Northern States Power 10-Q 26 Oct 18 2018 Q3 Quarterly report Financial data

Document and Entity Information

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	1,123,852
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	2,018
Document Fiscal Period Focus	Q3
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Sep. 30, 2018

CONSOLIDATED STATEMENTS OF INCO

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

CONSOLIDATED STATEMENTS OF IN_2

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

CONSOLIDATED STATEMENTS OF COMP

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

CONSOLIDATED STATEMENTS OF CO_2

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

CONSOLIDATED STATEMENTS OF CASH

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

CONSOLIDATED BALANCE SHEETS (UN

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

CONSOLIDATED BALANCE SHEETS (_2

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

Management's Opinion

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.

Summary of Significant Accounti

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.

Accounting Pronouncements

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 — I n 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.

Selected Balance Sheet Data

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

Income Taxes

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 .

Rate Matters

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.

Commitments and Contingencies

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.

Borrowings and Other Financing

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 .

Fair Value of Financial Assets

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 Pre-Tax (Gains) Losses Pre-Tax Gains Recognized (Thousands of Dollars) Accumulated Regulatory Accumulated Regulatory 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 Pre-Tax (Gains) Losses Pre-Tax Gains (Losses) (Thousands of Dollars) Accumulated Regulatory Accumulated Regulatory 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 Pre-Tax (Gains) Losses Pre-Tax Gains (Thousands of Dollars) Accumulated Regulatory Accumulated Regulatory 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 Pre-Tax (Gains) Losses Pre-Tax Gains (Losses) (Thousands of Dollars) Accumulated Regulatory Accumulated Regulatory 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 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.

Other (Expense) Income, Net

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 )

Segment Information

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 .

Benefit Plans and Other Postret

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.

Other Comprehensive Income

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 Unrealized Defined Benefit 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 Unrealized Defined Benefit 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 Unrealized Defined Benefit 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 (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 (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.

Revenues

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

Selected Balance Sheet Data (Ta

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

Income Taxes (Tables)

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 )

Commitments and Contingencies S

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

Borrowings and Other Financin_2

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

Fair Value of Financial Asset_2

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 Pre-Tax (Gains) Losses Pre-Tax Gains Recognized (Thousands of Dollars) Accumulated Regulatory Accumulated Regulatory 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 Pre-Tax (Gains) Losses Pre-Tax Gains (Losses) (Thousands of Dollars) Accumulated Regulatory Accumulated Regulatory 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 Pre-Tax (Gains) Losses Pre-Tax Gains (Thousands of Dollars) Accumulated Regulatory Accumulated Regulatory 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 Pre-Tax (Gains) Losses Pre-Tax Gains (Losses) (Thousands of Dollars) Accumulated Regulatory Accumulated Regulatory 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 Fair Value Long-term debt, including current portion $ 4,936,135 $ 5,159,080 $ 4,933,018 $ 5,601,919

Other (Expense) Income, Net (Ta

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 )

Segment Information (Tables)

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 .

Benefit Plans and Other Postr_2

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.

Other Comprehensive Income (Tab

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 Unrealized Defined Benefit 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 Unrealized Defined Benefit 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 Unrealized Defined Benefit 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 (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 (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.

Revenues (Tables)

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

Accounting Pronouncements (Deta

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)

Selected Balance Sheet Data, Ac

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

Selected Balance Sheet Data, In

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

Selected Balance Sheet Data, Pr

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)

Income Taxes (Details)

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							2,009
State Jurisdiction (Minnesota)
Tax Audits [Abstract]
Year(s) under examination						2010 through 2014
Earliest year subject to examination		2,009
[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.

Rate Matters (Details)

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%

Commitments and Contingencies,

Commitments and Contingencies, Purchased Power Agreements (Details) $ in Millions	9 Months Ended
	Sep. 30, 2018USD ($)MW	Dec. 31, 2017USD ($)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)	2,027
Payment or Performance Guarantee
Purchased Power Agreements [Abstract]
Lease Guarantee Expiration (year)	2,019
Other MGP Sites [Member]
Purchased Power Agreements [Abstract]
Accrual for Environmental Loss Contingencies, Gross | $	$ 3	$ 3

Commitments and Contingencies C

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)	2,019
Guarantees issued and outstanding	$ 4.8	$ 4.8

Commitments and Contingencies_2

Commitments and Contingencies, Environmental Contingencies (Details) $ in Millions	9 Months Ended
	Sep. 30, 2018USD ($)	Dec. 31, 2017USD ($)
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

Borrowings and Other Financin_3

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%

Borrowings and Other Financin_4

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

Borrowings and Other Financin_5

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.

Fair Value of Financial Asset_3

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

Fair Value of Financial Asset_4

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.

Fair Value of Financial Asset_5

Fair Value of Financial Assets and Liabilities, Final Contractual Maturity Dates of Debt Securities in Nuclear Decommissioning Fund (Details) $ in Thousands	Sep. 30, 2018USD ($)
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

Fair Value of Financial Asset_6

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.

Fair Value of Financial Asset_7

Fair Value of Financial Assets and Liabilities, Derivative Instruments (Details) gal in Thousands, MWh in Thousands, MMBTU in Thousands, $ in Millions		Sep. 30, 2018USD ($)galMMBTUMWhCounterparty	Dec. 31, 2017galMMBTUMWh
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
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.

Fair Value of Financial Asset_8

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.

Fair Value of Financial Asset_9

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

Fair Value of Financial Asse_10

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.

Fair Value of Financial Asse_11

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.

Fair Value of Financial Asse_12

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

Other (Expense) Income, Net (De

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)

Segment Information (Details)

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.

Benefit Plans and Other Postr_3

Benefit Plans and Other Postretirement Benefits (Details) $ in Thousands		1 Months Ended	3 Months Ended				9 Months Ended				12 Months Ended
		Jan. 31, 2018USD ($)Plan	Sep. 30, 2018USD ($)		Sep. 30, 2017USD ($)		Sep. 30, 2018USD ($)		Sep. 30, 2017USD ($)		Dec. 31, 2017USD ($)
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.

Other Comprehensive Income (Det

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)

Other Comprehensive Income (Rec

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

Revenues (Details)

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.