. 30, 2018
(Thousands of Dollars)
 
Electric
 
Natural Gas
 
All Other
 
Total
Major revenue types
 
 
 
 
 
 
 
 
Revenue from contracts with customers:
 
 
 
 
 
 
 
 
Residential
 
$
1,014,549

 
$
198,920

 
$
20,032

 
$
1,233,501

Commercial and industrial (C&I)
 
1,587,034

 
152,662

 
127

 
1,739,823

Other
 
27,052

 
—

 
2,743

 
29,795

Total retail
 
2,628,635

 
351,582

 
22,902

 
3,003,119

Wholesale
 
136,106

 
—

 
—

 
136,106

Transmission
 
180,915

 
—

 
—

 
180,915

Interchange
 
357,025

 
—

 
—

 
357,025

Other
 
22,164

 
10,808

 
—

 
32,972

Total revenue from contracts with customers
 
3,324,845

 
362,390

 
22,902

 
3,710,137

Alternative revenue and other
 
123,324

 
16,779

 
—

 
140,103

Total revenues
 
$
3,448,169

 
$
379,169

 
$
22,902

 
$
3,850,240


 
 
Nine Months Ended Sept. 30, 2017
(Thousands of Dollars)
 
Electric
 
Natural Gas
 
All Other
 
Total
Major revenue types
 
 
 
 
 
 
 
 
Revenue from contracts with customers:
 
 
 
 
 
 
 
 
Residential
 
$
950,355

 
$
182,748

 
$
19,417

 
$
1,152,520

C&I
 
1,638,560

 
142,262

 
169

 
1,780,991

Other
 
25,384

 
—

 
1,862

 
27,246

Total retail
 
2,614,299

 
325,010

 
21,448

 
2,960,757

Wholesale
 
111,358

 
—

 
—

 
111,358

Transmission
 
175,803

 
—

 
—

 
175,803

Interchange
 
366,596

 
—

 
—

 
366,596

Other
 
22,040

 
4,286

 
—

 
26,326

Total revenue from contracts with customers
 
3,290,096

 
329,296

 
21,448

 
3,640,840

Alternative revenue and other
 
159,684

 
27,335

 
—

 
187,019

Total revenues
 
$
3,449,780

 
$
356,631

 
$
21,448

 
$
3,827,859
v3.10.0.1
Accounting Pronouncements (Details) - Accounting Standards Update 2017-07 - USD ($)
$ in Millions
 3 Months Ended 9 Months Ended
Sep. 30, 2017
Sep. 30, 2017
Operating and Maintenance Expense    
New Accounting Pronouncements or Change in Accounting Principle [Line Items]    
Net benefit cost recognized for financial reporting $ 3.4 $ 10.2
Other Income    
New Accounting Pronouncements or Change in Accounting Principle [Line Items]    
Net benefit cost recognized for financial reporting $ (3.4) $ (10.2)
v3.10.0.1
Selected Balance Sheet Data, Accounts Receivable (Details) - USD ($)
$ in Thousands
Sep. 30, 2018
Dec. 31, 2017
Accounts receivable, net    
Accounts receivable $ 409,740 $ 366,388
Less allowance for bad debts (21,885) (21,278)
Accounts receivable, net $ 387,855 $ 345,110
v3.10.0.1
Selected Balance Sheet Data, Inventories (Details) - USD ($)
$ in Thousands
Sep. 30, 2018
Dec. 31, 2017
Public Utilities, Inventory [Line Items]    
Inventories $ 288,678 $ 337,712
Materials and supplies    
Public Utilities, Inventory [Line Items]    
Inventories 176,183 209,236
Fuel    
Public Utilities, Inventory [Line Items]    
Inventories 76,962 94,483
Natural gas    
Public Utilities, Inventory [Line Items]    
Inventories $ 35,533 $ 33,993
v3.10.0.1
Selected Balance Sheet Data, Property, Plant and Equipment (Details) - USD ($)
$ in Thousands
Sep. 30, 2018
Dec. 31, 2017
Public Utility, Property, Plant and Equipment [Line Items]    
Property, plant and equipment, gross $ 20,201,339 $ 19,649,447
Less accumulated depreciation (7,314,826) (7,018,249)
Property, plant and equipment, net 13,215,718 13,033,612
Electric plant    
Public Utility, Property, Plant and Equipment [Line Items]    
Property, plant and equipment, gross 17,363,519 17,024,925
Natural gas plant    
Public Utility, Property, Plant and Equipment [Line Items]    
Property, plant and equipment, gross 1,426,646 1,370,330
Common and other property    
Public Utility, Property, Plant and Equipment [Line Items]    
Property, plant and equipment, gross 726,127 724,066
Construction work in progress    
Public Utility, Property, Plant and Equipment [Line Items]    
Property, plant and equipment, gross 685,047 530,126
Nuclear fuel    
Public Utility, Property, Plant and Equipment [Line Items]    
Property, plant and equipment, gross 2,716,595 2,697,412
Less accumulated depreciation $ (2,387,390) $ (2,294,998)
v3.10.0.1
Income Taxes (Details) - USD ($)
 9 Months Ended 12 Months Ended
Sep. 30, 2018
Sep. 30, 2017
Sep. 30, 2015
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2012
Income Tax Examination [Line Items]            
Unrecognized Tax Benefits, Interest on Income Taxes Accrued $ (2,000,000)     $ (900,000) $ (2,000,000)  
Interest Expense related to unrecognized tax benefits $ (1,100,000)     1,100,000    
Tax Audits [Abstract]            
Effective Income Tax Rate Reconciliation, at Federal Statutory Income Tax Rate, Percent 21.00% 35.00%        
Effective Income Tax Rate Reconciliation, State and Local Income Taxes, Percent 7.10% 5.80%        
Effective Income Tax Rate Reconciliation, Tax Credit, Percent [1] (12.10%) (11.30%)        
Regulatory differences - ARAM (b) [2] (9.50%) (0.10%)        
Regulatory differences - ARAM deferral (c) [3] 7.70% 0.00%        
Regulatory differences - reversal of prior quarters’ ARAM deferral (c) [3] (7.10%) 0.00%        
Effective Income Tax Rate Reconciliation, Other Regulatory Items, Percent 0.20% (0.30%)        
Effective Income Tax Rate Reconciliation Regulatory Differences Utility Plant Items (1.40%) (1.10%)        
Effective Income Tax Rate Reconciliation, Other Adjustments, Percent 1.20% (2.50%)        
Effective Income Tax Rate Reconciliation, Percent 7.10% 25.50%        
Unrecognized Tax Benefits [Abstract]            
Unrecognized tax benefit — Permanent tax positions $ 11,400,000     10,200,000    
Unrecognized tax benefit — Temporary tax positions 5,400,000     7,900,000    
Total unrecognized tax benefit 16,800,000     18,100,000    
NOL and tax credit carryforwards (14,000,000)     (12,800,000)    
Upper bound of decrease in unrecognized tax benefit that is reasonably possible 13,000,000          
Amounts accrued for penalties related to unrecognized tax benefits $ 0     $ 0    
Internal Revenue Service (IRS)            
Tax Audits [Abstract]            
Year(s) under examination     2012 and 2013     2010 and 2011
Year of carryback claim under examination           2009
State Jurisdiction (Minnesota)            
Tax Audits [Abstract]            
Year(s) under examination         2010 through 2014  
Earliest year subject to examination 2009          
[1] Quarterly PTCs may vary due to production and timing differences. Annual 2018 PTCs are forecasted to exceed 2017.
[2] The average rate assumption method (ARAM); a method to flow back excess deferred taxes to customers.
[3] ARAM has been deferred when regulatory treatment has not been established. As NSP-Minnesota received direction from its regulatory commissions regarding the return of excess deferred taxes to customers, the ARAM deferral was reversed. This resulted in a reduction to tax expense with a corresponding reduction to revenue.
v3.10.0.1
Rate Matters (Details) - USD ($)
$ in Millions
 1 Months Ended
Oct. 26, 2018
Aug. 31, 2018
Jul. 31, 2018
Jun. 30, 2018
Sep. 30, 2016
Jun. 30, 2016
Feb. 28, 2015
Nov. 30, 2013
Rate Matters [Abstract]                
Public Utilities, PPA Termination Expected Customer Savings, Amount       $ 600        
Public Utilities, Length of Savings Period, in Years       10 years        
FERC Proceeding, MISO ROE Complaint                
Rate Matters [Abstract]                
Public Utilities, Base Return On Equity Charged To Customers Through Transmission Formula Rates             12.38% 12.38%
Public Utilities, ROE Applicable To Transmission Formula Rates In The MISO Region, Recommended By Third Parties             8.67% 9.15%
Benson purchase power agreement termination [Member]                
Rate Matters [Abstract]                
Regulatory Assets       $ 103        
Public Utilities, Purchase Power Agreement Termination Payments       $ 93        
Regulatory Asset, Amortization Period       10 years        
Laurentian purchase power agreement termination [Member]                
Rate Matters [Abstract]                
Regulatory Assets       $ 109        
Regulatory Asset, Amortization Period       6 years        
Minnesota Public Utilities Commission | MPUC Proceeding - Tax Cuts and Jobs Act of 2017 [Member]                
Rate Matters [Abstract]                
Tax Cuts and Jobs Act of 2017, Proposed Low Income Program Funding from TCJA Benefits   $ 2            
Federal Energy Regulatory Commission (FERC) | FERC Proceeding, MISO ROE Complaint                
Rate Matters [Abstract]                
Public Utilities, ROE Applicable To Transmission Formula Rates In The MISO Region, Approved         10.32%      
Public Utilities, Length of Refund Period, In Months         15 months      
Public Utilities, ROE Applicable To Transmission Formula Rates In The MISO Region, with RTO Adder, Approved         10.82%      
Public Utilities, ROE Basis Point Adder, Approved         50      
Administrative Law Judge | FERC Proceeding, MISO ROE Complaint                
Rate Matters [Abstract]                
Public Utilities, ROE Applicable To Transmission Formula Rates In The MISO Region, Recommended By Third Parties           9.70%    
South Dakota Public Utilities Commission [Member] | SDPUC Proceeding - Tax Cuts and Jobs Act of 2017 [Member]                
Rate Matters [Abstract]                
Tax Cuts and Jobs Act of 2017, Approved Refund of 2018 TCJA Benefits     $ 11          
Public Utilities, Length of Stay-out Provision, In Years     2 years          
North Dakota Public Service Commission [Member] | NDPSC Proceeding - Tax Cuts and Jobs Act of 2017 [Member]                
Rate Matters [Abstract]                
Tax Cuts and Jobs Act of 2017, Approved Amortization of Regulatory Asset for the Remediation of an MGP   1            
Regulated Natural Gas [Member] | Minnesota Public Utilities Commission | MPUC Proceeding - Tax Cuts and Jobs Act of 2017 [Member]                
Rate Matters [Abstract]                
Tax Cuts and Jobs Act of 2017, Approved Refund of 2018 TCJA Benefits   5            
Regulated Electric [Member] | Minnesota Public Utilities Commission | MPUC Proceeding - Tax Cuts and Jobs Act of 2017 [Member]                
Rate Matters [Abstract]                
Tax Cuts and Jobs Act of 2017, Approved Refund of 2018 TCJA Benefits   $ 131            
Subsequent Event [Member] | North Dakota Public Service Commission [Member] | NDPSC Proceeding - Tax Cuts and Jobs Act of 2017 [Member]                
Rate Matters [Abstract]                
Tax Cuts and Jobs Act of 2017, Approved Refund of 2018 TCJA Benefits $ 10              
Public Utilities, Length of Stay-out Provision, In Years 2 years              
Public Utilities, Approved Return on Equity, Percentage 9.85%              
v3.10.0.1
Commitments and Contingencies, Purchased Power Agreements (Details)
$ in Millions
 9 Months Ended
Sep. 30, 2018
USD ($)
MW
Dec. 31, 2017
USD ($)
MW
Independent Power Producing Entities    
Purchased Power Agreements [Abstract]    
Generating capacity under long term purchased power agreements (in MW) | MW 1,002 1,069
Purchase Power Agreement Expiration (year) 2027  
Payment or Performance Guarantee    
Purchased Power Agreements [Abstract]    
Lease Guarantee Expiration (year) 2019  
Other MGP Sites [Member]    
Purchased Power Agreements [Abstract]    
Accrual for Environmental Loss Contingencies, Gross | $ $ 3 $ 3
v3.10.0.1
Commitments and Contingencies Commitments and Contingencies, Guarantees and Indemnifications (Details) - Payment or Performance Guarantee - USD ($)
$ in Millions
 9 Months Ended
Sep. 30, 2018
Dec. 31, 2017
Guarantor Obligations [Line Items]    
Lease Guarantee Expiration (year) 2019  
Guarantees issued and outstanding $ 4.8 $ 4.8
v3.10.0.1
Commitments and Contingencies, Environmental Contingencies (Details)
$ in Millions
 9 Months Ended
Sep. 30, 2018
USD ($)
Dec. 31, 2017
USD ($)
Fargo MGP Site    
Environmental Requirements [Abstract]    
Accrual for Environmental Loss Contingencies, Gross $ 6 $ 16
Current Cost Estimate for Site Remediation 25  
Estimated amount spent on cleanup $ 19  
Percentage of Response Costs Allocable to the Minnesota Jurisdiction 88.00%  
Other MGP Sites [Member]    
Environmental Requirements [Abstract]    
Accrual for Environmental Loss Contingencies, Gross $ 3 $ 3
Number of identified MGP sites under current investigation and/or remediation in addition to those separately disclosed 5  
v3.10.0.1
Borrowings and Other Financing Instruments, Short-Term Borrowings (Details) - USD ($)
 3 Months Ended 12 Months Ended
Sep. 30, 2018
Dec. 31, 2017
Short-term Debt [Line Items]    
Amount outstanding at period end $ 24,000,000 $ 20,000,000
Money Pool    
Short-term Debt [Line Items]    
Borrowing limit 250,000,000 250,000,000
Amount outstanding at period end 0 85,000,000
Average amount outstanding 34,000,000 25,000,000
Maximum amount outstanding $ 143,000,000 $ 142,000,000
Weighted average interest rate, computed on a daily basis (percentage) 1.97% 1.14%
Weighted average interest rate at period end (percentage)   1.18%
Commercial Paper    
Short-term Debt [Line Items]    
Borrowing limit $ 500,000,000 $ 500,000,000
Amount outstanding at period end 24,000,000 20,000,000
Average amount outstanding 6,000,000 62,000,000
Maximum amount outstanding $ 73,000,000 $ 237,000,000
Weighted average interest rate, computed on a daily basis (percentage) 2.12% 1.10%
Weighted average interest rate at period end (percentage) 2.30% 1.93%
v3.10.0.1
Borrowings and Other Financing Instruments, Letters of Credit (Details) - USD ($)
$ in Thousands
 9 Months Ended
Sep. 30, 2018
Dec. 31, 2017
Line of Credit Facility [Line Items]    
Amount outstanding at period end $ 24,000 $ 20,000
Letter of Credit    
Line of Credit Facility [Line Items]    
Amount outstanding at period end $ 37,000 $ 24,000
Letter of Credit | Letter of Credit    
Line of Credit Facility [Line Items]    
Term of letters of credit (in years) 1 year  
v3.10.0.1
Borrowings and Other Financing Instruments, Credit Facility (Details) - Credit Facility - USD ($)
 9 Months Ended
Sep. 30, 2018
Dec. 31, 2017
Line of Credit Facility [Line Items]    
Credit Facility [1] $ 500,000,000  
Drawn [2] 61,000,000  
Available $ 439,000,000  
Maturity Date Jun. 30, 2021  
Direct advances on the credit facility outstanding $ 0 $ 0
[1] This credit facility expires in June 2021.
[2] Includes outstanding letters of credit.
v3.10.0.1
Fair Value of Financial Assets and Liabilities Fair Value of Financial Assets and Liabilities (Details)
 9 Months Ended
Sep. 30, 2018
Minimum | Commingled and international equity funds  
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]  
Notice period for investment redemption (in days) 1 day
Minimum | Real Estate Funds  
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]  
Notice period for investment redemption (in days) 45 days
Maximum | Commingled and international equity funds  
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]  
Notice period for investment redemption (in days) 90 days
Maximum | Real Estate Funds  
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]  
Notice period for investment redemption (in days) 90 days
v3.10.0.1
Fair Value of Financial Assets and Liabilities, Cost and Fair Value of Nuclear Decommissioning Fund (Details) - USD ($)
$ in Thousands
 9 Months Ended 12 Months Ended
Sep. 30, 2018
Dec. 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Unrealized Gain on Securities $ 600,000 $ 560,000
Unrealized Loss on Securities 22,000 7,000
Investments [Abstract]    
Miscellaneous investments 52,000 49,000
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Cost    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Cash equivalents 33,040 [1] 28,741 [2]
Investments [Abstract]    
Equity Securities 1,633,349 [1] 1,590,849 [2]
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Cost | Non U.S. equities    
Investments [Abstract]    
Equity Securities 262,345 [1] 263,694 [2]
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Cost | Emerging market debt funds    
Investments [Abstract]    
Equity Securities 162,835 [1] 156,057 [2]
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Cost | Private equity investments    
Investments [Abstract]    
Equity Securities 169,676 [1] 141,413 [2]
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Cost | Real estate    
Investments [Abstract]    
Equity Securities 125,422 [1] 130,787 [2]
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Cost | Other commingled funds    
Investments [Abstract]    
Equity Securities [2]   9,340
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Cost | Government securities    
Investments [Abstract]    
Debt Securities, Available-for-sale 75,799 [1] 67,760 [2]
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Cost | U.S. corporate bonds    
Investments [Abstract]    
Debt Securities, Available-for-sale 334,219 [1] 319,809 [2]
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Cost | Non U.S. corporate bonds    
Investments [Abstract]    
Debt Securities, Available-for-sale 55,856 [1] 50,121 [2]
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Cost | U.S. equities    
Investments [Abstract]    
Equity Securities 257,932 [1] 271,166 [2]
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Cost | Non U.S. equities    
Investments [Abstract]    
Equity Securities 156,225 [1] 151,961 [2]
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Fair Value    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Cash equivalents 33,040 [1] 28,741 [2]
Alternative Investment 704,501 [1],[3] 659,086 [2],[4]
Investments [Abstract]    
Equity Securities 2,210,912 [1] 2,143,281 [2]
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Fair Value | Cash equivalents    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Alternative Investment 0 [1],[3] 0 [2],[4]
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Fair Value | Non U.S. equities    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Alternative Investment 91,426 [1],[3] 89,857 [2],[4]
Investments [Abstract]    
Equity Securities 287,385 [1] 306,408 [2]
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Fair Value | Emerging market debt funds    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Alternative Investment 164,772 [1],[3] 166,375 [2],[4]
Investments [Abstract]    
Equity Securities 164,772 [1] 166,375 [2]
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Fair Value | Private equity investments    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Alternative Investment 249,881 [1],[3] 198,037 [2],[4]
Investments [Abstract]    
Equity Securities 249,881 [1] 198,037 [2]
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Fair Value | Real estate    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Alternative Investment 198,422 [1],[3] 201,842 [2],[4]
Investments [Abstract]    
Equity Securities 198,422 [1] 201,842 [2]
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Fair Value | Other commingled funds    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Alternative Investment [2],[4]   2,975
Investments [Abstract]    
Equity Securities [2]   9,261
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Fair Value | Government securities    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Alternative Investment 0 [1],[3] 0 [2],[4]
Investments [Abstract]    
Debt Securities, Available-for-sale 73,317 [1] 69,413 [2]
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Fair Value | U.S. corporate bonds    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Alternative Investment 0 [1],[3] 0 [2],[4]
Investments [Abstract]    
Debt Securities, Available-for-sale 329,727 [1] 322,129 [2]
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Fair Value | Non U.S. corporate bonds    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Alternative Investment 0 [1],[3] 0 [2],[4]
Investments [Abstract]    
Debt Securities, Available-for-sale 54,813 [1] 50,102 [2]
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Fair Value | U.S. equities    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Alternative Investment 0 [1],[3] 0 [2],[4]
Investments [Abstract]    
Equity Securities 590,310 [1] 556,974 [2]
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Fair Value | Non U.S. equities    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Alternative Investment 0 [1],[3] 0 [2],[4]
Investments [Abstract]    
Equity Securities 229,245 [1] 233,999 [2]
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Fair Value | Level 1    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Cash equivalents 33,040 [1] 28,741 [2]
Investments [Abstract]    
Equity Securities 1,048,554 [1] 1,042,551 [2]
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Fair Value | Level 1 | Non U.S. equities    
Investments [Abstract]    
Equity Securities 195,959 [1] 216,551 [2]
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Fair Value | Level 1 | Emerging market debt funds    
Investments [Abstract]    
Equity Securities 0 [1] 0 [2]
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Fair Value | Level 1 | Private equity investments    
Investments [Abstract]    
Equity Securities 0 [1] 0 [2]
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Fair Value | Level 1 | Real estate    
Investments [Abstract]    
Equity Securities 0 [1] 0 [2]
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Fair Value | Level 1 | Other commingled funds    
Investments [Abstract]    
Equity Securities [2]   6,286
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Fair Value | Level 1 | Government securities    
Investments [Abstract]    
Debt Securities, Available-for-sale 0 [1] 0 [2]
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Fair Value | Level 1 | U.S. corporate bonds    
Investments [Abstract]    
Debt Securities, Available-for-sale 0 [1] 0 [2]
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Fair Value | Level 1 | Non U.S. corporate bonds    
Investments [Abstract]    
Debt Securities, Available-for-sale 0 [1] 0 [2]
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Fair Value | Level 1 | U.S. equities    
Investments [Abstract]    
Equity Securities 590,310 [1] 556,974 [2]
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Fair Value | Level 1 | Non U.S. equities    
Investments [Abstract]    
Equity Securities 229,245 [1] 233,999 [2]
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Fair Value | Level 2    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Cash equivalents 0 [1] 0 [2]
Investments [Abstract]    
Equity Securities 457,857 [1] 441,644 [2]
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Fair Value | Level 2 | Non U.S. equities    
Investments [Abstract]    
Equity Securities 0 [1] 0 [2]
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Fair Value | Level 2 | Emerging market debt funds    
Investments [Abstract]    
Equity Securities 0 [1] 0 [2]
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Fair Value | Level 2 | Private equity investments    
Investments [Abstract]    
Equity Securities 0 [1] 0 [2]
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Fair Value | Level 2 | Real estate    
Investments [Abstract]    
Equity Securities 0 [1] 0 [2]
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Fair Value | Level 2 | Other commingled funds    
Investments [Abstract]    
Equity Securities [2]   0
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Fair Value | Level 2 | Government securities    
Investments [Abstract]    
Debt Securities, Available-for-sale 73,317 [1] 69,413 [2]
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Fair Value | Level 2 | U.S. corporate bonds    
Investments [Abstract]    
Debt Securities, Available-for-sale 329,727 [1] 322,129 [2]
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Fair Value | Level 2 | Non U.S. corporate bonds    
Investments [Abstract]    
Debt Securities, Available-for-sale 54,813 [1] 50,102 [2]
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Fair Value | Level 2 | U.S. equities    
Investments [Abstract]    
Equity Securities 0 [1] 0 [2]
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Fair Value | Level 2 | Non U.S. equities    
Investments [Abstract]    
Equity Securities 0 [1] 0 [2]
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Fair Value | Level 3    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Cash equivalents 0 [1] 0 [2]
Investments [Abstract]    
Equity Securities 0 [1] 0 [2]
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Fair Value | Level 3 | Non U.S. equities    
Investments [Abstract]    
Equity Securities 0 [1] 0 [2]
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Fair Value | Level 3 | Emerging market debt funds    
Investments [Abstract]    
Equity Securities 0 [1] 0 [2]
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Fair Value | Level 3 | Private equity investments    
Investments [Abstract]    
Equity Securities 0 [1] 0 [2]
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Fair Value | Level 3 | Real estate    
Investments [Abstract]    
Equity Securities 0 [1] 0 [2]
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Fair Value | Level 3 | Other commingled funds    
Investments [Abstract]    
Equity Securities [2]   0
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Fair Value | Level 3 | Government securities    
Investments [Abstract]    
Debt Securities, Available-for-sale 0 [1] 0 [2]
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Fair Value | Level 3 | U.S. corporate bonds    
Investments [Abstract]    
Debt Securities, Available-for-sale 0 [1] 0 [2]
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Fair Value | Level 3 | Non U.S. corporate bonds    
Investments [Abstract]    
Debt Securities, Available-for-sale 0 [1] 0 [2]
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Fair Value | Level 3 | U.S. equities    
Investments [Abstract]    
Equity Securities 0 [1] 0 [2]
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Fair Value | Level 3 | Non U.S. equities    
Investments [Abstract]    
Equity Securities $ 0 [1] $ 0 [2]
[1] Reported in nuclear decommissioning fund and other investments on the consolidated balance sheet, which also includes $52 million of rabbi trust assets and miscellaneous investments.
[2] Reported in nuclear decommissioning fund and other investments on the consolidated balance sheet, which also includes $49 million of rabbi trust assets and miscellaneous investments.
[3] Due to limited availability of published pricing and a lack of immediate redeemability, certain fund investments measured at NAV are not required to be categorized within the fair value hierarchy.
[4] Due to limited availability of published pricing and a lack of immediate redeemability, certain fund investments measured at NAV are not required to be categorized within the fair value hierarchy.
v3.10.0.1
Fair Value of Financial Assets and Liabilities, Final Contractual Maturity Dates of Debt Securities in Nuclear Decommissioning Fund (Details)
$ in Thousands
Sep. 30, 2018
USD ($)
Final Contractual Maturity [Abstract]  
Due in 1 Year or Less $ 14,757
Due in 1 to 5 Years 111,480
Due in 5 to 10 Years 205,563
Due after 10 Years 126,057
Total 457,857
Government securities  
Final Contractual Maturity [Abstract]  
Due in 1 Year or Less 0
Due in 1 to 5 Years 0
Due in 5 to 10 Years 1,750
Due after 10 Years 71,567
Total 73,317
U.S. corporate bonds  
Final Contractual Maturity [Abstract]  
Due in 1 Year or Less 12,759
Due in 1 to 5 Years 90,982
Due in 5 to 10 Years 175,662
Due after 10 Years 50,324
Total 329,727
Non U.S. corporate bonds  
Final Contractual Maturity [Abstract]  
Due in 1 Year or Less 1,998
Due in 1 to 5 Years 20,498
Due in 5 to 10 Years 28,151
Due after 10 Years 4,166
Total $ 54,813
v3.10.0.1
Fair Value of Financial Assets and Liabilities Fair Value of Financial Assets and Liabilities, Cost and Fair Value of Rabbi Trust (Details) - Fair Value Measured on a Recurring Basis - USD ($)
$ in Thousands
Sep. 30, 2018
Dec. 31, 2017
Cost    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Debt Securities, Trading, and Equity Securities, FV-NI [1] $ 12,795 $ 11,115
Cost | Rabbi Trust [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Cash equivalents [1] 2,321 783
Cost | Mutual Funds [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Debt Securities, Trading, and Equity Securities, FV-NI [1] 10,474 10,332
Fair Value    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Debt Securities, Trading, and Equity Securities, FV-NI [1] 14,069 12,066
Fair Value | Level 1    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Debt Securities, Trading, and Equity Securities, FV-NI [1] 14,069 12,066
Fair Value | Level 2    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Debt Securities, Trading, and Equity Securities, FV-NI [1] 0 0
Fair Value | Level 3    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Debt Securities, Trading, and Equity Securities, FV-NI [1] 0 0
Fair Value | Rabbi Trust [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Cash equivalents [1] 2,321 783
Fair Value | Rabbi Trust [Member] | Level 1    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Cash equivalents [1] 2,321 783
Fair Value | Rabbi Trust [Member] | Level 2    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Cash equivalents [1] 0 0
Fair Value | Rabbi Trust [Member] | Level 3    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Cash equivalents [1] 0 0
Fair Value | Mutual Funds [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Debt Securities, Trading, and Equity Securities, FV-NI [1] 11,748 11,283
Fair Value | Mutual Funds [Member] | Level 1    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Debt Securities, Trading, and Equity Securities, FV-NI [1] 11,748 11,283
Fair Value | Mutual Funds [Member] | Level 2    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Debt Securities, Trading, and Equity Securities, FV-NI [1] 0 0
Fair Value | Mutual Funds [Member] | Level 3    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Debt Securities, Trading, and Equity Securities, FV-NI [1] $ 0 $ 0
[1] Reported in nuclear decommissioning fund and other investments on the consolidated balance sheet.
v3.10.0.1
Fair Value of Financial Assets and Liabilities, Derivative Instruments (Details)
gal in Thousands, MWh in Thousands, MMBTU in Thousands, $ in Millions
Sep. 30, 2018
USD ($)
gal
MMBTU
MWh
Counterparty
Dec. 31, 2017
gal
MMBTU
MWh
Credit Concentration Risk    
Consideration of Credit Risk and Concentrations [Abstract]    
Number of most significant counterparties for wholesale, trading and non-trading commodity activities with credit exposure 10  
Credit Concentration Risk | Municipal or Cooperative Entities or Other Utilities [Member]    
Consideration of Credit Risk and Concentrations [Abstract]    
Number of most significant counterparties for wholesale, trading and non-trading commodity activities with credit exposure 10  
Credit Concentration Risk | External Credit Rating, Investment Grade [Member]    
Consideration of Credit Risk and Concentrations [Abstract]    
Number of most significant counterparties for wholesale, trading and non-trading commodity activities with credit exposure 8  
Wholesale, trading and non-trading commodity credit exposure for the most significant counterparties | $ $ 41.8  
Percentage of wholesale, trading and non-trading commodity credit exposure for the most significant counterparties (in hundredths) 51.00%  
Credit Concentration Risk | No Investment Grade Ratings from External Credit Rating Agencies [Member]    
Consideration of Credit Risk and Concentrations [Abstract]    
Number of most significant counterparties for wholesale, trading and non-trading commodity activities with credit exposure 2  
Wholesale, trading and non-trading commodity credit exposure for the most significant counterparties | $ $ 14.0  
Percentage of wholesale, trading and non-trading commodity credit exposure for the most significant counterparties (in hundredths) 17.00%  
Interest Rate Derivatives    
Interest Rate Derivatives [Abstract]    
Amount of accumulated other comprehensive gains (losses) related to interest rate derivatives expected to be reclassified into earnings within the next twelve months | $ $ (0.8)  
Electric Commodity (in megawatt hours)    
Gross Notional Amounts of Commodity Forwards, Options and FTRs [Abstract]    
Derivative, Nonmonetary Notional amount | MWh [1],[2] 61,746 41,711
Natural Gas Commodity (in million British thermal units)    
Gross Notional Amounts of Commodity Forwards, Options and FTRs [Abstract]    
Derivative, Nonmonetary Notional amount | MMBTU [1],[2] 11,131 23,829
Vehicle Fuel Commodity (in gallons)    
Gross Notional Amounts of Commodity Forwards, Options and FTRs [Abstract]    
Derivative, Nonmonetary Notional amount | gal [1],[2] 60 240
[1] Amounts are not reflective of net positions in the underlying commodities.
[2] Notional amounts for options are included on a gross basis, but are weighted for the probability of exercise.
v3.10.0.1
Fair Value of Financial Assets and Liabilities, Impact of Derivative Activity (Details) - USD ($)
 3 Months Ended 9 Months Ended
Sep. 30, 2018
Sep. 30, 2017
Sep. 30, 2018
Sep. 30, 2017
Financial Impact of Qualifying Fair Value Hedges on Earnings [Abstract]        
Derivative instruments designated as fair value hedges $ 0 $ 0 $ 0 $ 0
Recognized gains (losses) from fair value hedges or related hedged transactions 0 0 0 0
Designated as Hedging Instrument | Cash Flow Hedges        
Impact of Derivative Activity on Accumulated Other Comprehensive Loss, Regulatory Assets and Liabilities, and Income [Abstract]        
Pre-tax fair value gains (losses) recognized during the period in accumulated other comprehensive loss 7,000 38,000 45,000 81,000
Pre-tax fair value gains (losses) recognized during the period in regulatory (assets) and liabilities 0 0 0 0
Pre-tax (gains) losses reclassified into income during the period from accumulated other comprehensive loss 228,000 601,000 689,000 1,288,000
Pre-tax (gains) losses reclassified into income during the period from regulatory assets and (liabilities) 0 0 0 0
Pre-tax gains (losses) recognized during the period in income 0 0 0 0
Designated as Hedging Instrument | Cash Flow Hedges | Interest Rate        
Impact of Derivative Activity on Accumulated Other Comprehensive Loss, Regulatory Assets and Liabilities, and Income [Abstract]        
Pre-tax fair value gains (losses) recognized during the period in accumulated other comprehensive loss 0 0 0 0
Pre-tax fair value gains (losses) recognized during the period in regulatory (assets) and liabilities 0 0 0 0
Pre-tax (gains) losses reclassified into income during the period from accumulated other comprehensive loss [1] 267,000 612,000 792,000 1,304,000
Pre-tax (gains) losses reclassified into income during the period from regulatory assets and (liabilities) 0 0 0 0
Pre-tax gains (losses) recognized during the period in income 0 0 0 0
Designated as Hedging Instrument | Cash Flow Hedges | Vehicle Fuel And Other Commodity        
Impact of Derivative Activity on Accumulated Other Comprehensive Loss, Regulatory Assets and Liabilities, and Income [Abstract]        
Pre-tax fair value gains (losses) recognized during the period in accumulated other comprehensive loss 7,000 38,000 45,000 81,000
Pre-tax fair value gains (losses) recognized during the period in regulatory (assets) and liabilities 0 0 0 0
Pre-tax (gains) losses reclassified into income during the period from accumulated other comprehensive loss [2] (39,000) (11,000) (103,000) (16,000)
Pre-tax (gains) losses reclassified into income during the period from regulatory assets and (liabilities) 0 0 0 0
Pre-tax gains (losses) recognized during the period in income 0 0 0 0
Other Derivative Instruments        
Impact of Derivative Activity on Accumulated Other Comprehensive Loss, Regulatory Assets and Liabilities, and Income [Abstract]        
Pre-tax fair value gains (losses) recognized during the period in accumulated other comprehensive loss 0 0 0 0
Pre-tax fair value gains (losses) recognized during the period in regulatory (assets) and liabilities 757,000 19,833,000 (4,027,000) 16,434,000
Pre-tax (gains) losses reclassified into income during the period from accumulated other comprehensive loss 0 0 0 0
Pre-tax (gains) losses reclassified into income during the period from regulatory assets and (liabilities) 447,000 (5,356,000) 3,237,000 (8,595,000)
Pre-tax gains (losses) recognized during the period in income 3,447,000 1,493,000 10,710,000 7,147,000
Other Derivative Instruments | Commodity Trading        
Impact of Derivative Activity on Accumulated Other Comprehensive Loss, Regulatory Assets and Liabilities, and Income [Abstract]        
Pre-tax fair value gains (losses) recognized during the period in accumulated other comprehensive loss 0 0 0 0
Pre-tax fair value gains (losses) recognized during the period in regulatory (assets) and liabilities 0 0 0 0
Pre-tax (gains) losses reclassified into income during the period from accumulated other comprehensive loss 0 0 0 0
Pre-tax (gains) losses reclassified into income during the period from regulatory assets and (liabilities) 0 0 0 0
Pre-tax gains (losses) recognized during the period in income [3] 3,447,000 1,493,000 11,114,000 8,092,000
Other Derivative Instruments | Electric Commodity        
Impact of Derivative Activity on Accumulated Other Comprehensive Loss, Regulatory Assets and Liabilities, and Income [Abstract]        
Pre-tax fair value gains (losses) recognized during the period in accumulated other comprehensive loss 0 0 0 0
Pre-tax fair value gains (losses) recognized during the period in regulatory (assets) and liabilities 1,127,000 20,216,000 (4,487,000) 17,444,000
Pre-tax (gains) losses reclassified into income during the period from accumulated other comprehensive loss 0 0 0 0
Pre-tax (gains) losses reclassified into income during the period from regulatory assets and (liabilities) [4] 447,000 (5,356,000) 3,758,000 (9,293,000)
Pre-tax gains (losses) recognized during the period in income 0 0 0 0
Other Derivative Instruments | Natural Gas Commodity        
Impact of Derivative Activity on Accumulated Other Comprehensive Loss, Regulatory Assets and Liabilities, and Income [Abstract]        
Pre-tax fair value gains (losses) recognized during the period in accumulated other comprehensive loss 0 0 0 0
Pre-tax fair value gains (losses) recognized during the period in regulatory (assets) and liabilities (370,000) (383,000) 460,000 (1,010,000)
Pre-tax (gains) losses reclassified into income during the period from accumulated other comprehensive loss 0 0 0 0
Pre-tax (gains) losses reclassified into income during the period from regulatory assets and (liabilities) 0 0 (521,000) [5] 698,000 [5]
Pre-tax gains (losses) recognized during the period in income $ 0 $ 0 $ (404,000) [5] $ (945,000) [5]
[1] Amounts are recorded to interest charges.
[2] Amounts are recorded to operating and maintenance (O&M) expenses.
[3] Amounts are recorded to electric operating revenues. Portions of these gains and losses are subject to sharing with electric customers through margin-sharing mechanisms and deducted from gross revenue, as appropriate.
[4] Amounts are recorded to electric fuel and purchased power. These derivative settlement gains and losses are shared with electric customers through fuel and purchased energy cost-recovery mechanisms, and reclassified out of income as regulatory assets or liabilities, as appropriate.
[5] Amounts are recorded to cost of natural gas sold and transported. These derivative settlement gains and losses are shared with natural gas customers through purchased natural gas cost-recovery mechanisms, and reclassified out of income as regulatory assets or liabilities, as appropriate.
v3.10.0.1
Fair Value of Financial Assets and Liabilities, Credit Related Contingent Features (Details) - USD ($)
Sep. 30, 2018
Dec. 31, 2017
Fair Value Disclosures [Abstract]    
Derivative instruments in a gross liability position $ 0 $ 0
Collateral posted on derivative instruments 0 0
Collateral posted related to adequate assurance clauses in derivative contracts $ 0 $ 0
v3.10.0.1
Fair Value of Financial Assets and Liabilities, Derivative Assets and Liabilities at Fair Value (Details) - USD ($)
$ in Thousands
Sep. 30, 2018
Dec. 31, 2017
Derivatives, Fair Value [Line Items]    
Derivative Asset, Collateral, Obligation to Return Cash, Offset $ 0 $ 0
Derivative Liability, Collateral, Right to Reclaim Cash, Offset 4,900 3,100
Other Current Assets    
Derivatives, Fair Value [Line Items]    
Derivative Asset, Fair Value, Gross Asset 30,450 25,230
Other Noncurrent Assets    
Derivatives, Fair Value [Line Items]    
Derivative Asset, Fair Value, Gross Asset 22,083 28,102
Other Current Liabilities    
Derivatives, Fair Value [Line Items]    
Derivative Liability, Fair Value, Gross Liability 18,664 17,697
Other Noncurrent Liabilities    
Derivatives, Fair Value [Line Items]    
Derivative Liability, Fair Value, Gross Liability 86,748 102,742
Fair Value Measured on a Recurring Basis | Other Current Assets    
Derivatives, Fair Value [Line Items]    
Derivative Asset, Fair Value, Gross Asset 30,425 24,573
Derivative Asset, Fair Value, Gross Liability and Obligation to Return Cash, Offset (8,334) [1] (12,169) [2]
Fair Value Measured on a Recurring Basis | Other Current Assets | Other Derivative Instruments | Commodity Trading    
Derivatives, Fair Value [Line Items]    
Derivative Asset, Fair Value, Gross Asset 11,776 7,233
Derivative Asset, Fair Value, Gross Liability and Obligation to Return Cash, Offset (8,162) [1] (11,744) [2]
Fair Value Measured on a Recurring Basis | Other Current Assets | Other Derivative Instruments | Electric Commodity    
Derivatives, Fair Value [Line Items]    
Derivative Asset, Fair Value, Gross Asset 17,830 17,156
Derivative Asset, Fair Value, Gross Liability and Obligation to Return Cash, Offset (172) [1] (425) [2]
Fair Value Measured on a Recurring Basis | Other Current Assets | Other Derivative Instruments | Natural Gas Commodity    
Derivatives, Fair Value [Line Items]    
Derivative Asset, Fair Value, Gross Asset 771 77
Derivative Asset, Fair Value, Gross Liability and Obligation to Return Cash, Offset 0 [1] 0 [2]
Fair Value Measured on a Recurring Basis | Other Noncurrent Assets    
Derivatives, Fair Value [Line Items]    
Derivative Asset, Fair Value, Gross Asset 21,982 27,982
Derivative Asset, Fair Value, Gross Liability and Obligation to Return Cash, Offset (11,561) [1] (6,502) [2]
Fair Value Measured on a Recurring Basis | Other Noncurrent Assets | Other Derivative Instruments | Commodity Trading    
Derivatives, Fair Value [Line Items]    
Derivative Asset, Fair Value, Gross Asset 21,982 27,982
Derivative Asset, Fair Value, Gross Liability and Obligation to Return Cash, Offset (11,561) [1] (6,502) [2]
Fair Value Measured on a Recurring Basis | Other Current Liabilities    
Derivatives, Fair Value [Line Items]    
Derivative Liability, Fair Value, Gross Liability 4,813 3,592
Derivative Liability, Fair Value, Gross Asset and Right to Reclaim Cash, Offset (12,040) [1] (12,399) [2]
Fair Value Measured on a Recurring Basis | Other Current Liabilities | Other Derivative Instruments | Commodity Trading    
Derivatives, Fair Value [Line Items]    
Derivative Liability, Fair Value, Gross Liability 4,813 3,592
Derivative Liability, Fair Value, Gross Asset and Right to Reclaim Cash, Offset (11,868) [1] (11,974) [2]
Fair Value Measured on a Recurring Basis | Other Current Liabilities | Other Derivative Instruments | Electric Commodity    
Derivatives, Fair Value [Line Items]    
Derivative Liability, Fair Value, Gross Liability 0 0
Derivative Liability, Fair Value, Gross Asset and Right to Reclaim Cash, Offset (172) [1] (425) [2]
Fair Value Measured on a Recurring Basis | Other Noncurrent Liabilities    
Derivatives, Fair Value [Line Items]    
Derivative Liability, Fair Value, Gross Liability 7,224 12,829
Derivative Liability, Fair Value, Gross Asset and Right to Reclaim Cash, Offset (12,696) [1] (9,334) [2]
Fair Value Measured on a Recurring Basis | Other Noncurrent Liabilities | Other Derivative Instruments | Commodity Trading    
Derivatives, Fair Value [Line Items]    
Derivative Liability, Fair Value, Gross Liability 7,224 12,829
Derivative Liability, Fair Value, Gross Asset and Right to Reclaim Cash, Offset (12,696) [1] (9,334) [2]
Fair Value Measured on a Recurring Basis | Level 1 | Other Current Assets    
Derivatives, Fair Value [Line Items]    
Derivative Asset, Fair Value, Gross Asset 422 1,691
Fair Value Measured on a Recurring Basis | Level 1 | Other Current Assets | Other Derivative Instruments | Commodity Trading    
Derivatives, Fair Value [Line Items]    
Derivative Asset, Fair Value, Gross Asset 422 1,691
Fair Value Measured on a Recurring Basis | Level 1 | Other Current Assets | Other Derivative Instruments | Electric Commodity    
Derivatives, Fair Value [Line Items]    
Derivative Asset, Fair Value, Gross Asset 0 0
Fair Value Measured on a Recurring Basis | Level 1 | Other Current Assets | Other Derivative Instruments | Natural Gas Commodity    
Derivatives, Fair Value [Line Items]    
Derivative Asset, Fair Value, Gross Asset 0 0
Fair Value Measured on a Recurring Basis | Level 1 | Other Noncurrent Assets    
Derivatives, Fair Value [Line Items]    
Derivative Asset, Fair Value, Gross Asset 19 0
Fair Value Measured on a Recurring Basis | Level 1 | Other Noncurrent Assets | Other Derivative Instruments | Commodity Trading    
Derivatives, Fair Value [Line Items]    
Derivative Asset, Fair Value, Gross Asset 19 0
Fair Value Measured on a Recurring Basis | Level 1 | Other Current Liabilities    
Derivatives, Fair Value [Line Items]    
Derivative Liability, Fair Value, Gross Liability 135 1,713
Fair Value Measured on a Recurring Basis | Level 1 | Other Current Liabilities | Other Derivative Instruments | Commodity Trading    
Derivatives, Fair Value [Line Items]    
Derivative Liability, Fair Value, Gross Liability 135 1,713
Fair Value Measured on a Recurring Basis | Level 1 | Other Current Liabilities | Other Derivative Instruments | Electric Commodity    
Derivatives, Fair Value [Line Items]    
Derivative Liability, Fair Value, Gross Liability 0 0
Fair Value Measured on a Recurring Basis | Level 1 | Other Noncurrent Liabilities    
Derivatives, Fair Value [Line Items]    
Derivative Liability, Fair Value, Gross Liability 2 0
Fair Value Measured on a Recurring Basis | Level 1 | Other Noncurrent Liabilities | Other Derivative Instruments | Commodity Trading    
Derivatives, Fair Value [Line Items]    
Derivative Liability, Fair Value, Gross Liability 2 0
Fair Value Measured on a Recurring Basis | Level 2 | Other Current Assets    
Derivatives, Fair Value [Line Items]    
Derivative Asset, Fair Value, Gross Asset 18,384 17,328
Fair Value Measured on a Recurring Basis | Level 2 | Other Current Assets | Other Derivative Instruments | Commodity Trading    
Derivatives, Fair Value [Line Items]    
Derivative Asset, Fair Value, Gross Asset 17,565 17,144
Fair Value Measured on a Recurring Basis | Level 2 | Other Current Assets | Other Derivative Instruments | Electric Commodity    
Derivatives, Fair Value [Line Items]    
Derivative Asset, Fair Value, Gross Asset 0 0
Fair Value Measured on a Recurring Basis | Level 2 | Other Current Assets | Other Derivative Instruments | Natural Gas Commodity    
Derivatives, Fair Value [Line Items]    
Derivative Asset, Fair Value, Gross Asset 771 77
Fair Value Measured on a Recurring Basis | Level 2 | Other Noncurrent Assets    
Derivatives, Fair Value [Line Items]    
Derivative Asset, Fair Value, Gross Asset 28,125 29,121
Fair Value Measured on a Recurring Basis | Level 2 | Other Noncurrent Assets | Other Derivative Instruments | Commodity Trading    
Derivatives, Fair Value [Line Items]    
Derivative Asset, Fair Value, Gross Asset 28,125 29,121
Fair Value Measured on a Recurring Basis | Level 2 | Other Current Liabilities    
Derivatives, Fair Value [Line Items]    
Derivative Liability, Fair Value, Gross Liability 14,792 13,853
Fair Value Measured on a Recurring Basis | Level 2 | Other Current Liabilities | Other Derivative Instruments | Commodity Trading    
Derivatives, Fair Value [Line Items]    
Derivative Liability, Fair Value, Gross Liability 14,792 13,853
Fair Value Measured on a Recurring Basis | Level 2 | Other Current Liabilities | Other Derivative Instruments | Electric Commodity    
Derivatives, Fair Value [Line Items]    
Derivative Liability, Fair Value, Gross Liability 0 0
Fair Value Measured on a Recurring Basis | Level 2 | Other Noncurrent Liabilities    
Derivatives, Fair Value [Line Items]    
Derivative Liability, Fair Value, Gross Liability 19,507 22,163
Fair Value Measured on a Recurring Basis | Level 2 | Other Noncurrent Liabilities | Other Derivative Instruments | Commodity Trading    
Derivatives, Fair Value [Line Items]    
Derivative Liability, Fair Value, Gross Liability 19,507 22,163
Fair Value Measured on a Recurring Basis | Level 3 | Other Current Assets    
Derivatives, Fair Value [Line Items]    
Derivative Asset, Fair Value, Gross Asset 19,953 17,723
Fair Value Measured on a Recurring Basis | Level 3 | Other Current Assets | Other Derivative Instruments | Commodity Trading    
Derivatives, Fair Value [Line Items]    
Derivative Asset, Fair Value, Gross Asset 1,951 142
Fair Value Measured on a Recurring Basis | Level 3 | Other Current Assets | Other Derivative Instruments | Electric Commodity    
Derivatives, Fair Value [Line Items]    
Derivative Asset, Fair Value, Gross Asset 18,002 17,581
Fair Value Measured on a Recurring Basis | Level 3 | Other Current Assets | Other Derivative Instruments | Natural Gas Commodity    
Derivatives, Fair Value [Line Items]    
Derivative Asset, Fair Value, Gross Asset 0 0
Fair Value Measured on a Recurring Basis | Level 3 | Other Noncurrent Assets    
Derivatives, Fair Value [Line Items]    
Derivative Asset, Fair Value, Gross Asset 5,399 5,363
Fair Value Measured on a Recurring Basis | Level 3 | Other Noncurrent Assets | Other Derivative Instruments | Commodity Trading    
Derivatives, Fair Value [Line Items]    
Derivative Asset, Fair Value, Gross Asset 5,399 5,363
Fair Value Measured on a Recurring Basis | Level 3 | Other Current Liabilities    
Derivatives, Fair Value [Line Items]    
Derivative Liability, Fair Value, Gross Liability 1,926 425
Fair Value Measured on a Recurring Basis | Level 3 | Other Current Liabilities | Other Derivative Instruments | Commodity Trading    
Derivatives, Fair Value [Line Items]    
Derivative Liability, Fair Value, Gross Liability 1,754 0
Fair Value Measured on a Recurring Basis | Level 3 | Other Current Liabilities | Other Derivative Instruments | Electric Commodity    
Derivatives, Fair Value [Line Items]    
Derivative Liability, Fair Value, Gross Liability 172 425
Fair Value Measured on a Recurring Basis | Level 3 | Other Noncurrent Liabilities    
Derivatives, Fair Value [Line Items]    
Derivative Liability, Fair Value, Gross Liability 411 0
Fair Value Measured on a Recurring Basis | Level 3 | Other Noncurrent Liabilities | Other Derivative Instruments | Commodity Trading    
Derivatives, Fair Value [Line Items]    
Derivative Liability, Fair Value, Gross Liability 411 0
Fair Value, Measurements, Nonrecurring | Other Current Assets | Purchased Power Agreements    
Derivatives, Fair Value [Line Items]    
Derivative Asset, Fair Value, Gross Asset 25 [3] 657 [4]
Fair Value, Measurements, Nonrecurring | Other Noncurrent Assets | Purchased Power Agreements    
Derivatives, Fair Value [Line Items]    
Derivative Asset, Fair Value, Gross Asset 101 [3] 120 [4]
Fair Value, Measurements, Nonrecurring | Other Current Liabilities | Purchased Power Agreements    
Derivatives, Fair Value [Line Items]    
Derivative Liability, Fair Value, Gross Liability 13,851 [3] 14,105 [4]
Fair Value, Measurements, Nonrecurring | Other Noncurrent Liabilities | Purchased Power Agreements    
Derivatives, Fair Value [Line Items]    
Derivative Liability, Fair Value, Gross Liability 79,524 [3] 89,913 [4]
Estimate of Fair Value Measurement [Member] | Fair Value Measured on a Recurring Basis | Other Current Assets    
Derivatives, Fair Value [Line Items]    
Derivative Asset, Fair Value, Gross Asset 38,759 36,742
Estimate of Fair Value Measurement [Member] | Fair Value Measured on a Recurring Basis | Other Current Assets | Other Derivative Instruments | Commodity Trading    
Derivatives, Fair Value [Line Items]    
Derivative Asset, Fair Value, Gross Asset 19,938 18,977
Estimate of Fair Value Measurement [Member] | Fair Value Measured on a Recurring Basis | Other Current Assets | Other Derivative Instruments | Electric Commodity    
Derivatives, Fair Value [Line Items]    
Derivative Asset, Fair Value, Gross Asset 18,002 17,581
Estimate of Fair Value Measurement [Member] | Fair Value Measured on a Recurring Basis | Other Current Assets | Other Derivative Instruments | Natural Gas Commodity    
Derivatives, Fair Value [Line Items]    
Derivative Asset, Fair Value, Gross Asset 771 77
Estimate of Fair Value Measurement [Member] | Fair Value Measured on a Recurring Basis | Other Noncurrent Assets    
Derivatives, Fair Value [Line Items]    
Derivative Asset, Fair Value, Gross Asset 33,543 34,484
Estimate of Fair Value Measurement [Member] | Fair Value Measured on a Recurring Basis | Other Noncurrent Assets | Other Derivative Instruments | Commodity Trading    
Derivatives, Fair Value [Line Items]    
Derivative Asset, Fair Value, Gross Asset 33,543 34,484
Estimate of Fair Value Measurement [Member] | Fair Value Measured on a Recurring Basis | Other Current Liabilities    
Derivatives, Fair Value [Line Items]    
Derivative Liability, Fair Value, Gross Liability 16,853 15,991
Estimate of Fair Value Measurement [Member] | Fair Value Measured on a Recurring Basis | Other Current Liabilities | Other Derivative Instruments | Commodity Trading    
Derivatives, Fair Value [Line Items]    
Derivative Liability, Fair Value, Gross Liability 16,681 15,566
Estimate of Fair Value Measurement [Member] | Fair Value Measured on a Recurring Basis | Other Current Liabilities | Other Derivative Instruments | Electric Commodity    
Derivatives, Fair Value [Line Items]    
Derivative Liability, Fair Value, Gross Liability 172 425
Estimate of Fair Value Measurement [Member] | Fair Value Measured on a Recurring Basis | Other Noncurrent Liabilities    
Derivatives, Fair Value [Line Items]    
Derivative Liability, Fair Value, Gross Liability 19,920 22,163
Estimate of Fair Value Measurement [Member] | Fair Value Measured on a Recurring Basis | Other Noncurrent Liabilities | Other Derivative Instruments | Commodity Trading    
Derivatives, Fair Value [Line Items]    
Derivative Liability, Fair Value, Gross Liability 19,920 22,163
Cash Flow Hedges | Fair Value Measured on a Recurring Basis | Other Current Assets | Designated as Hedging Instrument | Vehicle Fuel And Other Commodity    
Derivatives, Fair Value [Line Items]    
Derivative Asset, Fair Value, Gross Asset 48 107
Derivative Asset, Fair Value, Gross Liability and Obligation to Return Cash, Offset 0 [1] 0
Cash Flow Hedges | Fair Value Measured on a Recurring Basis | Level 1 | Other Current Assets | Designated as Hedging Instrument | Vehicle Fuel And Other Commodity    
Derivatives, Fair Value [Line Items]    
Derivative Asset, Fair Value, Gross Asset 0 0
Cash Flow Hedges | Fair Value Measured on a Recurring Basis | Level 2 | Other Current Assets | Designated as Hedging Instrument | Vehicle Fuel And Other Commodity    
Derivatives, Fair Value [Line Items]    
Derivative Asset, Fair Value, Gross Asset 48 107
Cash Flow Hedges | Fair Value Measured on a Recurring Basis | Level 3 | Other Current Assets | Designated as Hedging Instrument | Vehicle Fuel And Other Commodity    
Derivatives, Fair Value [Line Items]    
Derivative Asset, Fair Value, Gross Asset 0 0
Cash Flow Hedges | Estimate of Fair Value Measurement [Member] | Fair Value Measured on a Recurring Basis | Other Current Assets | Designated as Hedging Instrument | Vehicle Fuel And Other Commodity    
Derivatives, Fair Value [Line Items]    
Derivative Asset, Fair Value, Gross Asset $ 48 $ 107
[1] NSP-Minnesota nets derivative instruments and related collateral in its consolidated balance sheet when supported by a legally enforceable master netting agreement, and all derivative instruments and related collateral amounts were subject to master netting agreements at Sept. 30, 2018. At Sept. 30, 2018, derivative assets and liabilities include no obligations to return cash collateral and rights to reclaim cash collateral of $4.9 million. The counterparty netting amounts presented exclude settlement receivables and payables and non-derivative amounts that may be subject to the same master netting agreements.
[2] NSP-Minnesota nets derivative instruments and related collateral in its consolidated balance sheet when supported by a legally enforceable master netting agreement, and all derivative instruments and related collateral amounts were subject to master netting agreements at Dec. 31, 2017. At Dec. 31, 2017, derivative assets and liabilities include no obligations to return cash collateral and rights to reclaim cash collateral of $3.1 million. The counterparty netting amounts presented exclude settlement receivables and payables and non-derivative amounts that may be subject to the same master netting agreements.
[3] During 2006, NSP-Minnesota qualified these contracts under the normal purchase exception. Based on this qualification, the contracts are no longer adjusted to fair value and the previous carrying value of these contracts is being amortized over the remaining contract lives along with the offsetting regulatory assets and liabilities.
[4] During 2006, NSP-Minnesota qualified these contracts under the normal purchase exception. Based on this qualification, the contracts are no longer adjusted to fair value and the previous carrying value of these contracts is being amortized over the remaining contract lives along with the offsetting regulatory assets and liabilities.
v3.10.0.1
Fair Value of Financial Assets and Liabilities, Changes in Level 3 Commodity Derivatives (Details) - Commodity Contract - USD ($)
 3 Months Ended 9 Months Ended
Sep. 30, 2018
Sep. 30, 2017
Sep. 30, 2018
Sep. 30, 2017
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]        
Balance at beginning of period $ 28,029,000 $ 40,572,000 $ 22,661,000 $ 15,320,000
Purchases     26,397,000 40,740,000
Settlements (8,694,000) (23,186,000) (15,798,000) (34,681,000)
Gains (losses) recognized in earnings [1] 303,000 527,000 (70,000) 5,742,000
Net losses recognized as regulatory assets and liabilities 3,377,000 27,500,000 (10,175,000) 18,292,000
Balance at end of period 23,015,000 45,413,000 23,015,000 45,413,000
Transfers into Level 3 0 0 0 0
Transfers out of Level 3 $ 0 $ 0 $ 0 $ 0
[1] (a) These amounts relate to commodity derivatives held at the end of the period.
v3.10.0.1
Fair Value of Financial Assets and Liabilities, Fair Value of Long-Term Debt (Details) - USD ($)
$ in Thousands
Sep. 30, 2018
Dec. 31, 2017
Carrying Amount    
Financial Liabilities, Balance Sheet Groupings [Abstract]    
Long-term debt, including current portion $ 4,936,135 $ 4,933,018
Fair Value    
Financial Liabilities, Balance Sheet Groupings [Abstract]    
Long-term debt, including current portion $ 5,159,080 $ 5,601,919
v3.10.0.1
Other (Expense) Income, Net (Details) - USD ($)
$ in Thousands
 3 Months Ended 9 Months Ended
Sep. 30, 2018
Sep. 30, 2017
Sep. 30, 2018
Sep. 30, 2017
Other Income and Expenses [Abstract]        
Interest income $ 1,596 $ 2,936 $ 5,053 $ 6,250
Other Nonoperating Expense (722) (429) (24) (402)
Insurance policy income (expense) 157 (387) (1,038) (2,098)
Defined Benefit Plan, Non-service Costs (3,007) (3,398) (7,985) (10,194)
Other expense, net $ (1,976) $ (1,278) $ (3,994) $ (6,444)
v3.10.0.1
Segment Information (Details) - USD ($)
$ in Thousands
 3 Months Ended 9 Months Ended
Sep. 30, 2018
Sep. 30, 2017
Sep. 30, 2018
Sep. 30, 2017
Segment Reporting Information [Line Items]        
Total operating revenues $ 1,351,754 $ 1,355,779 $ 3,850,240 $ 3,827,859
Net income (loss) 201,176 229,003 405,303 410,831
Regulated Electric [Member]        
Segment Reporting Information [Line Items]        
Total operating revenues 1,289,717 1,291,131 3,448,681 3,450,292
Net income (loss) 207,316 232,078 381,913 399,637
Regulated Natural Gas [Member]        
Segment Reporting Information [Line Items]        
Total operating revenues 54,763 57,542 379,567 357,002
Net income (loss) (8,416) (6,242) 17,428 7,903
All Other        
Segment Reporting Information [Line Items]        
Total operating revenues 7,631 7,366 22,902 21,448
Net income (loss) 2,276 3,167 5,962 3,291
Total revenues        
Segment Reporting Information [Line Items]        
Total operating revenues 1,351,754 [1],[2] 1,355,779 [1],[2] 3,850,240 [3],[4] 3,827,859 [3],[4]
Total revenues | Regulated Electric [Member]        
Segment Reporting Information [Line Items]        
Total operating revenues 1,289,498 [1] 1,290,971 [1] 3,448,169 [3] 3,449,780 [3]
Total revenues | Regulated Natural Gas [Member]        
Segment Reporting Information [Line Items]        
Total operating revenues 54,625 [2] 57,442 [2] 379,169 [4] 356,631 [4]
Total revenues | All Other        
Segment Reporting Information [Line Items]        
Total operating revenues 7,631 7,366 22,902 21,448
Intersegment Eliminations        
Segment Reporting Information [Line Items]        
Total operating revenues (357) (260) (910) (883)
Net income (loss) 0 0 0 0
Intersegment Eliminations | Regulated Electric [Member]        
Segment Reporting Information [Line Items]        
Total operating revenues 219 160 512 512
Intersegment Eliminations | Regulated Natural Gas [Member]        
Segment Reporting Information [Line Items]        
Total operating revenues 138 100 398 371
Intersegment Eliminations | All Other        
Segment Reporting Information [Line Items]        
Total operating revenues $ 0 $ 0 $ 0 $ 0
[1] Operating revenues include $123 million and $124 million of affiliate electric revenue for the three months ended Sept. 30, 2018 and 2017.
[2] Operating revenues include an immaterial amount of affiliate gas revenue for the three months ended Sept. 30, 2018 and 2017.
[3] Operating revenues include $357 million and $367 million of affiliate electric revenue for the nine months ended Sept. 30, 2018 and 2017.
[4] Operating revenues include an immaterial amount of affiliate gas revenue for the nine months ended Sept. 30, 2018 and 2017.
v3.10.0.1
Benefit Plans and Other Postretirement Benefits (Details)
$ in Thousands
 1 Months Ended 3 Months Ended 9 Months Ended 12 Months Ended
Jan. 31, 2018
USD ($)
Plan
Sep. 30, 2018
USD ($)
Sep. 30, 2017
USD ($)
Sep. 30, 2018
USD ($)
Sep. 30, 2017
USD ($)
Dec. 31, 2017
USD ($)
Pension Plan [Member]            
Components of Net Periodic Benefit Cost [Abstract]            
Service cost   $ 6,982 $ 6,958 $ 20,946 $ 20,874  
Interest cost (a) [1]   8,805 10,177 26,413 30,531  
Expected return on plan assets (a) [1]   (14,542) (15,016) (43,625) (45,050)  
Amortization of prior service (credit) cost (a) [1]   (29) 265 (86) 795  
Amortization of net loss (a) [1]   9,614 9,902 28,843 29,706  
Defined Benefit Plan, Net Periodic Benefit Cost (Credit), Gain (Loss) Due to Settlement   34,907 [2] 0 [2] 34,907 [2] 0 [2] $ 48,000
Net periodic benefit cost   45,737 12,286 67,398 36,856  
Costs not recognized due to the effects of regulation   (42,037) (4,899) (47,869) (14,696)  
Net benefit cost recognized for financial reporting   3,700 7,387 19,529 22,160  
Total contributions to the pension plans during the period $ 63,000          
Pension Plan [Member] | Xcel Energy Inc.            
Components of Net Periodic Benefit Cost [Abstract]            
Total contributions to the pension plans during the period $ 150,000          
Number of Xcel Energy's pension plans to which contributions were made | Plan 4          
Other Postretirement Benefits Plan [Member]            
Components of Net Periodic Benefit Cost [Abstract]            
Service cost   42 36 128 108  
Interest cost (a) [1]   768 854 2,306 2,562  
Expected return on plan assets (a) [1]   (95) (53) (287) (161)  
Amortization of prior service (credit) cost (a) [1]   (759) (759) (2,277) (2,277)  
Amortization of net loss (a) [1]   596 506 1,786 1,520  
Defined Benefit Plan, Net Periodic Benefit Cost (Credit), Gain (Loss) Due to Settlement [2]   0 0 0 0  
Net periodic benefit cost   552 584 1,656 1,752  
Costs not recognized due to the effects of regulation   0 0 0 0  
Net benefit cost recognized for financial reporting   $ 552 $ 584 $ 1,656 $ 1,752  
[1] The components of net periodic cost other than the service cost component are included in the line item “other expense, net” in the income statement or capitalized on the balance sheet as a regulatory asset.
[2] A settlement charge is required when the amount of lump-sum distributions during the year is greater than the sum of the service and interest cost components of the annual net periodic pension cost. In the third quarter of 2018 as a result of lump-sum distributions during the 2018 plan year, NSP-Minnesota recorded a total pension settlement charge of $35 million, which was not recognized due to the effects of rate making. In the fourth quarter of 2017 as a result of lump-sum distributions during the 2017 plan year, NSP-Minnesota recorded a total pension settlement charge of $48 million, which was not recognized due to the effects of rate making.
v3.10.0.1
Other Comprehensive Income (Details) - USD ($)
$ in Thousands
 3 Months Ended 9 Months Ended
Sep. 30, 2018
Sep. 30, 2017
Sep. 30, 2018
Sep. 30, 2017
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]        
Accumulated other comprehensive (loss) income at beginning of period     $ 5,475,570  
(Gains) losses reclassified from net accumulated other comprehensive loss $ 228 $ 418 539 $ 896
Accumulated other comprehensive (loss) income at end of period 5,533,123   5,533,123  
Gains and Losses on Cash Flow Hedges        
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]        
Accumulated other comprehensive (loss) income at beginning of period (20,536) (17,775) (20,895) (18,208)
Other comprehensive income (loss) before reclassifications 5 22 32 48
(Gains) losses reclassified from net accumulated other comprehensive loss 172 379 504 786
Net current period other comprehensive income (loss) 177 401 536 834
Accumulated other comprehensive (loss) income at end of period (20,359) (17,374) (20,359) (17,374)
Unrealized Gains and Losses on Marketable Securities        
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]        
Accumulated other comprehensive (loss) income at beginning of period 0 105 128 105
Other comprehensive income (loss) before reclassifications 0 0 0 0
(Gains) losses reclassified from net accumulated other comprehensive loss 0 0 (128) 0
Net current period other comprehensive income (loss) 0 0 (128) 0
Accumulated other comprehensive (loss) income at end of period 0 105 0 105
Defined Benefit Pension and Postretirement Items        
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]        
Accumulated other comprehensive (loss) income at beginning of period (3,663) (2,609) (3,770) (2,680)
Other comprehensive income (loss) before reclassifications 0 0 0 0
(Gains) losses reclassified from net accumulated other comprehensive loss 56 39 163 110
Net current period other comprehensive income (loss) 56 39 163 110
Accumulated other comprehensive (loss) income at end of period (3,607) (2,570) (3,607) (2,570)
Total        
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]        
Accumulated other comprehensive (loss) income at beginning of period (24,199) (20,279) (24,537) (20,783)
Other comprehensive income (loss) before reclassifications 5 22 32 48
(Gains) losses reclassified from net accumulated other comprehensive loss 228 418 539 896
Net current period other comprehensive income (loss) 233 440 571 944
Accumulated other comprehensive (loss) income at end of period $ (23,966) $ (19,839) $ (23,966) $ (19,839)
v3.10.0.1
Other Comprehensive Income (Reclassification from AOCI) (Details) - USD ($)
$ in Thousands
 3 Months Ended 9 Months Ended
Sep. 30, 2018
Sep. 30, 2017
Sep. 30, 2018
Sep. 30, 2017
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]        
Operating and maintenance expenses $ 310,536 $ 285,372 $ 912,027 $ 899,187
Total, pre-tax (210,507) (296,946) (436,319) (551,559)
Tax benefit 9,331 67,943 31,016 140,728
Total amounts reclassified, net of tax 228 418 539 896
Gains and Losses on Cash Flow Hedges        
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]        
Total amounts reclassified, net of tax 172 379 504 786
Gains and Losses on Cash Flow Hedges | Amounts Reclassified from Accumulated Other Comprehensive Loss        
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]        
Total, pre-tax 228 601 689 1,288
Tax benefit (56) (222) (185) (502)
Total, net of tax 172 379 504 786
Gains and Losses on Cash Flow Hedges | Interest Rate Derivatives | Amounts Reclassified from Accumulated Other Comprehensive Loss        
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]        
Interest charges 267 612 792 1,304
Gains and Losses on Cash Flow Hedges | Vehicle Fuel Derivatives | Amounts Reclassified from Accumulated Other Comprehensive Loss        
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]        
Operating and maintenance expenses (39) (11) (103) (16)
Amortization of net loss | Amounts Reclassified from Accumulated Other Comprehensive Loss        
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]        
Total, pre-tax 125 109 375 327
Prior service credit | Amounts Reclassified from Accumulated Other Comprehensive Loss        
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]        
Total, pre-tax (49) (49) (147) (147)
Defined Benefit Pension and Postretirement Items        
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]        
Total amounts reclassified, net of tax 56 39 163 110
Defined Benefit Pension and Postretirement Items | Amounts Reclassified from Accumulated Other Comprehensive Loss        
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]        
Total, pre-tax 76 60 228 180
Tax (benefit) expense (20) (21) (65) (70)
Total amounts reclassified, net of tax 56 39 163 110
Marketable securities        
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]        
Total amounts reclassified, net of tax 0 0 (128) 0
Marketable securities | Amounts Reclassified from Accumulated Other Comprehensive Loss        
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]        
Realization of gains 0 0 (179) 0
Total, pre-tax 0 0 (179) 0
Tax benefit 0 0 51 0
Total, net of tax $ 0 $ 0 $ (128) $ 0
v3.10.0.1
Revenues (Details) - USD ($)
$ in Thousands
 3 Months Ended 9 Months Ended
Sep. 30, 2018
Sep. 30, 2017
Sep. 30, 2018
Sep. 30, 2017
Disaggregation of Revenue [Line Items]        
Total operating revenues $ 1,351,754 $ 1,355,779 $ 3,850,240 $ 3,827,859
Maximum number of months following end of annual period in which revenues are earned to be included in incentive programs     24 months  
Total revenue from contracts with customers        
Disaggregation of Revenue [Line Items]        
Total revenue from contracts with customers 1,306,536 1,283,365 $ 3,710,137 3,640,840
Retail        
Disaggregation of Revenue [Line Items]        
Total revenue from contracts with customers 1,060,160 1,047,356 3,003,119 2,960,757
Retail | Residential        
Disaggregation of Revenue [Line Items]        
Total revenue from contracts with customers 424,196 388,475 1,233,501 1,152,520
Retail | Commercial and industrial (C&I)        
Disaggregation of Revenue [Line Items]        
Total revenue from contracts with customers 626,680 649,018 1,739,823 1,780,991
Retail | Other        
Disaggregation of Revenue [Line Items]        
Total revenue from contracts with customers 9,284 9,863 29,795 27,246
Wholesale        
Disaggregation of Revenue [Line Items]        
Total revenue from contracts with customers 47,708 36,598 136,106 111,358
Transmission        
Disaggregation of Revenue [Line Items]        
Total revenue from contracts with customers 66,166 65,667 180,915 175,803
Interchange [Member]        
Disaggregation of Revenue [Line Items]        
Total revenue from contracts with customers 122,636 123,523 357,025 366,596
Other        
Disaggregation of Revenue [Line Items]        
Total revenue from contracts with customers 9,866 10,221 32,972 26,326
Alternative revenue and other        
Disaggregation of Revenue [Line Items]        
Total operating revenues 45,218 72,414 140,103 187,019
Regulated Electric [Member]        
Disaggregation of Revenue [Line Items]        
Total operating revenues 1,289,717 1,291,131 3,448,681 3,450,292
Regulated Electric [Member] | Total revenue from contracts with customers        
Disaggregation of Revenue [Line Items]        
Total revenue from contracts with customers 1,246,425 1,227,546 3,324,845 3,290,096
Regulated Electric [Member] | Retail        
Disaggregation of Revenue [Line Items]        
Total revenue from contracts with customers 1,005,329 993,126 2,628,635 2,614,299
Regulated Electric [Member] | Retail | Residential        
Disaggregation of Revenue [Line Items]        
Total revenue from contracts with customers 392,334 357,209 1,014,549 950,355
Regulated Electric [Member] | Retail | Commercial and industrial (C&I)        
Disaggregation of Revenue [Line Items]        
Total revenue from contracts with customers 604,301 626,590 1,587,034 1,638,560
Regulated Electric [Member] | Retail | Other        
Disaggregation of Revenue [Line Items]        
Total revenue from contracts with customers 8,694 9,327 27,052 25,384
Regulated Electric [Member] | Wholesale        
Disaggregation of Revenue [Line Items]        
Total revenue from contracts with customers 47,708 36,598 136,106 111,358
Regulated Electric [Member] | Transmission        
Disaggregation of Revenue [Line Items]        
Total revenue from contracts with customers 66,166 65,667 180,915 175,803
Regulated Electric [Member] | Interchange [Member]        
Disaggregation of Revenue [Line Items]        
Total revenue from contracts with customers 122,636 123,523 357,025 366,596
Regulated Electric [Member] | Other        
Disaggregation of Revenue [Line Items]        
Total revenue from contracts with customers 4,586 8,632 22,164 22,040
Regulated Electric [Member] | Alternative revenue and other        
Disaggregation of Revenue [Line Items]        
Total operating revenues 43,073 63,425 123,324 159,684
Regulated Natural Gas [Member]        
Disaggregation of Revenue [Line Items]        
Total operating revenues 54,763 57,542 379,567 357,002
Regulated Natural Gas [Member] | Total revenue from contracts with customers        
Disaggregation of Revenue [Line Items]        
Total revenue from contracts with customers 52,480 48,453 362,390 329,296
Regulated Natural Gas [Member] | Retail        
Disaggregation of Revenue [Line Items]        
Total revenue from contracts with customers 47,200 46,864 351,582 325,010
Regulated Natural Gas [Member] | Retail | Residential        
Disaggregation of Revenue [Line Items]        
Total revenue from contracts with customers 24,863 24,475 198,920 182,748
Regulated Natural Gas [Member] | Retail | Commercial and industrial (C&I)        
Disaggregation of Revenue [Line Items]        
Total revenue from contracts with customers 22,337 22,389 152,662 142,262
Regulated Natural Gas [Member] | Retail | Other        
Disaggregation of Revenue [Line Items]        
Total revenue from contracts with customers 0 0 0 0
Regulated Natural Gas [Member] | Wholesale        
Disaggregation of Revenue [Line Items]        
Total revenue from contracts with customers 0 0 0 0
Regulated Natural Gas [Member] | Transmission        
Disaggregation of Revenue [Line Items]        
Total revenue from contracts with customers 0 0 0 0
Regulated Natural Gas [Member] | Interchange [Member]        
Disaggregation of Revenue [Line Items]        
Total revenue from contracts with customers 0 0 0 0
Regulated Natural Gas [Member] | Other        
Disaggregation of Revenue [Line Items]        
Total revenue from contracts with customers 5,280 1,589 10,808 4,286
Regulated Natural Gas [Member] | Alternative revenue and other        
Disaggregation of Revenue [Line Items]        
Total operating revenues 2,145 8,989 16,779 27,335
All Other        
Disaggregation of Revenue [Line Items]        
Total operating revenues 7,631 7,366 22,902 21,448
All Other | Total revenue from contracts with customers        
Disaggregation of Revenue [Line Items]        
Total revenue from contracts with customers 7,631 7,366 22,902 21,448
All Other | Retail        
Disaggregation of Revenue [Line Items]        
Total revenue from contracts with customers 7,631 7,366 22,902 21,448
All Other | Retail | Residential        
Disaggregation of Revenue [Line Items]        
Total revenue from contracts with customers 6,999 6,791 20,032 19,417
All Other | Retail | Commercial and industrial (C&I)        
Disaggregation of Revenue [Line Items]        
Total revenue from contracts with customers 42 39 127 169
All Other | Retail | Other        
Disaggregation of Revenue [Line Items]        
Total revenue from contracts with customers 590 536 2,743 1,862
All Other | Wholesale        
Disaggregation of Revenue [Line Items]        
Total revenue from contracts with customers 0 0 0 0
All Other | Transmission        
Disaggregation of Revenue [Line Items]        
Total revenue from contracts with customers 0 0 0 0
All Other | Interchange [Member]        
Disaggregation of Revenue [Line Items]        
Total revenue from contracts with customers 0 0 0 0
All Other | Other        
Disaggregation of Revenue [Line Items]        
Total revenue from contracts with customers 0 0 0 0
All Other | Alternative revenue and other        
Disaggregation of Revenue [Line Items]        
Total operating revenues 0 0 0 0
Total revenues        
Disaggregation of Revenue [Line Items]        
Total operating revenues 1,351,754 [1],[2] 1,355,779 [1],[2] 3,850,240 [3],[4] 3,827,859 [3],[4]
Total revenues | Regulated Electric [Member]        
Disaggregation of Revenue [Line Items]        
Total operating revenues 1,289,498 [1] 1,290,971 [1] 3,448,169 [3] 3,449,780 [3]
Total revenues | Regulated Natural Gas [Member]        
Disaggregation of Revenue [Line Items]        
Total operating revenues 54,625 [2] 57,442 [2] 379,169 [4] 356,631 [4]
Total revenues | All Other        
Disaggregation of Revenue [Line Items]        
Total operating revenues $ 7,631 $ 7,366 $ 22,902 $ 21,448
[1] Operating revenues include $123 million and $124 million of affiliate electric revenue for the three months ended Sept. 30, 2018 and 2017.
[2] Operating revenues include an immaterial amount of affiliate gas revenue for the three months ended Sept. 30, 2018 and 2017.
[3] Operating revenues include $357 million and $367 million of affiliate electric revenue for the nine months ended Sept. 30, 2018 and 2017.
[4] Operating revenues include an immaterial amount of affiliate gas revenue for the nine months ended Sept. 30, 2018 and 2017.