Northern States Power 10-Q 27 Jul 18 2018 Q2 Quarterly report Financial data

Document and Entity Information

Document and Entity Information - shares	6 Months Ended
	Jun. 30, 2018	Jul. 27, 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 Well-known Seasoned Issuer	Yes
Entity Voluntary Filers	No
Entity Current Reporting Status	Yes
Entity Filer Category	Non-accelerated Filer
Entity Common Stock, Shares Outstanding		1,000,000
Document Fiscal Year Focus	2,018
Document Fiscal Period Focus	Q2
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Jun. 30, 2018

CONSOLIDATED STATEMENTS OF INCO

CONSOLIDATED STATEMENTS OF INCOME (UNAUDITED) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Operating revenues
Electric, non-affiliates	$ 978,991	$ 960,394	$ 1,924,282	$ 1,915,735
Electric, affiliates	117,417	119,385	234,389	243,074
Natural gas	83,112	78,006	324,544	299,189
Other	8,215	7,155	15,271	14,082
Total operating revenues	1,187,735	1,164,940	2,498,486	2,472,080
Operating expenses
Electric fuel and purchased power	416,502	383,947	824,274	780,068
Cost of natural gas sold and transported	36,818	35,500	192,084	178,245
Cost of sales — other	4,655	4,594	8,836	8,772
Operating and maintenance expenses	309,196	305,189	601,491	613,815
Conservation program expenses	27,518	28,065	58,328	60,564
Depreciation and amortization	178,654	173,152	360,132	345,331
Taxes (other than income taxes)	64,285	61,445	131,830	129,769
Total operating expenses	1,037,628	991,892	2,176,975	2,116,564
Operating income	150,107	173,048	321,511	355,516
Other expense, net	(2,234)	(3,632)	(2,018)	(5,166)
Allowance for funds used during construction — equity	6,903	6,425	13,651	12,708
Interest charges and financing costs
Interest charges — includes other financing costs of $1,833 $1,806, $3,648 and $3,592, respectively	56,767	57,707	114,119	114,971
Allowance for funds used during construction — debt	(3,352)	(3,298)	(6,787)	(6,526)
Total interest charges and financing costs	53,415	54,409	107,332	108,445
Income before income taxes	101,361	121,432	225,812	254,613
Income taxes	8,974	33,770	21,685	72,785
Net income	$ 92,387	$ 87,662	$ 204,127	$ 181,828

CONSOLIDATED STATEMENTS OF INC3

CONSOLIDATED STATEMENTS OF INCOME (UNAUDITED) (Parenthetical) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Interest charges and financing costs
Other financing costs	$ 1,833	$ 1,806	$ 3,648	$ 3,592

CONSOLIDATED STATEMENTS OF COMP

CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME (UNAUDITED) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Comprehensive income:
Net income	$ 92,387	$ 87,662	$ 204,127	$ 181,828
Pension and retiree medical benefits:
Amortization of losses included in net periodic benefit cost, net of tax of $22, $22, $45 and $47, respectively	54	36	107	71
Derivative instruments:
Net fair value decrease, net of tax of $9, $17, $11 and $17, respectively	22	26	27	26
Reclassification of losses to net income, net of tax of $64, $141, $129 and $280, respectively	165	204	332	407
Total derivative instruments, net of tax	187	230	359	433
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	241	266	338	504
Comprehensive income	$ 92,628	$ 87,928	$ 204,465	$ 182,332

CONSOLIDATED STATEMENTS OF COM5

CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME (UNAUDITED) (Parenthetical) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Pension and retiree medical benefits:
Amortization of losses included in net periodic benefit cost, tax	$ 22	$ 22	$ 45	$ 47
Derivative instruments:
Net fair value (decrease) increase, tax	9	17	11	17
Reclassification of losses to net income, tax	64	141	129	280
Other Comprehensive Income (Loss), Available-for-sale Securities, 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	6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017
Operating activities
Net income	$ 204,127	$ 181,828
Adjustments to reconcile net income to cash provided by operating activities:
Depreciation and amortization	363,310	348,442
Nuclear fuel amortization	62,294	57,003
Deferred income taxes	41,204	90,927
Amortization of Investment Tax Credits	(817)	(827)
Allowance for equity funds used during construction	(13,651)	(12,708)
Net realized and unrealized hedging and derivative transactions	(1,497)	(2,973)
Other, net	(998)	(946)
Changes in operating assets and liabilities:
Accounts receivable	(16,025)	(23,008)
Accrued unbilled revenues	31,354	45,497
Inventories	44,093	18,901
Other current assets	59,683	(56,145)
Accounts payable	(31,673)	(17,828)
Net regulatory assets and liabilities	43,782	2,613
Other current liabilities	(105,681)	(94,500)
Pension and other employee benefit obligations	(59,371)	(57,698)
Change in other noncurrent assets	1,224	(2,505)
Change in other noncurrent liabilities	(17,885)	(11,878)
Net cash provided by operating activities	603,473	464,195
Investing activities
Utility capital/construction expenditures	(439,868)	(484,172)
Allowance for equity funds used during construction	13,651	12,708
Purchases of investment securities	(367,159)	(361,881)
Proceeds from the sale of investment securities	356,973	350,446
Investments in utility money pool arrangement	(627,000)	(192,000)
Repayments from utility money pool arrangement	627,000	192,000
Other, net	(2,065)	(4,097)
Net cash used in investing activities	(438,468)	(486,996)
Financing activities
Repayments of short-term borrowings, net	(20,000)	(2,000)
Borrowings under utility money pool arrangement	131,000	465,000
Repayments under utility money pool arrangement	(154,000)	(374,000)
Repayments of long-term debt	(9)	0
Capital contributions from parent	49,622	89,487
Dividends paid to parent	(183,245)	(175,115)
Other	(68)	0
Net cash (used in) provided by financing activities	(176,700)	3,372
Net change in cash and cash equivalents	(11,695)	(19,429)
Cash and cash equivalents at beginning of period	43,781	47,595
Cash and cash equivalents at end of period	32,086	28,166
Supplemental disclosure of cash flow information:
Cash paid for interest (net of amounts capitalized)	(101,005)	(104,020)
Cash received (paid) for income taxes, net	64,212	(33,014)
Supplemental disclosure of non-cash investing transactions:
Property, plant and equipment additions in accounts payable	$ 50,370	$ 60,361

CONSOLIDATED BALANCE SHEETS (UN

CONSOLIDATED BALANCE SHEETS (UNAUDITED) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Current assets
Cash and cash equivalents	$ 32,086	$ 43,781
Accounts receivable, net	361,144	345,110
Accounts receivable from affiliates	19,462	48,494
Accrued unbilled revenues	246,362	277,716
Inventories	294,602	337,712
Regulatory assets	300,150	276,392
Derivative instruments	31,268	25,230
Prepaid Taxes	41,763	79,145
Prepayments and other	36,717	43,682
Total current assets	1,363,554	1,477,262
Property, plant and equipment, net	13,075,676	13,033,612
Other assets
Nuclear decommissioning fund and other investments	2,195,458	2,192,344
Regulatory assets	1,343,820	1,190,429
Derivative instruments	26,790	28,102
Other	11,936	4,142
Total other assets	3,578,004	3,415,017
Total assets	18,017,234	17,925,891
Current liabilities
Current portion of long-term debt	8	7
Short-term debt	0	20,000
Borrowings under utility money pool arrangement	62,000	85,000
Accounts payable	348,475	368,342
Accounts payable to affiliates	53,790	80,070
Regulatory liabilities	135,725	83,403
Taxes accrued	193,465	229,335
Accrued interest	67,765	65,896
Dividends payable to parent	88,714	98,687
Derivative instruments	18,029	17,697
Customer deposits	66,764	95,369
Other	153,256	152,965
Total current liabilities	1,187,991	1,296,771
Deferred credits and other liabilities
Deferred income taxes	1,655,863	1,612,341
Deferred investment tax credits	21,710	22,528
Regulatory liabilities	2,013,411	1,978,527
Asset retirement obligations	2,134,741	2,083,874
Derivative instruments	92,122	102,742
Pension and employee benefit obligations	271,228	331,087
Other	178,347	89,440
Total deferred credits and other liabilities	6,367,422	6,220,539
Commitments and contingencies
Capitalization
Long-term debt	4,935,060	4,933,011
Common stock — authorized 5,000,000 shares of $0.01 par value; 1,000,000 shares outstanding at June 30, 2018 and Dec. 31, 2017, respectively	10	10
Additional paid in capital	3,600,234	3,580,234
Retained earnings	1,950,716	1,919,863
Accumulated other comprehensive loss	(24,199)	(24,537)
Total common stockholder’s equity	5,526,761	5,475,570
Total liabilities and equity	$ 18,017,234	$ 17,925,891

CONSOLIDATED BALANCE SHEETS (U8

CONSOLIDATED BALANCE SHEETS (UNAUDITED) (Parenthetical) - $ / shares	Jun. 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	6 Months Ended
	Jun. 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 June 30, 2018 and Dec. 31, 2017 ; the results of its operations, including the components of net income and comprehensive income, for the three and six months ended June 30, 2018 and 2017 ; and its cash flows for the six months ended June 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 June 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	6 Months Ended
	Jun. 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	6 Months Ended
	Jun. 30, 2017
New Accounting Pronouncements and Changes in Accounting Principles [Abstract]
Accounting Pronouncements	Accounting Pronouncements Recently Issued Leases — I n 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. NSP-Minnesota has not yet fully determined the impacts of implementation. However, adoption is expected to occur on Jan. 1, 2019 utilizing the practical expedients provided by the standard and proposed in Targeted Improvements, Topic 842 (Proposed ASU 2018-200) . On Jan. 1, 2019 agreements considered leases for the use of office space, equipment and natural gas storage assets, as well as certain purchased power agreements (PPAs) for fossil-fueled generating facilities are expected to be recognized on the consolidated balance sheet. 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 significant 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, $6.8 million of pension costs were retrospectively reclassified from operating and maintenance expenses to other income, net on the consolidated income statement for the six months ended June 30, 2017. 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	6 Months Ended
	Jun. 30, 2018
Balance Sheet Related Disclosures [Abstract]
Selected Balance Sheet Data	Selected Balance Sheet Data (Thousands of Dollars) June 30, 2018 Dec. 31, 2017 Accounts receivable, net Accounts receivable $ 380,606 $ 366,388 Less allowance for bad debts (19,462 ) (21,278 ) $ 361,144 $ 345,110 (Thousands of Dollars) June 30, 2018 Dec. 31, 2017 Inventories Materials and supplies $ 208,596 $ 209,236 Fuel 73,737 94,483 Natural gas 12,269 33,993 $ 294,602 $ 337,712 (Thousands of Dollars) June 30, 2018 Dec. 31, 2017 Property, plant and equipment, net Electric plant $ 17,276,273 $ 17,024,925 Natural gas plant 1,407,523 1,370,330 Common and other property 719,069 724,066 Construction work in progress 528,079 530,126 Total property, plant and equipment 19,930,944 19,649,447 Less accumulated depreciation (7,210,566 ) (7,018,249 ) Nuclear fuel 2,712,590 2,697,412 Less accumulated amortization (2,357,292 ) (2,294,998 ) $ 13,075,676 $ 13,033,612

Income Taxes

Income Taxes	6 Months Ended
	Jun. 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: Six Months Ended June 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) (17.1 ) (11.4 ) Regulatory differences - ARAM (a) (8.5 ) (0.2 ) Regulatory differences - ARAM deferral (b) 7.9 — Other tax credits, net of federal income tax expense (1.5 ) (1.0 ) Other, net 0.7 0.4 Effective income tax rate 9.6 % 28.6 % (a) The average rate assumption method (ARAM); a method to flow back excess deferred taxes to customers. (b) As we receive direction from our regulatory commissions regarding the return of excess deferred taxes (to our customers resulting from the Tax Cuts and Jobs Act (TCJA)), the ARAM deferral may decrease during the year, which would result 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 - 2011 December 2018 2012 - 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. The IRS proposed an adjustment to the federal tax loss carryback claims and in 2015 the IRS forwarded the issue to the Office of Appeals (Appeals). In 2017 Xcel Energy and 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). After evaluating the proposed adjustment Xcel Energy filed a protest with the IRS. As of June 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 June 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 June 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) June 30, 2018 Dec. 31, 2017 Unrecognized tax benefit — Permanent tax positions $ 10.2 $ 10.2 Unrecognized tax benefit — Temporary tax positions 5.5 7.9 Total unrecognized tax benefit $ 15.7 $ 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) June 30, 2018 Dec. 31, 2017 NOL and tax credit carryforwards $ (15.0 ) $ (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) June 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 (1.0 ) 1.1 Payable for interest related to unrecognized tax benefits at end of period $ (1.9 ) $ (0.9 ) No amounts were accrued for penalties related to unrecognized tax benefits as of June 30, 2018 or Dec. 31, 2017 .

Rate Matters

Rate Matters	6 Months Ended
	Jun. 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 Report on Form 10-Q for the quarterly period ended March 31, 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. Each of the states in NSP-Minnesota’s service areas have opened dockets to address the impacts of the TCJA. In April 2018, NSP-Minnesota updated the estimated impact of the TCJA, which reflected an overall reduction in 2018 revenue requirements of approximately $136 million for electric and $7 million for natural gas, and made recommendations regarding the sharing of those benefits with ratepayers. The proposed electric options included: customer refunds and rider impacts of $68 million , deferral of $44 million to allow for a rate case stay-out for 2020, acceleration of depreciation for the King coal plant of $22 million and low income program funding of $2 million . The proposed natural gas options included customer refunds and rider impacts of $3 million , with the remaining TCJA benefits deferred to mitigate increased costs in the next natural gas rate case. In June 2018, the Minnesota Department of Commerce (DOC) recommended to implement refunds for the current tax impacts (approximately $90 million ), and incorporate the deferred tax impacts (approximately $53 million ) in NSP-Minnesota’s next electric and gas rate cases. A decision from the Minnesota Public Utilities Commission (MPUC) is expected in 2018. North and South Dakota — In February 2018, NSP-Minnesota proposed using the reduced revenue requirements from the TCJA to defer planned future rate filings in North Dakota and South Dakota. In July 2018, the South Dakota Public Utilities Commission (SDPUC) approved a settlement which proposed a one-time customer refund of $11 million for the 2018 impact of the TCJA and a 2 -year rate case moratorium. Dockets have also been opened in North Dakota and South Dakota. In February 2018, NSP-Minnesota proposed using the reduced revenue requirements from the TCJA to defer planned future rate filings in both jurisdictions. Recently Concluded Regulatory Proceedings — MPUC and the North Dakota Public Service Commission (NDPSC) PPA Terminations and Amendments — In June 2018, NSP-Minnesota executed the terminations of 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 6 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. 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	6 Months Ended
	Jun. 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 Report of Form 10-Q for the quarterly period ended March 31, 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 June 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) June 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 — In May 2015, underground pipes, tars and impacted soils were discovered in a right-of-way in Fargo, N.D. that appeared to be associated with a former MGP operated by NSP-Minnesota or prior companies. NSP-Minnesota removed impacted soils and other materials and commenced an investigation of the historic MGP and adjacent properties (the Fargo MGP Site). The North Dakota Department of Health approved NSP-Minnesota’s proposed cleanup plan in January 2017, which involves targeted source removal of impacted soils and historic MGP infrastructure. Remediation activities commenced in June 2018. 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 $10 million as of June 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 $22 million , of which approximately $12 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 December 2017, NSP-Minnesota filed a request with the MPUC to defer post-2017 MGP remediation expenditures allocable to the Minnesota jurisdiction, including the Fargo MGP Site. In March 2018, the DOC recommended that the MPUC deny NSP-Minnesota’s deferral request. A MPUC decision is expected in the third quarter of 2018. 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 contamination is present and 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 2018. NSP-Minnesota accrued $3 million as of June 30, 2018 and Dec. 31, 2017 for all of these sites. There may be insurance recovery and/or recovery from other PRPs that will offset any costs incurred. Xcel Energy anticipates that any amounts spent will be fully recovered from customers.
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	6 Months Ended
	Jun. 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 June 30, 2018 Year Ended Dec. 31, 2017 Borrowing limit $ 250 $ 250 Amount outstanding at period end 62 85 Average amount outstanding 1 25 Maximum amount outstanding 62 142 Weighted average interest rate, computed on a daily basis 1.85 % 1.14 % Weighted average interest rate at period end 1.85 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 June 30, 2018 Year Ended Dec. 31, 2017 Borrowing limit $ 500 $ 500 Amount outstanding at period end — 20 Average amount outstanding — 62 Maximum amount outstanding — 237 Weighted average interest rate, computed on a daily basis N/A 1.10 % Weighted average interest rate at period end N/A 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 June 30, 2018 and Dec. 31, 2017 , there were $36 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 June 30, 2018 , NSP-Minnesota had the following committed credit facility available (in millions of dollars): Credit Facility (a) Drawn (b) Available $ 500 $ 36 $ 464 (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 June 30, 2018 and Dec. 31, 2017 .

Fair Value of Financial Assets

Fair Value of Financial Assets and Liabilities	6 Months Ended
	Jun. 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 $547 million and $560 million as of June 30, 2018 and Dec. 31, 2017 , respectively, and unrealized losses and amounts recorded as other-than-temporary impairments were $23 million and $7 million as of June 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 June 30, 2018 and Dec. 31, 2017 : June 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 $ 30,868 $ 30,868 $ — $ — $ — $ 30,868 Commingled funds: Non U.S. equities 262,468 197,545 — — 90,402 287,947 Emerging market debt funds 158,296 — — — 158,075 158,075 Private equity investments 150,565 — — — 220,460 220,460 Real estate 128,115 — — — 197,032 197,032 Debt securities: Government securities 75,806 — 75,477 — — 75,477 U.S. corporate bonds 330,170 — 322,689 — — 322,689 Non U.S. corporate bonds 57,902 — 56,422 — — 56,422 Equity securities: U.S. equities 269,483 568,134 — — — 568,134 Non U.S. equities 156,756 227,226 — — — 227,226 Total $ 1,620,429 $ 1,023,773 $ 454,588 $ — $ 665,969 $ 2,144,330 (a) Reported in nuclear decommissioning fund and other investments on the consolidated balance sheet, which also includes $51 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 six months ended June 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 June 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 $ — $ 3,930 $ 2,300 $ 69,247 $ 75,477 U.S. corporate bonds 4,919 89,594 172,423 55,753 322,689 Non U.S. corporate bonds 2,007 19,925 30,316 4,174 56,422 Debt securities $ 6,926 $ 113,449 $ 205,039 $ 129,174 $ 454,588 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 June 30, 2018 and Dec. 31, 2017 : June 30, 2018 Fair Value (Thousands of Dollars) Cost Level 1 Level 2 Level 3 Total Rabbi Trust (a) Cash equivalents $ 394 $ 394 $ — $ — $ 394 Mutual funds 10,423 11,374 — — 11,374 Total $ 10,817 $ 11,768 $ — $ — $ 11,768 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 June 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 June 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 six months ended June 30, 2018 and 2017. At June 30, 2018 , net gains related to commodity derivative cash flow hedges recorded as a component of accumulated other comprehensive losses included $0.1 million of 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 June 30, 2018 and Dec. 31, 2017 : (Amounts in Thousands) (a)(b) June 30, 2018 Dec. 31, 2017 Megawatt hours of electricity 73,771 41,711 Million British thermal units of natural gas 7,403 23,829 Gallons of vehicle fuel 120 240 (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 six months ended June 30, 2018 and 2017 on accumulated other comprehensive loss, regulatory assets and liabilities and income: Three Months Ended June 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 $ — $ — $ 264 (a) $ — $ — Vehicle fuel and other commodity 31 — (35 ) (b) — — Total $ 31 $ — $ 229 $ — $ — Other derivative instruments Commodity trading $ — $ — $ — $ — $ 2,263 (c) Electric commodity — 24,262 — 1,142 (d) — Natural gas commodity — (18 ) — — — Total $ — $ 24,244 $ — $ 1,142 $ 2,263 Six Months Ended June 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 $ — $ — $ 525 (a) $ — $ — Vehicle fuel and other commodity 38 — (64 ) (b) — — Total $ 38 $ — $ 461 $ — $ — Other derivative instruments Commodity trading $ — $ — $ — $ — $ 8,981 (c) Electric commodity — (5,615 ) — 3,311 (d) — Natural gas commodity — 830 — (521 ) (e) (404 ) (e) Total $ — $ (4,785 ) $ — $ 2,790 $ 8,577 Three Months Ended June 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 $ — $ — $ 350 (a) $ — $ — Vehicle fuel and other commodity 43 — (5 ) (b) — — Total $ 43 $ — $ 345 $ — $ — Other derivative instruments Commodity trading $ — $ — $ — $ — $ 5,977 (c) Electric commodity — (1,526 ) — (1,149 ) (d) — Natural gas commodity — (51 ) — — — Total $ — $ (1,577 ) $ — $ (1,149 ) $ 5,977 Six Months Ended June 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 $ — $ — $ 692 (a) $ — $ — Vehicle fuel and other commodity 43 — (5 ) (b) — — Total $ 43 $ — $ 687 $ — $ — Other derivative instruments Commodity trading $ — $ — $ — $ — $ 6,599 (c) Electric commodity — (2,772 ) — (3,937 ) (d) — Natural gas commodity — (717 ) — 698 (e) (945 ) (e) Total $ — $ (3,489 ) $ — $ (3,239 ) $ 5,654 (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 six months ended June 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 June 30, 2018 , five of NSP-Minnesota’s 10 most significant counterparties for these activities, comprising $39.7 million or 48 percent of this credit exposure, had investment grade credit ratings from Standard & Poor’s, Moody’s or Fitch Ratings. Five of the 10 most significant counterparties, comprising $20.1 million or 24 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 June 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 June 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 June 30, 2018 : June 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 $ — $ 81 $ — $ 81 $ — $ 81 Other derivative instruments: Commodity trading 480 14,737 1,661 16,878 (8,601 ) 8,277 Electric commodity — — 22,896 22,896 (177 ) 22,719 Natural gas commodity — 167 — 167 — 167 Total current derivative assets $ 480 $ 14,985 $ 24,557 $ 40,022 $ (8,778 ) 31,244 PPAs (a) 24 Current derivative instruments $ 31,268 Noncurrent derivative assets Other derivative instruments: Commodity trading $ — $ 33,171 $ 5,596 $ 38,767 $ (12,085 ) $ 26,682 Total noncurrent derivative assets $ — $ 33,171 $ 5,596 $ 38,767 $ (12,085 ) 26,682 PPAs (a) 108 Noncurrent derivative instruments $ 26,790 June 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 $ 286 $ 12,147 $ 1,550 $ 13,983 $ (9,805 ) $ 4,178 Electric commodity — — 177 177 (177 ) — Total current derivative liabilities $ 286 $ 12,147 $ 1,727 $ 14,160 $ (9,982 ) 4,178 PPAs (a) 13,851 Current derivative instruments $ 18,029 Noncurrent derivative liabilities Other derivative instruments: Commodity trading $ — $ 24,476 $ 397 $ 24,873 $ (15,738 ) $ 9,135 Total noncurrent derivative liabilities $ — $ 24,476 $ 397 $ 24,873 $ (15,738 ) 9,135 PPAs (a) 82,987 Noncurrent derivative instruments $ 92,122 (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 June 30, 2018 . At June 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 six months ended June 30, 2018 and 2017 : Three Months Ended June 30 (Thousands of Dollars) 2018 2017 Balance at April 1 $ 13,516 $ 4,643 Purchases 26,395 40,460 Settlements (5,170 ) (8,166 ) Net transactions recorded during the period: (Losses) gains recognized in earnings (a) (2,654 ) 6,007 Net losses recognized as regulatory assets and liabilities (4,058 ) (2,372 ) Balance at June 30 $ 28,029 $ 40,572 Six Months Ended June 30 (Thousands of Dollars) 2018 2017 Balance at Jan. 1 $ 22,662 $ 15,320 Purchases 26,397 40,740 Settlements (7,104 ) (11,592 ) Net transactions recorded during the period: (Losses) gains recognized in earnings (a) (374 ) 5,215 Net gains recognized as regulatory assets and liabilities (13,552 ) (9,111 ) Balance at June 30 $ 28,029 $ 40,572 (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 six months ended June 30, 2018 and 2017 . Fair Value of Long-Term Debt As of June 30, 2018 and Dec. 31, 2017 , other financial instruments for which the carrying amount did not equal fair value were as follows: June 30, 2018 Dec. 31, 2017 (Thousands of Dollars) Carrying Amount Fair Value Carrying Fair Value Long-term debt, including current portion $ 4,935,068 $ 5,192,385 $ 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 June 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	6 Months Ended
	Jun. 30, 2018
Other Income and Expenses [Abstract]
Other (Expense) Income, Net	Other Expense, Net Other expense, net consisted of the following: Three Months Ended June 30 Six Months Ended June 30 (Thousands of Dollars) 2018 2017 2018 2017 Interest income $ 522 $ 605 $ 3,457 $ 3,314 Other nonoperating income 697 17 698 27 Insurance policy expense (1,537 ) (856 ) (1,195 ) (1,711 ) Benefits non-service cost (1,916 ) (3,398 ) (4,978 ) (6,796 ) Other expense, net $ (2,234 ) $ (3,632 ) $ (2,018 ) $ (5,166 )

Segment Information

Segment Information	6 Months Ended
	Jun. 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 June 30, 2018 Operating revenues (a)(b) $ 1,096,408 $ 83,112 $ 8,215 $ — $ 1,187,735 Intersegment revenues 128 116 — (244 ) — Total revenues $ 1,096,536 $ 83,228 $ 8,215 $ (244 ) $ 1,187,735 Net income $ 88,875 $ 399 $ 3,113 $ — $ 92,387 (Thousands of Dollars) Regulated Electric Regulated Natural Gas All Other Reconciling Eliminations Consolidated Total Three Months Ended June 30, 2017 Operating revenues (a)(b) $ 1,079,779 $ 78,006 $ 7,155 $ — $ 1,164,940 Intersegment revenues 243 177 — (420 ) — Total revenues $ 1,080,022 $ 78,183 $ 7,155 $ (420 ) $ 1,164,940 Net income (loss) $ 89,477 $ (3,360 ) $ 1,545 $ — $ 87,662 (a) Operating revenues include 117 million and 119 million of affiliate electric revenue for the three months ended June 30, 2018 and 2017 . (b) Operating revenues include an immaterial amount of affiliate gas revenue for the three months ended June 30, 2018 and 2017 . (Thousands of Dollars) Regulated Electric Regulated Natural Gas All Other Reconciling Eliminations Consolidated Total Six Months Ended June 30, 2018 Operating revenues (a)(b) $ 2,158,671 $ 324,544 $ 15,271 $ — $ 2,498,486 Intersegment revenues 293 260 — (553 ) — Total revenues $ 2,158,964 $ 324,804 $ 15,271 $ (553 ) $ 2,498,486 Net income $ 174,597 $ 25,844 $ 3,686 $ — $ 204,127 (Thousands of Dollars) Regulated Electric Regulated Natural Gas All Other Reconciling Eliminations Consolidated Total Six Months Ended June 30, 2017 Operating revenues (a)(b) $ 2,158,809 $ 299,189 $ 14,082 $ — $ 2,472,080 Intersegment revenues 352 271 — (623 ) — Total revenues $ 2,159,161 $ 299,460 $ 14,082 $ (623 ) $ 2,472,080 Net income $ 167,559 $ 14,145 $ 124 $ — 181,828 (a) Operating revenues include $234 million and $243 million of affiliate electric revenue for the six months ended June 30, 2018 and 2017 . (b) Operating revenues include an immaterial amount of affiliate gas revenue for the six months ended June 30, 2018 and 2017 .

Benefit Plans and Other Postret

Benefit Plans and Other Postretirement Benefits	6 Months Ended
	Jun. 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 June 30 2018 2017 2018 2017 (Thousands of Dollars) Pension Benefits Postretirement Health Care Benefits Service cost $ 6,982 $ 6,958 $ 43 $ 36 Interest cost (a) 8,804 10,177 769 854 Expected return on plan assets (a) (14,542 ) (15,017 ) (96 ) (54 ) Amortization of prior service (credit) cost (a) (28 ) 265 (759 ) (759 ) Amortization of net loss (a) 9,614 9,902 595 507 Net periodic benefit cost 10,830 12,285 552 584 Costs not recognized due to the effects of regulation (3,069 ) (4,899 ) — — Net benefit cost recognized for financial reporting $ 7,761 $ 7,386 $ 552 $ 584 Six Months Ended June 30 2018 2017 2018 2017 (Thousands of Dollars) Pension Benefits Postretirement Health Care Benefits Service cost $ 13,964 $ 13,916 $ 86 $ 72 Interest cost (a) 17,608 20,354 1,538 1,708 Expected return on plan assets (a) (29,083 ) (30,034 ) (192 ) (108 ) Amortization of prior service (credit) cost (a) (57 ) 530 (1,518 ) (1,518 ) Amortization of net loss (a) 19,229 19,804 1,190 1,014 Net periodic benefit cost 21,661 24,570 1,104 1,168 Costs not recognized due to the effects of regulation (5,832 ) (9,798 ) — — Net benefit cost recognized for financial reporting $ 15,829 $ 14,772 $ 1,104 $ 1,168 (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. 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	6 Months Ended
	Jun. 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 six months ended June 30, 2018 and 2017 were as follows: Three Months Ended June 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 April 1 $ (20,723 ) $ — $ (3,717 ) $ (24,440 ) Other comprehensive income before reclassifications 22 — — 22 Losses reclassified from net accumulated other comprehensive loss 165 — 54 219 Net current period other comprehensive income 187 — 54 241 Accumulated other comprehensive loss at June 30 $ (20,536 ) $ — $ (3,663 ) $ (24,199 ) Three Months Ended June 30, 2017 (Thousands of Dollars) Gains and Unrealized Defined Benefit Total Accumulated other comprehensive loss at April 1 $ (18,005 ) $ 105 $ (2,645 ) $ (20,545 ) Other comprehensive income before reclassifications 26 — — 26 Losses reclassified from net accumulated other comprehensive loss 204 — 36 240 Net current period other comprehensive income 230 — 36 266 Accumulated other comprehensive loss at June 30 $ (17,775 ) $ 105 $ (2,609 ) $ (20,279 ) Six Months Ended June 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 27 — — 27 Losses (gains) reclassified from net accumulated other comprehensive loss 332 (128 ) 107 311 Net current period other comprehensive income 359 (128 ) 107 338 Accumulated other comprehensive loss at June 30 $ (20,536 ) $ — $ (3,663 ) $ (24,199 ) Six Months Ended June 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 26 — — 26 Losses reclassified from net accumulated other comprehensive loss 407 — 71 478 Net current period other comprehensive income 433 — 71 504 Accumulated other comprehensive loss at June 30 $ (17,775 ) $ 105 $ (2,609 ) $ (20,279 ) Reclassifications from accumulated other comprehensive loss for the three and six months ended June 30, 2018 and 2017 were as follows: Amounts Reclassified from (Thousands of Dollars) Three Months Ended June 30, 2018 Three Months Ended June 30, 2017 Losses (gains) on cash flow hedges: Interest rate derivatives $ 264 (a) $ 350 (a) Vehicle fuel derivatives (35 ) (b) (5 ) (b) Total, pre-tax 229 345 Tax benefit (64 ) (141 ) Total, net of tax 165 204 Defined benefit pension and postretirement losses: Amortization of net loss 125 (c) 109 (c) Prior service credit (49 ) (c) (51 ) (c) Total, pre-tax 76 58 Tax benefit (22 ) (22 ) Total, net of tax 54 36 Marketable securities: Realization of gains — — Total, pre-tax — — Tax expense — — Total, net of tax — — Total amounts reclassified, net of tax $ 219 $ 240 Amounts Reclassified from (Thousands of Dollars) Six Months Ended June 30, 2018 Six Months Ended June 30, 2017 Losses (gains) on cash flow hedges: Interest rate derivatives $ 525 (a) $ 692 (a) Vehicle fuel derivatives (64 ) (b) (5 ) (b) Total, pre-tax 461 687 Tax benefit (129 ) (280 ) Total, net of tax 332 407 Defined benefit pension and postretirement losses: Amortization of net loss 250 (c) 218 (c) Prior service credit (98 ) (c) (100 ) (c) Total, pre-tax 152 118 Tax benefit (45 ) (47 ) Total, net of tax 107 71 Marketable securities: Realization of gains (179 ) — Total, pre-tax (179 ) — Tax expense 51 — Total, net of tax (128 ) — Total amounts reclassified, net of tax $ 311 $ 478 (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	6 Months Ended
	Jun. 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 June 30, 2018 (Thousands of Dollars) Electric Natural Gas All Other Total Major revenue types Revenue from contracts with customers: Residential $ 311,086 $ 42,398 $ 6,710 $ 360,194 Commercial and industrial (C&I) 514,710 33,417 42 548,169 Other 8,639 — 1,463 10,102 Total retail 834,435 75,815 8,215 918,465 Wholesale 42,033 — — 42,033 Transmission 59,951 — — 59,951 Interchange 117,417 — — 117,417 Other 6,033 3,317 — 9,350 Total revenue from contracts with customers 1,059,869 79,132 8,215 1,147,216 Alternative revenue and other 36,539 3,980 — 40,519 Total revenues $ 1,096,408 $ 83,112 $ 8,215 $ 1,187,735 Three Months Ended June 30, 2017 (Thousands of Dollars) Electric Natural Gas All Other Total Major revenue types Revenue from contracts with customers: Residential $ 288,733 $ 36,860 $ 6,416 $ 332,009 C&I 526,867 31,382 92 558,341 Other 7,854 — 647 8,501 Total retail 823,454 68,242 7,155 898,851 Wholesale 28,841 — — 28,841 Transmission 56,177 — — 56,177 Interchange 119,384 — — 119,384 Other 7,445 1,452 — 8,897 Total revenue from contracts with customers 1,035,301 69,694 7,155 1,112,150 Alternative revenue and other 44,478 8,312 — 52,790 Total revenues $ 1,079,779 $ 78,006 $ 7,155 $ 1,164,940 Six Months Ended June 30, 2018 (Thousands of Dollars) Electric Natural Gas All Other Total Major revenue types Revenue from contracts with customers: Residential $ 622,215 $ 174,057 $ 13,033 $ 809,305 Commercial and industrial (C&I) 982,733 130,325 85 1,113,143 Other 18,358 — 2,153 20,511 Total retail 1,623,306 304,382 15,271 1,942,959 Wholesale 88,398 — — 88,398 Transmission 114,749 — — 114,749 Interchange 234,389 — — 234,389 Other 17,578 5,528 — 23,106 Total revenue from contracts with customers 2,078,420 309,910 15,271 2,403,601 Alternative revenue and other 80,251 14,634 — 94,885 Total revenues $ 2,158,671 $ 324,544 $ 15,271 $ 2,498,486 Six Months Ended June 30, 2017 (Thousands of Dollars) Electric Natural Gas All Other Total Major revenue types Revenue from contracts with customers: Residential $ 593,146 $ 158,273 $ 12,626 $ 764,045 C&I 1,011,970 119,873 130 1,131,973 Other 16,057 — 1,326 17,383 Total retail 1,621,173 278,146 14,082 1,913,401 Wholesale 74,760 — — 74,760 Transmission 110,136 — — 110,136 Interchange 243,073 — — 243,073 Other 13,408 2,697 — 16,105 Total revenue from contracts with customers 2,062,550 280,843 14,082 2,357,475 Alternative revenue and other 96,259 18,346 — 114,605 Total revenues $ 2,158,809 $ 299,189 $ 14,082 $ 2,472,080

Selected Balance Sheet Data (Ta

Selected Balance Sheet Data (Tables)	6 Months Ended
	Jun. 30, 2018
Balance Sheet Related Disclosures [Abstract]
Accounts receivable, net	(Thousands of Dollars) June 30, 2018 Dec. 31, 2017 Accounts receivable, net Accounts receivable $ 380,606 $ 366,388 Less allowance for bad debts (19,462 ) (21,278 ) $ 361,144 $ 345,110
Inventories	(Thousands of Dollars) June 30, 2018 Dec. 31, 2017 Inventories Materials and supplies $ 208,596 $ 209,236 Fuel 73,737 94,483 Natural gas 12,269 33,993 $ 294,602 $ 337,712
Property, plant and equipment, net	(Thousands of Dollars) June 30, 2018 Dec. 31, 2017 Property, plant and equipment, net Electric plant $ 17,276,273 $ 17,024,925 Natural gas plant 1,407,523 1,370,330 Common and other property 719,069 724,066 Construction work in progress 528,079 530,126 Total property, plant and equipment 19,930,944 19,649,447 Less accumulated depreciation (7,210,566 ) (7,018,249 ) Nuclear fuel 2,712,590 2,697,412 Less accumulated amortization (2,357,292 ) (2,294,998 ) $ 13,075,676 $ 13,033,612

Income Taxes (Tables)

Income Taxes (Tables)	6 Months Ended
	Jun. 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: Six Months Ended June 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) (17.1 ) (11.4 ) Regulatory differences - ARAM (a) (8.5 ) (0.2 ) Regulatory differences - ARAM deferral (b) 7.9 — Other tax credits, net of federal income tax expense (1.5 ) (1.0 ) Other, net 0.7 0.4 Effective income tax rate 9.6 % 28.6 % (a) The average rate assumption method (ARAM); a method to flow back excess deferred taxes to customers. (b) As we receive direction from our regulatory commissions regarding the return of excess deferred taxes (to our customers resulting from the Tax Cuts and Jobs Act (TCJA)), the ARAM deferral may decrease during the year, which would result 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 - 2011 December 2018 2012 - 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) June 30, 2018 Dec. 31, 2017 Unrecognized tax benefit — Permanent tax positions $ 10.2 $ 10.2 Unrecognized tax benefit — Temporary tax positions 5.5 7.9 Total unrecognized tax benefit $ 15.7 $ 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) June 30, 2018 Dec. 31, 2017 NOL and tax credit carryforwards $ (15.0 ) $ (12.8 )
Interest Payable related to Unrecognized Tax Benefits [Table Text Block]	(Millions of Dollars) June 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 (1.0 ) 1.1 Payable for interest related to unrecognized tax benefits at end of period $ (1.9 ) $ (0.9 )

Commitments and Contingencies S

Commitments and Contingencies Schedule of Guarantor Obligations (Tables)	6 Months Ended
	Jun. 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) June 30, 2018 Dec. 31, 2017 Guarantee issued and outstanding $ 4.8 $ 4.8

Borrowings and Other Financin26

Borrowings and Other Financing Instruments (Tables)	6 Months Ended
	Jun. 30, 2018
Borrowings and Other Financing Instruments [Abstract]
Credit Facilities	At June 30, 2018 , NSP-Minnesota had the following committed credit facility available (in millions of dollars): Credit Facility (a) Drawn (b) Available $ 500 $ 36 $ 464 (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 June 30, 2018 Year Ended Dec. 31, 2017 Borrowing limit $ 250 $ 250 Amount outstanding at period end 62 85 Average amount outstanding 1 25 Maximum amount outstanding 62 142 Weighted average interest rate, computed on a daily basis 1.85 % 1.14 % Weighted average interest rate at period end 1.85 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 June 30, 2018 Year Ended Dec. 31, 2017 Borrowing limit $ 500 $ 500 Amount outstanding at period end — 20 Average amount outstanding — 62 Maximum amount outstanding — 237 Weighted average interest rate, computed on a daily basis N/A 1.10 % Weighted average interest rate at period end N/A 1.93

Fair Value of Financial Asset27

Fair Value of Financial Assets and Liabilities (Tables)	6 Months Ended
	Jun. 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 June 30, 2018 and Dec. 31, 2017 : June 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 $ 30,868 $ 30,868 $ — $ — $ — $ 30,868 Commingled funds: Non U.S. equities 262,468 197,545 — — 90,402 287,947 Emerging market debt funds 158,296 — — — 158,075 158,075 Private equity investments 150,565 — — — 220,460 220,460 Real estate 128,115 — — — 197,032 197,032 Debt securities: Government securities 75,806 — 75,477 — — 75,477 U.S. corporate bonds 330,170 — 322,689 — — 322,689 Non U.S. corporate bonds 57,902 — 56,422 — — 56,422 Equity securities: U.S. equities 269,483 568,134 — — — 568,134 Non U.S. equities 156,756 227,226 — — — 227,226 Total $ 1,620,429 $ 1,023,773 $ 454,588 $ — $ 665,969 $ 2,144,330 (a) Reported in nuclear decommissioning fund and other investments on the consolidated balance sheet, which also includes $51 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 June 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 $ — $ 3,930 $ 2,300 $ 69,247 $ 75,477 U.S. corporate bonds 4,919 89,594 172,423 55,753 322,689 Non U.S. corporate bonds 2,007 19,925 30,316 4,174 56,422 Debt securities $ 6,926 $ 113,449 $ 205,039 $ 129,174 $ 454,588
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 June 30, 2018 and Dec. 31, 2017 : June 30, 2018 Fair Value (Thousands of Dollars) Cost Level 1 Level 2 Level 3 Total Rabbi Trust (a) Cash equivalents $ 394 $ 394 $ — $ — $ 394 Mutual funds 10,423 11,374 — — 11,374 Total $ 10,817 $ 11,768 $ — $ — $ 11,768 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 June 30, 2018 and Dec. 31, 2017 : (Amounts in Thousands) (a)(b) June 30, 2018 Dec. 31, 2017 Megawatt hours of electricity 73,771 41,711 Million British thermal units of natural gas 7,403 23,829 Gallons of vehicle fuel 120 240 (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 six months ended June 30, 2018 and 2017 on accumulated other comprehensive loss, regulatory assets and liabilities and income: Three Months Ended June 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 $ — $ — $ 264 (a) $ — $ — Vehicle fuel and other commodity 31 — (35 ) (b) — — Total $ 31 $ — $ 229 $ — $ — Other derivative instruments Commodity trading $ — $ — $ — $ — $ 2,263 (c) Electric commodity — 24,262 — 1,142 (d) — Natural gas commodity — (18 ) — — — Total $ — $ 24,244 $ — $ 1,142 $ 2,263 Six Months Ended June 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 $ — $ — $ 525 (a) $ — $ — Vehicle fuel and other commodity 38 — (64 ) (b) — — Total $ 38 $ — $ 461 $ — $ — Other derivative instruments Commodity trading $ — $ — $ — $ — $ 8,981 (c) Electric commodity — (5,615 ) — 3,311 (d) — Natural gas commodity — 830 — (521 ) (e) (404 ) (e) Total $ — $ (4,785 ) $ — $ 2,790 $ 8,577 Three Months Ended June 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 $ — $ — $ 350 (a) $ — $ — Vehicle fuel and other commodity 43 — (5 ) (b) — — Total $ 43 $ — $ 345 $ — $ — Other derivative instruments Commodity trading $ — $ — $ — $ — $ 5,977 (c) Electric commodity — (1,526 ) — (1,149 ) (d) — Natural gas commodity — (51 ) — — — Total $ — $ (1,577 ) $ — $ (1,149 ) $ 5,977 Six Months Ended June 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 $ — $ — $ 692 (a) $ — $ — Vehicle fuel and other commodity 43 — (5 ) (b) — — Total $ 43 $ — $ 687 $ — $ — Other derivative instruments Commodity trading $ — $ — $ — $ — $ 6,599 (c) Electric commodity — (2,772 ) — (3,937 ) (d) — Natural gas commodity — (717 ) — 698 (e) (945 ) (e) Total $ — $ (3,489 ) $ — $ (3,239 ) $ 5,654 (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 June 30, 2018 : June 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 $ — $ 81 $ — $ 81 $ — $ 81 Other derivative instruments: Commodity trading 480 14,737 1,661 16,878 (8,601 ) 8,277 Electric commodity — — 22,896 22,896 (177 ) 22,719 Natural gas commodity — 167 — 167 — 167 Total current derivative assets $ 480 $ 14,985 $ 24,557 $ 40,022 $ (8,778 ) 31,244 PPAs (a) 24 Current derivative instruments $ 31,268 Noncurrent derivative assets Other derivative instruments: Commodity trading $ — $ 33,171 $ 5,596 $ 38,767 $ (12,085 ) $ 26,682 Total noncurrent derivative assets $ — $ 33,171 $ 5,596 $ 38,767 $ (12,085 ) 26,682 PPAs (a) 108 Noncurrent derivative instruments $ 26,790 June 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 $ 286 $ 12,147 $ 1,550 $ 13,983 $ (9,805 ) $ 4,178 Electric commodity — — 177 177 (177 ) — Total current derivative liabilities $ 286 $ 12,147 $ 1,727 $ 14,160 $ (9,982 ) 4,178 PPAs (a) 13,851 Current derivative instruments $ 18,029 Noncurrent derivative liabilities Other derivative instruments: Commodity trading $ — $ 24,476 $ 397 $ 24,873 $ (15,738 ) $ 9,135 Total noncurrent derivative liabilities $ — $ 24,476 $ 397 $ 24,873 $ (15,738 ) 9,135 PPAs (a) 82,987 Noncurrent derivative instruments $ 92,122 (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 June 30, 2018 . At June 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 six months ended June 30, 2018 and 2017 : Three Months Ended June 30 (Thousands of Dollars) 2018 2017 Balance at April 1 $ 13,516 $ 4,643 Purchases 26,395 40,460 Settlements (5,170 ) (8,166 ) Net transactions recorded during the period: (Losses) gains recognized in earnings (a) (2,654 ) 6,007 Net losses recognized as regulatory assets and liabilities (4,058 ) (2,372 ) Balance at June 30 $ 28,029 $ 40,572 Six Months Ended June 30 (Thousands of Dollars) 2018 2017 Balance at Jan. 1 $ 22,662 $ 15,320 Purchases 26,397 40,740 Settlements (7,104 ) (11,592 ) Net transactions recorded during the period: (Losses) gains recognized in earnings (a) (374 ) 5,215 Net gains recognized as regulatory assets and liabilities (13,552 ) (9,111 ) Balance at June 30 $ 28,029 $ 40,572 (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 June 30, 2018 and Dec. 31, 2017 , other financial instruments for which the carrying amount did not equal fair value were as follows: June 30, 2018 Dec. 31, 2017 (Thousands of Dollars) Carrying Amount Fair Value Carrying Fair Value Long-term debt, including current portion $ 4,935,068 $ 5,192,385 $ 4,933,018 $ 5,601,919

Other (Expense) Income, Net (Ta

Other (Expense) Income, Net (Tables)	6 Months Ended
	Jun. 30, 2018
Other Income and Expenses [Abstract]
Other (Expense) Income, Net	Other expense, net consisted of the following: Three Months Ended June 30 Six Months Ended June 30 (Thousands of Dollars) 2018 2017 2018 2017 Interest income $ 522 $ 605 $ 3,457 $ 3,314 Other nonoperating income 697 17 698 27 Insurance policy expense (1,537 ) (856 ) (1,195 ) (1,711 ) Benefits non-service cost (1,916 ) (3,398 ) (4,978 ) (6,796 ) Other expense, net $ (2,234 ) $ (3,632 ) $ (2,018 ) $ (5,166 )

Segment Information (Tables)

Segment Information (Tables)	6 Months Ended
	Jun. 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 June 30, 2018 Operating revenues (a)(b) $ 1,096,408 $ 83,112 $ 8,215 $ — $ 1,187,735 Intersegment revenues 128 116 — (244 ) — Total revenues $ 1,096,536 $ 83,228 $ 8,215 $ (244 ) $ 1,187,735 Net income $ 88,875 $ 399 $ 3,113 $ — $ 92,387 (Thousands of Dollars) Regulated Electric Regulated Natural Gas All Other Reconciling Eliminations Consolidated Total Three Months Ended June 30, 2017 Operating revenues (a)(b) $ 1,079,779 $ 78,006 $ 7,155 $ — $ 1,164,940 Intersegment revenues 243 177 — (420 ) — Total revenues $ 1,080,022 $ 78,183 $ 7,155 $ (420 ) $ 1,164,940 Net income (loss) $ 89,477 $ (3,360 ) $ 1,545 $ — $ 87,662 (a) Operating revenues include 117 million and 119 million of affiliate electric revenue for the three months ended June 30, 2018 and 2017 . (b) Operating revenues include an immaterial amount of affiliate gas revenue for the three months ended June 30, 2018 and 2017 . (Thousands of Dollars) Regulated Electric Regulated Natural Gas All Other Reconciling Eliminations Consolidated Total Six Months Ended June 30, 2018 Operating revenues (a)(b) $ 2,158,671 $ 324,544 $ 15,271 $ — $ 2,498,486 Intersegment revenues 293 260 — (553 ) — Total revenues $ 2,158,964 $ 324,804 $ 15,271 $ (553 ) $ 2,498,486 Net income $ 174,597 $ 25,844 $ 3,686 $ — $ 204,127 (Thousands of Dollars) Regulated Electric Regulated Natural Gas All Other Reconciling Eliminations Consolidated Total Six Months Ended June 30, 2017 Operating revenues (a)(b) $ 2,158,809 $ 299,189 $ 14,082 $ — $ 2,472,080 Intersegment revenues 352 271 — (623 ) — Total revenues $ 2,159,161 $ 299,460 $ 14,082 $ (623 ) $ 2,472,080 Net income $ 167,559 $ 14,145 $ 124 $ — 181,828 (a) Operating revenues include $234 million and $243 million of affiliate electric revenue for the six months ended June 30, 2018 and 2017 . (b) Operating revenues include an immaterial amount of affiliate gas revenue for the six months ended June 30, 2018 and 2017 .

Benefit Plans and Other Postr30

Benefit Plans and Other Postretirement Benefits (Tables)	6 Months Ended
	Jun. 30, 2018
Retirement Benefits [Abstract]
Components of Net Periodic Benefit Cost (Credit)	Components of Net Periodic Benefit Cost Three Months Ended June 30 2018 2017 2018 2017 (Thousands of Dollars) Pension Benefits Postretirement Health Care Benefits Service cost $ 6,982 $ 6,958 $ 43 $ 36 Interest cost (a) 8,804 10,177 769 854 Expected return on plan assets (a) (14,542 ) (15,017 ) (96 ) (54 ) Amortization of prior service (credit) cost (a) (28 ) 265 (759 ) (759 ) Amortization of net loss (a) 9,614 9,902 595 507 Net periodic benefit cost 10,830 12,285 552 584 Costs not recognized due to the effects of regulation (3,069 ) (4,899 ) — — Net benefit cost recognized for financial reporting $ 7,761 $ 7,386 $ 552 $ 584 Six Months Ended June 30 2018 2017 2018 2017 (Thousands of Dollars) Pension Benefits Postretirement Health Care Benefits Service cost $ 13,964 $ 13,916 $ 86 $ 72 Interest cost (a) 17,608 20,354 1,538 1,708 Expected return on plan assets (a) (29,083 ) (30,034 ) (192 ) (108 ) Amortization of prior service (credit) cost (a) (57 ) 530 (1,518 ) (1,518 ) Amortization of net loss (a) 19,229 19,804 1,190 1,014 Net periodic benefit cost 21,661 24,570 1,104 1,168 Costs not recognized due to the effects of regulation (5,832 ) (9,798 ) — — Net benefit cost recognized for financial reporting $ 15,829 $ 14,772 $ 1,104 $ 1,168 (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.

Other Comprehensive Income (Tab

Other Comprehensive Income (Tables)	6 Months Ended
	Jun. 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 six months ended June 30, 2018 and 2017 were as follows: Three Months Ended June 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 April 1 $ (20,723 ) $ — $ (3,717 ) $ (24,440 ) Other comprehensive income before reclassifications 22 — — 22 Losses reclassified from net accumulated other comprehensive loss 165 — 54 219 Net current period other comprehensive income 187 — 54 241 Accumulated other comprehensive loss at June 30 $ (20,536 ) $ — $ (3,663 ) $ (24,199 ) Three Months Ended June 30, 2017 (Thousands of Dollars) Gains and Unrealized Defined Benefit Total Accumulated other comprehensive loss at April 1 $ (18,005 ) $ 105 $ (2,645 ) $ (20,545 ) Other comprehensive income before reclassifications 26 — — 26 Losses reclassified from net accumulated other comprehensive loss 204 — 36 240 Net current period other comprehensive income 230 — 36 266 Accumulated other comprehensive loss at June 30 $ (17,775 ) $ 105 $ (2,609 ) $ (20,279 ) Six Months Ended June 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 27 — — 27 Losses (gains) reclassified from net accumulated other comprehensive loss 332 (128 ) 107 311 Net current period other comprehensive income 359 (128 ) 107 338 Accumulated other comprehensive loss at June 30 $ (20,536 ) $ — $ (3,663 ) $ (24,199 ) Six Months Ended June 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 26 — — 26 Losses reclassified from net accumulated other comprehensive loss 407 — 71 478 Net current period other comprehensive income 433 — 71 504 Accumulated other comprehensive loss at June 30 $ (17,775 ) $ 105 $ (2,609 ) $ (20,279 )
Reclassifications out of Accumulated Other Comprehensive Loss	Reclassifications from accumulated other comprehensive loss for the three and six months ended June 30, 2018 and 2017 were as follows: Amounts Reclassified from (Thousands of Dollars) Three Months Ended June 30, 2018 Three Months Ended June 30, 2017 Losses (gains) on cash flow hedges: Interest rate derivatives $ 264 (a) $ 350 (a) Vehicle fuel derivatives (35 ) (b) (5 ) (b) Total, pre-tax 229 345 Tax benefit (64 ) (141 ) Total, net of tax 165 204 Defined benefit pension and postretirement losses: Amortization of net loss 125 (c) 109 (c) Prior service credit (49 ) (c) (51 ) (c) Total, pre-tax 76 58 Tax benefit (22 ) (22 ) Total, net of tax 54 36 Marketable securities: Realization of gains — — Total, pre-tax — — Tax expense — — Total, net of tax — — Total amounts reclassified, net of tax $ 219 $ 240 Amounts Reclassified from (Thousands of Dollars) Six Months Ended June 30, 2018 Six Months Ended June 30, 2017 Losses (gains) on cash flow hedges: Interest rate derivatives $ 525 (a) $ 692 (a) Vehicle fuel derivatives (64 ) (b) (5 ) (b) Total, pre-tax 461 687 Tax benefit (129 ) (280 ) Total, net of tax 332 407 Defined benefit pension and postretirement losses: Amortization of net loss 250 (c) 218 (c) Prior service credit (98 ) (c) (100 ) (c) Total, pre-tax 152 118 Tax benefit (45 ) (47 ) Total, net of tax 107 71 Marketable securities: Realization of gains (179 ) — Total, pre-tax (179 ) — Tax expense 51 — Total, net of tax (128 ) — Total amounts reclassified, net of tax $ 311 $ 478 (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 (Tables)

Revenues Revenues (Tables)	6 Months Ended
	Jun. 30, 2018
Revenue from Contract with Customer [Abstract]
Disaggregation of Revenue [Table Text Block]	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 June 30, 2018 (Thousands of Dollars) Electric Natural Gas All Other Total Major revenue types Revenue from contracts with customers: Residential $ 311,086 $ 42,398 $ 6,710 $ 360,194 Commercial and industrial (C&I) 514,710 33,417 42 548,169 Other 8,639 — 1,463 10,102 Total retail 834,435 75,815 8,215 918,465 Wholesale 42,033 — — 42,033 Transmission 59,951 — — 59,951 Interchange 117,417 — — 117,417 Other 6,033 3,317 — 9,350 Total revenue from contracts with customers 1,059,869 79,132 8,215 1,147,216 Alternative revenue and other 36,539 3,980 — 40,519 Total revenues $ 1,096,408 $ 83,112 $ 8,215 $ 1,187,735 Three Months Ended June 30, 2017 (Thousands of Dollars) Electric Natural Gas All Other Total Major revenue types Revenue from contracts with customers: Residential $ 288,733 $ 36,860 $ 6,416 $ 332,009 C&I 526,867 31,382 92 558,341 Other 7,854 — 647 8,501 Total retail 823,454 68,242 7,155 898,851 Wholesale 28,841 — — 28,841 Transmission 56,177 — — 56,177 Interchange 119,384 — — 119,384 Other 7,445 1,452 — 8,897 Total revenue from contracts with customers 1,035,301 69,694 7,155 1,112,150 Alternative revenue and other 44,478 8,312 — 52,790 Total revenues $ 1,079,779 $ 78,006 $ 7,155 $ 1,164,940 Six Months Ended June 30, 2018 (Thousands of Dollars) Electric Natural Gas All Other Total Major revenue types Revenue from contracts with customers: Residential $ 622,215 $ 174,057 $ 13,033 $ 809,305 Commercial and industrial (C&I) 982,733 130,325 85 1,113,143 Other 18,358 — 2,153 20,511 Total retail 1,623,306 304,382 15,271 1,942,959 Wholesale 88,398 — — 88,398 Transmission 114,749 — — 114,749 Interchange 234,389 — — 234,389 Other 17,578 5,528 — 23,106 Total revenue from contracts with customers 2,078,420 309,910 15,271 2,403,601 Alternative revenue and other 80,251 14,634 — 94,885 Total revenues $ 2,158,671 $ 324,544 $ 15,271 $ 2,498,486 Six Months Ended June 30, 2017 (Thousands of Dollars) Electric Natural Gas All Other Total Major revenue types Revenue from contracts with customers: Residential $ 593,146 $ 158,273 $ 12,626 $ 764,045 C&I 1,011,970 119,873 130 1,131,973 Other 16,057 — 1,326 17,383 Total retail 1,621,173 278,146 14,082 1,913,401 Wholesale 74,760 — — 74,760 Transmission 110,136 — — 110,136 Interchange 243,073 — — 243,073 Other 13,408 2,697 — 16,105 Total revenue from contracts with customers 2,062,550 280,843 14,082 2,357,475 Alternative revenue and other 96,259 18,346 — 114,605 Total revenues $ 2,158,809 $ 299,189 $ 14,082 $ 2,472,080

Accounting Pronouncements (Deta

Accounting Pronouncements (Details) - Accounting Standards Update 2017-07 $ in Millions	6 Months Ended
	Jun. 30, 2017USD ($)
Operating and Maintenance Expense
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Net benefit cost recognized for financial reporting	$ 6.8
Other Income
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Net benefit cost recognized for financial reporting	$ (6.8)

Selected Balance Sheet Data, Ac

Selected Balance Sheet Data, Accounts Receivable (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Accounts receivable, net
Accounts receivable	$ 380,606	$ 366,388
Less allowance for bad debts	(19,462)	(21,278)
Accounts receivable, net	$ 361,144	$ 345,110

Selected Balance Sheet Data, In

Selected Balance Sheet Data, Inventories (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Public Utilities, Inventory [Line Items]
Inventories	$ 294,602	$ 337,712
Materials and supplies
Public Utilities, Inventory [Line Items]
Inventories	208,596	209,236
Fuel
Public Utilities, Inventory [Line Items]
Inventories	73,737	94,483
Natural gas
Public Utilities, Inventory [Line Items]
Inventories	$ 12,269	$ 33,993

Selected Balance Sheet Data, Pr

Selected Balance Sheet Data, Property, Plant and Equipment (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Public Utility, Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	$ 19,930,944	$ 19,649,447
Less accumulated depreciation	(7,210,566)	(7,018,249)
Property, plant and equipment, net	13,075,676	13,033,612
Electric plant
Public Utility, Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	17,276,273	17,024,925
Natural gas plant
Public Utility, Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	1,407,523	1,370,330
Common and other property
Public Utility, Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	719,069	724,066
Construction work in progress
Public Utility, Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	528,079	530,126
Nuclear fuel
Public Utility, Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	2,712,590	2,697,412
Less accumulated depreciation	$ (2,357,292)	$ (2,294,998)

Income Taxes (Details)

Income Taxes (Details) - USD ($)	6 Months Ended		9 Months Ended	12 Months Ended
	Jun. 30, 2018	Jun. 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	$ (1,900,000)			$ (900,000)	$ (2,000,000)
Interest Expense related to unrecognized tax benefits	$ (1,000,000)			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	(17.10%)	(11.40%)
Effective Income Tax Rate Reconciliation, Change in Enacted Tax Rate, Percent	(8.50%)	(0.20%)
Effective Income Tax Rate Reconciliation, Tax Cuts and Jobs Act of 2017, Change in Tax Rate, Percent	7.90%	0.00%
Effective Income Tax Rate Reconciliation Regulatory Differences Utility Plant Items	(1.50%)	(1.00%)
Effective Income Tax Rate Reconciliation, Other Adjustments, Percent	0.70%	0.40%
Effective Income Tax Rate Reconciliation, Percent	9.60%	28.60%
Unrecognized Tax Benefits [Abstract]
Unrecognized tax benefit — Permanent tax positions	$ 10,200,000			10,200,000
Unrecognized tax benefit — Temporary tax positions	5,500,000			7,900,000
Total unrecognized tax benefit	15,700,000			18,100,000
NOL and tax credit carryforwards	(15,000,000)			(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

Rate Matters (Details)

Rate Matters (Details) - USD ($) $ in Millions	1 Months Ended							6 Months Ended
	Jul. 31, 2018	Jun. 30, 2018	Apr. 30, 2018	Sep. 30, 2016	Jun. 30, 2016	Feb. 28, 2015	Nov. 30, 2013	Jun. 30, 2018
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						$ 103
Public Utilities, Purchase Power Agreement Termination Payments		$ 93						93
Regulatory Asset, Amortization Period		10 years
Laurentian purchase power agreement termination [Member]
Rate Matters [Abstract]
Regulatory Assets		$ 109						$ 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, Estimated 2018 Electric Rate Reduction			$ 136
Tax Cuts and Jobs Act of 2017, Estimated 2018 Natural Gas Rate Reduction			7
Tax Cuts and Jobs Act of 2017, Proposed 2018 Electric Refund of TCJA Benefits			68
Tax Cuts and Jobs Act of 2017, Proposed Electric Deferral of TCJA Benefits			44
Tax Cuts and Jobs Act of 2017, Proposed Amount of Accelerated Depreciation			22
Tax Cuts and Jobs Act of 2017, Proposed Low Income Program Funding from TCJA Benefits			2
Tax Cuts and Jobs Act of 2017, Proposed 2018 Natural Gas Refund of TCJA Benefits			$ 3
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%
Minnesota Department of Commerce [Member] | MPUC Proceeding - Tax Cuts and Jobs Act of 2017 [Member]
Rate Matters [Abstract]
Tax Cuts and Jobs Act of 2017, Proposed 2018 Total Refund of TCJA Benefits		$ 90
Tax Cuts and Jobs Act of 2017, Proposed Total Deferral of TCJA Benefits		$ 53
Subsequent Event [Member] | 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

Commitments and Contingencies,

Commitments and Contingencies, Purchased Power Agreements (Details) $ in Millions	6 Months Ended
	Jun. 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	6 Months Ended
	Jun. 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 Contingencies41

Commitments and Contingencies, Environmental Contingencies (Details) $ in Millions	6 Months Ended
	Jun. 30, 2018USD ($)	Dec. 31, 2017USD ($)
Fargo MGP Site
Environmental Requirements [Abstract]
Accrual for Environmental Loss Contingencies, Gross	$ 10	$ 16
Current Cost Estimate for Site Remediation	22
Estimated amount spent on cleanup	$ 12
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 Financin42

Borrowings and Other Financing Instruments, Short-Term Borrowings (Details) - USD ($)	3 Months Ended	12 Months Ended
	Jun. 30, 2018	Dec. 31, 2017
Short-term Debt [Line Items]
Amount outstanding at period end	$ 0	$ 20,000,000
Money Pool
Short-term Debt [Line Items]
Borrowing limit	250,000,000	250,000,000
Amount outstanding at period end	62,000,000	85,000,000
Average amount outstanding	1,000,000	25,000,000
Maximum amount outstanding	$ 62,000,000	$ 142,000,000
Weighted average interest rate, computed on a daily basis (percentage)	1.85%	1.14%
Weighted average interest rate at period end (percentage)	1.85%	1.18%
Commercial Paper
Short-term Debt [Line Items]
Borrowing limit	$ 500,000,000	$ 500,000,000
Amount outstanding at period end	0	20,000,000
Average amount outstanding	0	62,000,000
Maximum amount outstanding	$ 0	$ 237,000,000
Weighted average interest rate, computed on a daily basis (percentage)		1.10%
Weighted average interest rate at period end (percentage)		1.93%

Borrowings and Other Financin43

Borrowings and Other Financing Instruments, Letters of Credit (Details) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2018	Dec. 31, 2017
Line of Credit Facility [Line Items]
Amount outstanding at period end	$ 0	$ 20,000
Letter of Credit
Line of Credit Facility [Line Items]
Amount outstanding at period end	$ 36,000	$ 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 Financin44

Borrowings and Other Financing Instruments, Credit Facility (Details) - Credit Facility - USD ($)		6 Months Ended
		Jun. 30, 2018	Dec. 31, 2017
Line of Credit Facility [Line Items]
Credit Facility	[1]	$ 500,000,000
Drawn	[2]	36,000,000
Available		$ 464,000,000
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 Asset45

Fair Value of Financial Assets and Liabilities Fair Value of Financial Assets and Liabilities (Details)	6 Months Ended
	Jun. 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 Asset46

Fair Value of Financial Assets and Liabilities, Cost and Fair Value of Nuclear Decommissioning Fund (Details) - USD ($) $ in Thousands		6 Months Ended		12 Months Ended
		Jun. 30, 2018		Dec. 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Unrealized Gain on Securities		$ 547,000		$ 560,000
Unrealized Loss on Securities		23,000		7,000
Investments [Abstract]
Miscellaneous investments		51,000		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		30,868	[1]	28,741	[2]
Investments [Abstract]
Equity Securities		1,620,429	[1]	1,590,849	[2]
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Cost | Non U.S. equities
Investments [Abstract]
Equity Securities		262,468	[1]	263,694	[2]
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Cost | Emerging market debt funds
Investments [Abstract]
Equity Securities		158,296	[1]	156,057	[2]
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Cost | Private equity investments
Investments [Abstract]
Equity Securities		150,565	[1]	141,413	[2]
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Cost | Real estate
Investments [Abstract]
Equity Securities		128,115	[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]
Available-for-sale Securities, Debt Securities		75,806	[1]	67,760	[2]
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Cost | U.S. corporate bonds
Investments [Abstract]
Available-for-sale Securities, Debt Securities		330,170	[1]	319,809	[2]
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Cost | Non U.S. corporate bonds
Investments [Abstract]
Available-for-sale Securities, Debt Securities		57,902	[1]	50,121	[2]
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Cost | U.S. equities
Investments [Abstract]
Equity Securities		269,483	[1]	271,166	[2]
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Cost | Non U.S. equities
Investments [Abstract]
Equity Securities		156,756	[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		30,868	[1]	28,741	[2]
Alternative Investment, Fair Value Disclosure		665,969	[1],[3]	659,086	[2],[4]
Investments [Abstract]
Equity Securities		2,144,330	[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, Fair Value Disclosure		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, Fair Value Disclosure		90,402	[1],[3]	89,857	[2],[4]
Investments [Abstract]
Equity Securities		287,947	[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, Fair Value Disclosure		158,075	[1],[3]	166,375	[2],[4]
Investments [Abstract]
Equity Securities		158,075	[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, Fair Value Disclosure		220,460	[1],[3]	198,037	[2],[4]
Investments [Abstract]
Equity Securities		220,460	[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, Fair Value Disclosure		197,032	[1],[3]	201,842	[2],[4]
Investments [Abstract]
Equity Securities		197,032	[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, Fair Value Disclosure	[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, Fair Value Disclosure		0	[1],[3]	0	[2],[4]
Investments [Abstract]
Available-for-sale Securities, Debt Securities		75,477	[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, Fair Value Disclosure		0	[1],[3]	0	[2],[4]
Investments [Abstract]
Available-for-sale Securities, Debt Securities		322,689	[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, Fair Value Disclosure		0	[1],[3]	0	[2],[4]
Investments [Abstract]
Available-for-sale Securities, Debt Securities		56,422	[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, Fair Value Disclosure		0	[1],[3]	0	[2],[4]
Investments [Abstract]
Equity Securities		568,134	[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, Fair Value Disclosure		0	[1],[3]	0	[2],[4]
Investments [Abstract]
Equity Securities		227,226	[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		30,868	[1]	28,741	[2]
Investments [Abstract]
Equity Securities		1,023,773	[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		197,545	[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]
Available-for-sale Securities, Debt Securities		0	[1]	0	[2]
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Fair Value | Level 1 | U.S. corporate bonds
Investments [Abstract]
Available-for-sale Securities, Debt Securities		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]
Available-for-sale Securities, Debt Securities		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		568,134	[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		227,226	[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		454,588	[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]
Available-for-sale Securities, Debt Securities		75,477	[1]	69,413	[2]
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Fair Value | Level 2 | U.S. corporate bonds
Investments [Abstract]
Available-for-sale Securities, Debt Securities		322,689	[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]
Available-for-sale Securities, Debt Securities		56,422	[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]
Available-for-sale Securities, Debt Securities		0	[1]	0	[2]
Nuclear Decommissioning Fund | Fair Value Measured on a Recurring Basis | Fair Value | Level 3 | U.S. corporate bonds
Investments [Abstract]
Available-for-sale Securities, Debt Securities		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]
Available-for-sale Securities, Debt Securities		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 $51 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 Asset47

Fair Value of Financial Assets and Liabilities, Final Contractual Maturity Dates of Debt Securities in Nuclear Decommissioning Fund (Details) $ in Thousands	Jun. 30, 2018USD ($)
Final Contractual Maturity [Abstract]
Due in 1 Year or Less	$ 6,926
Due in 1 to 5 Years	113,449
Due in 5 to 10 Years	205,039
Due after 10 Years	129,174
Total	454,588
Government securities
Final Contractual Maturity [Abstract]
Due in 1 Year or Less	0
Due in 1 to 5 Years	3,930
Due in 5 to 10 Years	2,300
Due after 10 Years	69,247
Total	75,477
U.S. corporate bonds
Final Contractual Maturity [Abstract]
Due in 1 Year or Less	4,919
Due in 1 to 5 Years	89,594
Due in 5 to 10 Years	172,423
Due after 10 Years	55,753
Total	322,689
Non U.S. corporate bonds
Final Contractual Maturity [Abstract]
Due in 1 Year or Less	2,007
Due in 1 to 5 Years	19,925
Due in 5 to 10 Years	30,316
Due after 10 Years	4,174
Total	$ 56,422

Fair Value of Financial Asset48

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		Jun. 30, 2018	Dec. 31, 2017
Cost
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading Securities	[1]	$ 10,817	$ 11,115
Cost | Rabbi Trust [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	[1]	394	783
Cost | Mutual Funds [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading Securities	[1]	10,423	10,332
Fair Value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading Securities	[1]	11,768	12,066
Fair Value | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading Securities	[1]	11,768	12,066
Fair Value | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading Securities	[1]	0	0
Fair Value | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading Securities	[1]	0	0
Fair Value | Rabbi Trust [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	[1]	394	783
Fair Value | Rabbi Trust [Member] | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	[1]	394	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]
Trading Securities	[1]	11,374	11,283
Fair Value | Mutual Funds [Member] | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading Securities	[1]	11,374	11,283
Fair Value | Mutual Funds [Member] | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading Securities	[1]	0	0
Fair Value | Mutual Funds [Member] | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading Securities	[1]	$ 0	$ 0
[1]	Reported in nuclear decommissioning fund and other investments on the consolidated balance sheet.

Fair Value of Financial Asset49

Fair Value of Financial Assets and Liabilities, Derivative Instruments (Details) gal in Thousands, MWh in Thousands, MMBTU in Thousands, $ in Millions		6 Months Ended
		Jun. 30, 2018USD ($)MMBTUgalMWhCounterparty	Dec. 31, 2017MMBTUgalMWh
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 | Counterparty		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 | Counterparty		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 | Counterparty		5
Wholesale, trading and non-trading commodity credit exposure for the most significant counterparties | $		$ 39.7
Percentage of wholesale, trading and non-trading commodity credit exposure for the most significant counterparties (in hundredths)		48.00%
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 | Counterparty		5
Wholesale, trading and non-trading commodity credit exposure for the most significant counterparties | $		$ 20.1
Percentage of wholesale, trading and non-trading commodity credit exposure for the most significant counterparties (in hundredths)		24.00%
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]	73,771	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]	7,403	23,829
Vehicle Fuel Commodity (in gallons)
Gross Notional Amounts of Commodity Forwards, Options and FTRs [Abstract]
Derivative, Nonmonetary Notional amount | gal	[1],[2]	120	240
Commodity Derivative
Reclassification of Cash Flow Hedge Gain (Loss) [Abstract]
Cash Flow Hedge Gain (Loss) to be Reclassified within Twelve Months | $		$ 0.1
[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 Asset50

Fair Value of Financial Assets and Liabilities, Impact of Derivative Activity (Details) - USD ($)		3 Months Ended		6 Months Ended
		Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018		Jun. 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		31,000	43,000	38,000		43,000
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		229,000	345,000	461,000		687,000
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]	264,000	350,000	525,000		692,000
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		31,000	43,000	38,000		43,000
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]	(35,000)	(5,000)	(64,000)		(5,000)
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		24,244,000	(1,577,000)	(4,785,000)		(3,489,000)
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)		1,142,000	(1,149,000)	2,790,000		(3,239,000)
Pre-tax gains (losses) recognized during the period in income		2,263,000	5,977,000	8,577,000		5,654,000
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]	2,263,000	5,977,000	8,981,000		6,599,000
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		24,262,000	(1,526,000)	(5,615,000)		(2,772,000)
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]	1,142,000	(1,149,000)	3,311,000		(3,937,000)
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		(18,000)	(51,000)	830,000		(717,000)
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 Asset51

Fair Value of Financial Assets and Liabilities, Credit Related Contingent Features (Details) - USD ($)	Jun. 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 Asset52

Fair Value of Financial Assets and Liabilities, Derivative Assets and Liabilities at Fair Value (Details) - USD ($) $ in Thousands	Jun. 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	31,268		25,230
Other Noncurrent Assets
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	26,790		28,102
Other Current Liabilities
Derivatives, Fair Value [Line Items]
Derivative Liability, Fair Value, Gross Liability	18,029		17,697
Other Noncurrent Liabilities
Derivatives, Fair Value [Line Items]
Derivative Liability, Fair Value, Gross Liability	92,122		102,742
Fair Value Measured on a Recurring Basis | Other Current Assets
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	31,244		24,573
Derivative Asset, Fair Value, Gross Liability and Obligation to Return Cash, Offset	(8,778)	[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	8,277		7,233
Derivative Asset, Fair Value, Gross Liability and Obligation to Return Cash, Offset	(8,601)	[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	22,719		17,156
Derivative Asset, Fair Value, Gross Liability and Obligation to Return Cash, Offset	(177)	[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	167		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	26,682		27,982
Derivative Asset, Fair Value, Gross Liability and Obligation to Return Cash, Offset	(12,085)	[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	26,682		27,982
Derivative Asset, Fair Value, Gross Liability and Obligation to Return Cash, Offset	(12,085)	[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	4,178		3,592
Derivative Liability, Fair Value, Gross Asset and Right to Reclaim Cash, Offset	(9,982)	[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	4,178		3,592
Derivative Liability, Fair Value, Gross Asset and Right to Reclaim Cash, Offset	(9,805)	[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	(177)	[1]	(425)	[2]
Fair Value Measured on a Recurring Basis | Other Noncurrent Liabilities
Derivatives, Fair Value [Line Items]
Derivative Liability, Fair Value, Gross Liability	9,135		12,829
Derivative Liability, Fair Value, Gross Asset and Right to Reclaim Cash, Offset	(15,738)	[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	9,135		12,829
Derivative Liability, Fair Value, Gross Asset and Right to Reclaim Cash, Offset	(15,738)	[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	480		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	480		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	0		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	0		0
Fair Value Measured on a Recurring Basis | Level 1 | Other Current Liabilities
Derivatives, Fair Value [Line Items]
Derivative Liability, Fair Value, Gross Liability	286		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	286		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	0		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	0		0
Fair Value Measured on a Recurring Basis | Level 2 | Other Current Assets
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	14,985		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	14,737		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	167		77
Fair Value Measured on a Recurring Basis | Level 2 | Other Noncurrent Assets
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	33,171		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	33,171		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	12,147		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	12,147		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	24,476		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	24,476		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	24,557		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	1,661		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	22,896		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	5,596		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	5,596		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	1,727		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	1,550		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	177		425
Fair Value Measured on a Recurring Basis | Level 3 | Other Noncurrent Liabilities
Derivatives, Fair Value [Line Items]
Derivative Liability, Fair Value, Gross Liability	397		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	397		0
Fair Value, Measurements, Nonrecurring | Other Current Assets | Purchased Power Agreements
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	24	[3]	657	[4]
Fair Value, Measurements, Nonrecurring | Other Noncurrent Assets | Purchased Power Agreements
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	108	[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	82,987	[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	40,022		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	16,878		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	22,896		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	167		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	38,767		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	38,767		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	14,160		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	13,983		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	177		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	24,873		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	24,873		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	81		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	81		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	$ 81		$ 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 June 30, 2018. At June 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 Asset53

Fair Value of Financial Assets and Liabilities, Changes in Level 3 Commodity Derivatives (Details) - Commodity Contract - USD ($)		3 Months Ended		6 Months Ended
		Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Balance at beginning of period		$ 13,516,000	$ 4,643,000	$ 22,662,000	$ 15,320,000
Purchases		26,395,000	40,460,000	26,397,000	40,740,000
Settlements		(5,170,000)	(8,166,000)	(7,104,000)	(11,592,000)
Gains (losses) recognized in earnings	[1]	(2,654,000)	6,007,000	(374,000)	5,215,000
Net losses recognized as regulatory assets and liabilities		(4,058,000)	(2,372,000)	(13,552,000)	(9,111,000)
Balance at end of period		28,029,000	40,572,000	28,029,000	40,572,000
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 Asset54

Fair Value of Financial Assets and Liabilities, Fair Value of Long-Term Debt (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Carrying Amount
Financial Liabilities, Balance Sheet Groupings [Abstract]
Long-term debt, including current portion	$ 4,935,068	$ 4,933,018
Fair Value
Financial Liabilities, Balance Sheet Groupings [Abstract]
Long-term debt, including current portion	$ 5,192,385	$ 5,601,919

Other (Expense) Income, Net (De

Other (Expense) Income, Net (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Other Income and Expenses [Abstract]
Interest income	$ 522	$ 605	$ 3,457	$ 3,314
Other nonoperating income	697	17	698	27
Insurance policy expense	(1,537)	(856)	(1,195)	(1,711)
Defined Benefit Plan, Non-service Costs	(1,916)	(3,398)	(4,978)	(6,796)
Other expense, net	$ (2,234)	$ (3,632)	$ (2,018)	$ (5,166)

Segment Information (Details)

Segment Information (Details) - USD ($) $ in Thousands	3 Months Ended				6 Months Ended
	Jun. 30, 2018		Jun. 30, 2017		Jun. 30, 2018		Jun. 30, 2017
Segment Reporting Information [Line Items]
Operating revenues	$ 1,187,735		$ 1,164,940		$ 2,498,486		$ 2,472,080
Net income (loss)	92,387		87,662		204,127		181,828
Affiliate electric revenue	117,417		119,385		234,389		243,074
Regulated Electric
Segment Reporting Information [Line Items]
Operating revenues	1,096,536		1,080,022		2,158,964		2,159,161
Net income (loss)	88,875		89,477		174,597		167,559
Regulated Natural Gas
Segment Reporting Information [Line Items]
Operating revenues	83,228		78,183		324,804		299,460
Net income (loss)	399		(3,360)		25,844		14,145
All Other
Segment Reporting Information [Line Items]
Operating revenues	8,215		7,155		15,271		14,082
Net income (loss)	3,113		1,545		3,686		124
Operating Segments
Segment Reporting Information [Line Items]
Operating revenues	1,187,735	[1],[2]	1,164,940	[1],[2]	2,498,486	[3],[4]	2,472,080	[3],[4]
Operating Segments | Regulated Electric
Segment Reporting Information [Line Items]
Operating revenues	1,096,408	[1]	1,079,779	[1]	2,158,671	[3]	2,158,809	[3]
Operating Segments | Regulated Natural Gas
Segment Reporting Information [Line Items]
Operating revenues	83,112	[2]	78,006	[2]	324,544	[4]	299,189	[4]
Operating Segments | All Other
Segment Reporting Information [Line Items]
Operating revenues	8,215		7,155		15,271		14,082
Intersegment Eliminations
Segment Reporting Information [Line Items]
Operating revenues	(244)		(420)		(553)		(623)
Net income (loss)	0		0		0		0
Intersegment Eliminations | Regulated Electric
Segment Reporting Information [Line Items]
Operating revenues	128		243		293		352
Intersegment Eliminations | Regulated Natural Gas
Segment Reporting Information [Line Items]
Operating revenues	116		177		260		271
Intersegment Eliminations | All Other
Segment Reporting Information [Line Items]
Operating revenues	$ 0		$ 0		$ 0		$ 0
[1]	Operating revenues include 117 million and 119 million of affiliate electric revenue for the three months ended June 30, 2018 and 2017.
[2]	Operating revenues include an immaterial amount of affiliate gas revenue for the three months ended June 30, 2018 and 2017.
[3]	Operating revenues include $234 million and $243 million of affiliate electric revenue for the six months ended June 30, 2018 and 2017.
[4]	Operating revenues include an immaterial amount of affiliate gas revenue for the six months ended June 30, 2018 and 2017.

Benefit Plans and Other Postr57

Benefit Plans and Other Postretirement Benefits (Details) $ in Thousands	1 Months Ended	3 Months Ended		6 Months Ended
	Jan. 31, 2018USD ($)Plan	Jun. 30, 2018USD ($)	Jun. 30, 2017USD ($)	Jun. 30, 2018USD ($)	Jun. 30, 2017USD ($)
Pension Plan [Member]
Components of Net Periodic Benefit Cost [Abstract]
Service cost		$ 6,982	$ 6,958	$ 13,964	$ 13,916
Interest cost (a)		8,804	10,177	17,608	20,354
Expected return on plan assets (a)		(14,542)	(15,017)	(29,083)	(30,034)
Amortization of prior service (credit) cost (a)		(28)	265	(57)	530
Amortization of net loss (a)		9,614	9,902	19,229	19,804
Net periodic benefit cost		10,830	12,285	21,661	24,570
Costs not recognized due to the effects of regulation		(3,069)	(4,899)	(5,832)	(9,798)
Net benefit cost recognized for financial reporting		7,761	7,386	15,829	14,772
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		43	36	86	72
Interest cost (a)		769	854	1,538	1,708
Expected return on plan assets (a)		(96)	(54)	(192)	(108)
Amortization of prior service (credit) cost (a)		(759)	(759)	(1,518)	(1,518)
Amortization of net loss (a)		595	507	1,190	1,014
Net periodic benefit cost		552	584	1,104	1,168
Costs not recognized due to the effects of regulation		0	0	0	0
Net benefit cost recognized for financial reporting		$ 552	$ 584	$ 1,104	$ 1,168

Other Comprehensive Income (Det

Other Comprehensive Income (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 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	$ 219	$ 240	311	$ 478
Accumulated other comprehensive (loss) income at end of period	5,526,761		5,526,761
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	(20,723)	(18,005)	(20,895)	(18,208)
Other comprehensive income (loss) before reclassifications	22	26	27	26
(Gains) losses reclassified from net accumulated other comprehensive loss	165	204	332	407
Net current period other comprehensive income (loss)	187	230	359	433
Accumulated other comprehensive (loss) income at end of period	(20,536)	(17,775)	(20,536)	(17,775)
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	(3,717)	(2,645)	(3,770)	(2,680)
Other comprehensive income (loss) before reclassifications	0	0	0	0
(Gains) losses reclassified from net accumulated other comprehensive loss	54	36	107	71
Net current period other comprehensive income (loss)	54	36	107	71
Accumulated other comprehensive (loss) income at end of period	(3,663)	(2,609)	(3,663)	(2,609)
Total
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Accumulated other comprehensive (loss) income at beginning of period	(24,440)	(20,545)	(24,537)	(20,783)
Other comprehensive income (loss) before reclassifications	22	26	27	26
(Gains) losses reclassified from net accumulated other comprehensive loss	219	240	311	478
Net current period other comprehensive income (loss)	241	266	338	504
Accumulated other comprehensive (loss) income at end of period	$ (24,199)	$ (20,279)	$ (24,199)	$ (20,279)

Other Comprehensive Income (Rec

Other Comprehensive Income (Reclassification from AOCI) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Operating and maintenance expenses	$ 309,196	$ 305,189	$ 601,491	$ 613,815
Total, pre-tax	(101,361)	(121,432)	(225,812)	(254,613)
Tax benefit	8,974	33,770	21,685	72,785
Total amounts reclassified, net of tax	219	240	311	478
Gains and Losses on Cash Flow Hedges
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total amounts reclassified, net of tax	165	204	332	407
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	229	345	461	687
Tax benefit	(64)	(141)	(129)	(280)
Total, net of tax	165	204	332	407
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	264	350	525	692
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	(35)	(5)	(64)	(5)
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	250	218
Prior service credit | Amounts Reclassified from Accumulated Other Comprehensive Loss
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total, pre-tax	(49)	(51)	(98)	(100)
Defined Benefit Pension and Postretirement Items
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total amounts reclassified, net of tax	54	36	107	71
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	58	152	118
Tax (benefit) expense	(22)	(22)	(45)	(47)
Total amounts reclassified, net of tax	54	36	107	71
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 Revenues (Details)

Revenues Revenues (Details) - USD ($) $ in Thousands	3 Months Ended				6 Months Ended
	Jun. 30, 2018		Jun. 30, 2017		Jun. 30, 2018		Jun. 30, 2017
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	$ 1,147,216		$ 1,112,150		$ 2,403,601		$ 2,357,475
Alternative revenue and other	40,519		52,790		94,885		114,605
Total operating revenues	1,187,735		1,164,940		$ 2,498,486		2,472,080
Maximum number of months following end of annual period in which revenues are earned to be included in incentive programs					24 months
Operating Segments
Disaggregation of Revenue [Line Items]
Total operating revenues	1,187,735	[1],[2]	1,164,940	[1],[2]	$ 2,498,486	[3],[4]	2,472,080	[3],[4]
Retail
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	918,465		898,851		1,942,959		1,913,401
Retail | Residential
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	360,194		332,009		809,305		764,045
Retail | Commercial and Industrial (C&I)
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	548,169		558,341		1,113,143		1,131,973
Retail | Other
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	10,102		8,501		20,511		17,383
Wholesale
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	42,033		28,841		88,398		74,760
Transmission
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	59,951		56,177		114,749		110,136
Interchange
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	117,417		119,384		234,389		243,073
Other
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	9,350		8,897		23,106		16,105
Regulated Electric
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	1,059,869		1,035,301		2,078,420		2,062,550
Alternative revenue and other	36,539		44,478		80,251		96,259
Total operating revenues	1,096,536		1,080,022		2,158,964		2,159,161
Regulated Electric | Operating Segments
Disaggregation of Revenue [Line Items]
Total operating revenues	1,096,408	[1]	1,079,779	[1]	2,158,671	[3]	2,158,809	[3]
Regulated Electric | Retail
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	834,435		823,454		1,623,306		1,621,173
Regulated Electric | Retail | Residential
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	311,086		288,733		622,215		593,146
Regulated Electric | Retail | Commercial and Industrial (C&I)
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	514,710		526,867		982,733		1,011,970
Regulated Electric | Retail | Other
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	8,639		7,854		18,358		16,057
Regulated Electric | Wholesale
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	42,033		28,841		88,398		74,760
Regulated Electric | Transmission
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	59,951		56,177		114,749		110,136
Regulated Electric | Interchange
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	117,417		119,384		234,389		243,073
Regulated Electric | Other
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	6,033		7,445		17,578		13,408
Regulated Natural Gas
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	79,132		69,694		309,910		280,843
Alternative revenue and other	3,980		8,312		14,634		18,346
Total operating revenues	83,228		78,183		324,804		299,460
Regulated Natural Gas | Operating Segments
Disaggregation of Revenue [Line Items]
Total operating revenues	83,112	[2]	78,006	[2]	324,544	[4]	299,189	[4]
Regulated Natural Gas | Retail
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	75,815		68,242		304,382		278,146
Regulated Natural Gas | Retail | Residential
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	42,398		36,860		174,057		158,273
Regulated Natural Gas | Retail | Commercial and Industrial (C&I)
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	33,417		31,382		130,325		119,873
Regulated Natural Gas | Retail | Other
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	0		0		0		0
Regulated Natural Gas | Wholesale
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	0		0		0		0
Regulated Natural Gas | Transmission
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	0		0		0		0
Regulated Natural Gas | Interchange
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	0		0		0		0
Regulated Natural Gas | Other
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	3,317		1,452		5,528		2,697
All Other
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	8,215		7,155		15,271		14,082
Alternative revenue and other	0		0		0		0
Total operating revenues	8,215		7,155		15,271		14,082
All Other | Operating Segments
Disaggregation of Revenue [Line Items]
Total operating revenues	8,215		7,155		15,271		14,082
All Other | Retail
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	8,215		7,155		15,271		14,082
All Other | Retail | Residential
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	6,710		6,416		13,033		12,626
All Other | Retail | Commercial and Industrial (C&I)
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	42		92		85		130
All Other | Retail | Other
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	1,463		647		2,153		1,326
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
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
[1]	Operating revenues include 117 million and 119 million of affiliate electric revenue for the three months ended June 30, 2018 and 2017.
[2]	Operating revenues include an immaterial amount of affiliate gas revenue for the three months ended June 30, 2018 and 2017.
[3]	Operating revenues include $234 million and $243 million of affiliate electric revenue for the six months ended June 30, 2018 and 2017.
[4]	Operating revenues include an immaterial amount of affiliate gas revenue for the six months ended June 30, 2018 and 2017.