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

Document and Entity Information

Document and Entity Information - shares	9 Months Ended
	Sep. 30, 2018	Oct. 26, 2018
Document and Entity Information [Abstract]
Entity Registrant Name	NORTHERN STATES POWER CO /WI/
Entity Central Index Key	72,909
Current Fiscal Year End Date	--12-31
Entity Filer Category	Non-accelerated Filer
Entity Emerging Growth Company	false
Entity Small Business	false
Document Type	10-Q
Document Period End Date	Sep. 30, 2018
Document Fiscal Year Focus	2,018
Document Fiscal Period Focus	Q3
Amendment Flag	false
Entity Common Stock, Shares Outstanding		933,000
Entity Current Reporting Status	Yes

CONSOLIDATED STATEMENTS OF COMM

CONSOLIDATED STATEMENTS OF COMMON STOCKHOLDER'S EQUITY - 9 months ended Sep. 30, 2018 $ in Thousands	USD ($)shares
Balance (in shares) at Dec. 31, 2017 | shares	933,000
Comprehensive income:
Net income | $	$ 77,613
Other comprehensive income | $	$ 69
Balance (in shares) at Sep. 30, 2018 | shares	933,000

CONSOLIDATED STATEMENTS OF INCO

CONSOLIDATED STATEMENTS OF INCOME (UNAUDITED) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Total operating revenues	$ 255,997	$ 247,511	$ 760,895	$ 742,468
Operating expenses
Purchased power, affiliates	106,753	106,697	309,693	314,948
Cost of natural gas sold and transported	6,157	6,032	43,387	40,582
Operating and maintenance expenses	50,057	51,130	149,335	148,846
Conservation expenses	3,058	3,298	9,180	9,529
Depreciation and amortization	31,669	27,979	93,705	82,656
Taxes (other than income taxes)	7,107	7,124	21,473	21,002
Total operating expenses	207,987	208,441	636,419	630,200
Operating income	48,010	39,070	124,476	112,268
Interest charges and financing costs
Interest charges — includes other financing costs of $495 and $465, $1,458, and $1,382 respectively	10,109	8,647	29,467	25,985
Allowance for funds used during construction — debt	(1,172)	(726)	(3,017)	(2,025)
Total interest charges and financing costs	8,937	7,921	26,450	23,960
Other expense, net	(782)	(583)	(1,651)	(1,610)
Allowance for funds used during construction — equity	2,631	1,705	6,732	4,743
Income before income taxes	40,922	32,271	103,107	91,441
Income taxes	9,914	9,946	25,494	32,456
Net income	31,008	22,325	77,613	58,985
Electric
Total operating revenues	240,344	232,802	666,212	659,853
Operating expenses
Electric fuel and purchased power, non afiiliates	3,186	6,181	9,646	12,637
Natural gas
Total operating revenues	15,355	14,394	93,836	81,768
Other
Total operating revenues	$ 298	$ 315	$ 847	$ 847

CONSOLIDATED STATEMENTS OF IN_2

CONSOLIDATED STATEMENTS OF INCOME (UNAUDITED) (Parenthetical) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Interest charges and financing costs
Other financing costs	$ 495	$ 465	$ 1,458	$ 1,382

CONSOLIDATED STATEMENTS OF COMP

CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME (UNAUDITED) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Comprehensive income:
Net income	$ 31,008	$ 22,325	$ 77,613	$ 58,985
Derivative instruments:
Reclassification of losses to net income, net of tax of $9, $13, $26 and $37, respectively	23	19	69	57
Other comprehensive income	23	19	69	57
Comprehensive income	$ 31,031	$ 22,344	$ 77,682	$ 59,042

CONSOLIDATED STATEMENTS OF CO_2

CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME (UNAUDITED) (Parenthetical) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Derivative instruments:
Reclassification of losses to net income, tax	$ 9	$ 13	$ 26	$ 37

CONSOLIDATED STATEMENTS OF CASH

CONSOLIDATED STATEMENTS OF CASH FLOWS (UNAUDITED) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017
Operating activities
Net income	$ 77,613	$ 58,985
Adjustments to reconcile net income to cash provided by operating activities:
Depreciation and amortization	94,925	83,797
Deferred income taxes	15,844	37,226
Amortization of investment tax credits	(393)	(392)
Allowance for equity funds used during construction	(6,732)	(4,743)
Net derivative losses	104	82
Other, net	498	(1,503)
Changes in operating assets and liabilities:
Accounts receivable	8,781	3,512
Accrued unbilled revenues	15,341	9,414
Inventories	(627)	(1,798)
Other current assets	5,109	4,990
Accounts payable	(7,546)	1,031
Net regulatory assets and liabilities	(13,611)	(19,309)
Other current liabilities	(4,924)	(9,700)
Pension and other employee benefit obligations	(8,631)	(8,189)
Change in other noncurrent assets	70	(362)
Change in other noncurrent liabilities	1,323	(4,165)
Net cash provided by operating activities	177,144	148,876
Investing activities
Utility capital/construction expenditures	(181,281)	(145,440)
Allowance for equity funds used during construction	6,732	4,743
Other, net	211	(10)
Net cash used in investing activities	(174,338)	(140,707)
Financing activities
(Repayments of) and proceeds from short-term borrowings, net	(11,000)	32,000
Proceeds from Issuance of Long-term Debt	197,110	0
Repayments of long-term debt	(19)	(71)
Capital contributions from parent	6,904	12,297
Dividends paid to parent	(47,986)	(52,640)
Other, net	(67)	(70)
Net cash provided by (used in) financing activities	144,942	(8,484)
Net change in cash and cash equivalents	147,748	(315)
Cash and cash equivalents at beginning of period	1,403	1,546
Cash and cash equivalents at end of period	149,151	1,231
Supplemental disclosure of cash flow information:
Cash paid for interest (net of amounts capitalized)	(22,016)	(21,496)
Cash paid for income taxes, net	(15,833)	(9,339)
Supplemental disclosure of non-cash investing transactions:
Property, plant and equipment additions in accounts payable	$ 22,731	$ 25,796

CONSOLIDATED BALANCE SHEETS (UN

CONSOLIDATED BALANCE SHEETS (UNAUDITED) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Current assets
Cash and cash equivalents	$ 149,151	$ 1,403
Accounts receivable, net	51,424	63,200
Accrued unbilled revenues	44,667	60,008
Other Receivables	15,339	15,144
Inventories	18,385	17,758
Regulatory assets	23,110	23,113
Prepaid taxes	18,941	23,606
Prepayments and other	3,474	3,450
Total current assets	324,491	207,682
Property, plant and equipment, net	2,190,260	2,088,728
Other assets
Regulatory assets	285,587	282,217
Other investments	2,682	2,892
Other	278	201
Total other assets	288,547	285,310
Total assets	2,803,298	2,581,720
Current liabilities
Current portion of long-term debt	151,061	151,080
Short-term debt	0	11,000
Notes payable to affiliates	500	500
Accounts payable	49,663	58,365
Accounts payable to affiliates	25,761	29,628
Dividends payable to parent	18,169	15,481
Regulatory liabilities	22,218	20,712
Environmental liabilities	14,046	10,469
Accrued interest	11,239	8,025
Other	24,535	34,474
Total current liabilities	317,192	339,734
Deferred credits and other liabilities
Deferred income taxes	272,118	256,687
Deferred investment tax credits	7,122	7,514
Regulatory liabilities	398,578	386,807
Environmental liabilities	13,723	19,190
Customer advances	18,647	16,325
Pension and employee benefit obligations	42,483	50,027
Other	18,101	18,747
Total deferred credits and other liabilities	770,772	755,297
Commitments and contingencies
Capitalization
Long-term debt	807,828	610,100
Common stock — 1,000,000 shares authorized of $100 par value; 933,000 shares outstanding at Sept. 30, 2018 and Dec. 31, 2017, respectively	93,300	93,300
Additional paid in capital	453,260	449,350
Retained earnings	360,946	334,008
Accumulated other comprehensive loss	0	(69)
Total common stockholder’s equity	907,506	876,589
Total liabilities and equity	$ 2,803,298	$ 2,581,720

CONSOLIDATED BALANCE SHEETS (_2

CONSOLIDATED BALANCE SHEETS (UNAUDITED) (Parenthetical) - $ / shares	Sep. 30, 2018	Dec. 31, 2017
Capitalization
Common stock, shares authorized (in shares)	1,000,000	1,000,000
Common stock, par value (in dollars per share)	$ 100	$ 100
Common stock, shares outstanding (in shares)	933,000	933,000

Accounting Pronouncements

Accounting Pronouncements	9 Months Ended
	Sep. 30, 2018
New Accounting Pronouncements and Changes in Accounting Principles [Abstract]
Accounting Pronouncements	Accounting Pronouncements Recently Issued Leases — In February 2016, the Financial Accounting Standards Board (FASB) issued Leases, Topic 842 (Accounting Standards Update (ASU) No. 2016-02) , which for lessees requires balance sheet recognition of right-of-use assets and lease liabilities for most leases. This guidance will be effective for interim and annual reporting periods beginning after Dec. 15, 2018. Adoption will occur on Jan. 1, 2019 utilizing the practical expedients provided by the standard and included in Targeted Improvements, Topic 842 ( ASU No. 2018-11 ). On Jan. 1, 2019, agreements historically disclosed as operating leases for the use of real estate, equipment and certain fossil-fueled generating facilities operated under purchased power agreements (PPAs) are expected to be recognized on the consolidated balance sheet. Other than first-time recognition of these types of operating leases on the consolidated balance sheet, the implementation is not expected to have a significant impact on NSP-Wisconsin’s consolidated financial statements. Recently Adopted Revenue Recognition — In May 2014, the FASB issued Revenue from Contracts with Customers, Topic 606 (ASU No. 2014-09) , which provides a new framework for the recognition of revenue. NSP-Wisconsin implemented the guidance on a modified retrospective basis on Jan. 1, 2018. Results for reporting periods beginning after Dec. 31, 2017 are presented in accordance with Topic 606, while prior period results have not been adjusted and continue to be reported in accordance with prior accounting guidance. Other than increased disclosures regarding revenues related to contracts with customers, the implementation did not have a material impact on NSP-Wisconsin’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-Wisconsin implemented the guidance on Jan. 1, 2018 and the implementation did not have a material impact on its consolidated financial statements. 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-Wisconsin implemented the new guidance on Jan. 1, 2018, and as a result, $0.7 million and $2.0 million of pension costs were retrospectively reclassified from operating and maintenance expenses to other income, net on the consolidated income statement for the three and nine months ended Sept. 30, 2017, respectively. Under a practical expedient permitted by the standard, NSP-Wisconsin used benefit cost amounts disclosed for prior periods as the basis for retrospective application.

Management's Opinion

Management's Opinion	9 Months Ended
	Sep. 30, 2018
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Management's Opinion	In the opinion of management, the accompanying unaudited consolidated financial statements contain all adjustments necessary to present fairly, in accordance with accounting principles generally accepted in the United States of America (GAAP), the financial position of NSP-Wisconsin and its subsidiaries as of Sept. 30, 2018 and Dec. 31, 2017 ; the results of its operations, including the components of net income and comprehensive income for the three and nine months ended Sept. 30, 2018 and 2017; and its cash flows for the nine months ended Sept. 30, 2018 and 2017. All adjustments are of a normal, recurring nature, except as otherwise disclosed. Management has also evaluated the impact of events occurring after Sept. 30, 2018 up to the date of issuance of these consolidated financial statements. These statements contain all necessary adjustments and disclosures resulting from that evaluation. The Dec. 31, 2017 balance sheet information has been derived from the audited 2017 consolidated financial statements included in the NSP-Wisconsin Annual Report on Form 10-K for the year ended Dec. 31, 2017 , as amended by the NSP-Wisconsin Amendment No. 1 to its Annual Report on Form 10-K/A, filed with the SEC on July 27, 2018. 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-Wisconsin Annual Report on Form 10-K for the year ended Dec. 31, 2017 , filed with the SEC on Feb. 26, 2018, as amended by the NSP-Wisconsin Amendment No. 1 to its Annual Report on Form 10-K/A, filed with the SEC on July 27, 2018. Due to the seasonality of NSP-Wisconsin’s electric and natural gas sales, interim results are not necessarily an appropriate base from which to project annual results.

Summary of Significant Accounti

Summary of Significant Accounting Policies	9 Months Ended
	Sep. 30, 2018
Accounting Policies [Abstract]
Summary of Significant Accounting Policies	Summary of Significant Accounting Policies The significant accounting policies set forth in Note 1 to the consolidated financial statements in the NSP-Wisconsin Annual Report on Form 10-K for the year ended Dec. 31, 2017 , as amended by the NSP-Wisconsin Amendment No. 1 to its Annual Report on Form 10-K/A, filed with the SEC on July 27, 2018, appropriately represent, in all material respects, the current status of accounting policies and are incorporated herein by reference.

Selected Balance Sheet Data

Selected Balance Sheet Data	9 Months Ended
	Sep. 30, 2018
Balance Sheet Related Disclosures [Abstract]
Selected Balance Sheet Data	Selected Balance Sheet Data (Thousands of Dollars) Sept. 30, 2018 Dec. 31, 2017 Accounts receivable, net (a) Accounts receivable $ 56,498 $ 68,073 Less allowance for bad debts (5,074 ) (4,873 ) $ 51,424 $ 63,200 (a) Accounts receivable, net includes $0.3 million and $3.4 million due from affiliates as of Sept. 30, 2018 and Dec. 31, 2017 , respectively. (Thousands of Dollars) Sept. 30, 2018 Dec. 31, 2017 Inventories Materials and supplies $ 6,770 $ 6,916 Fuel 3,718 3,866 Natural gas 7,897 6,976 $ 18,385 $ 17,758 (Thousands of Dollars) Sept. 30, 2018 Dec. 31, 2017 Property, plant and equipment, net Electric plant $ 2,653,711 $ 2,602,671 Natural gas plant 339,399 326,723 Common and other property 188,934 181,105 Construction work in progress 235,065 148,770 Total property, plant and equipment 3,417,109 3,259,269 Less accumulated depreciation (1,226,849 ) (1,170,541 ) $ 2,190,260 $ 2,088,728

Income Taxes

Income Taxes	9 Months Ended
	Sep. 30, 2018
Income Tax Disclosure [Abstract]
Income Taxes	Income Taxes Except to the extent noted below, Note 6 to the consolidated financial statements included in the NSP-Wisconsin Annual Report on Form 10-K for the year ended Dec. 31, 2017, as amended by the NSP-Wisconsin Amendment No. 1 to its Annual Report on Form 10-K/A, filed with the SEC on July 27, 2018, appropriately represents, in all material respects, the current status of other income tax matters, and are incorporated herein by reference. Total income tax expense from operations differs from the amount computed by applying the statutory federal income tax rate to income before income tax expense. The following reconciles such differences: Nine Months Ended Sept. 30 2018 2017 Federal statutory rate 21.0 % 35.0 % State tax (net of federal tax effect) 6.2 5.1 Increases (decreases) in tax from: Regulatory differences - ARAM (a) (4.5 ) (0.1 ) Regulatory differences - ARAM deferral (b) 4.4 — Regulatory difference - reversal of prior quarters' ARAM deferral (b) (0.1 ) — Regulatory differences - other utility plant items (1.4 ) (1.3 ) Tax credits (net of federal income tax expense) (0.8 ) (1.0 ) Other (net) (0.1 ) (2.2 ) Effective income tax rate 24.7 % 35.5 % (a) The average rate assumption method (ARAM); a method to flow back excess deferred taxes to customers. (b) ARAM has been deferred when regulatory treatment has not been established. As NSP-Wisconsin received direction from its regulatory commissions regarding the return of excess deferred taxes to customers, the ARAM deferral was reversed. This resulted in a reduction to tax expense with a corresponding reduction to revenue. Federal Audits — NSP-Wisconsin is a member of the Xcel Energy affiliated group that files a consolidated federal income tax return. The statute of limitations applicable to Xcel Energy’s federal income tax returns expire as follows: Tax Year(s) Expiration 2009 - 2014 October 2019 2015 September 2019 2016 September 2020 In 2012, the Internal Revenue Service (IRS) commenced an examination of tax years 2010 and 2011 , including the 2009 carryback claim. In 2017 Xcel Energy and the Office of Appeals (Appeals) reached an agreement and the benefit related to the agreed upon portions was recognized. NSP-Wisconsin did not accrue any income tax benefit related to this adjustment. In the second quarter of 2018, the Joint Committee on Taxation completed its review and took no exception to the agreement. As a result, the remaining unrecognized tax benefit was released and recorded as a payable to the IRS. In the third quarter of 2015, the IRS commenced an examination of tax years 2012 and 2013 . In the third quarter of 2017, the IRS concluded the audit of tax years 2012 and 2013 and proposed an adjustment that would impact Xcel Energy’s net operating loss (NOL) and effective tax rate (ETR). Xcel Energy filed a protest with the IRS. As of Sept. 30, 2018 , the case has been forwarded to Appeals and Xcel Energy has recognized its best estimate of income tax expense that will result from a final resolution of this issue; however, the outcome and timing of a resolution is unknown. State Audits — NSP-Wisconsin is a member of the Xcel Energy affiliated group that files consolidated state income tax returns. As of Sept. 30, 2018 , NSP-Wisconsin’s earliest open tax year that is subject to examination by state taxing authorities under applicable statutes of limitations is 2012 . In 2016, the state of Wisconsin began an audit of years 2012 and 2013 . The audit concluded in the third quarter of 2018 with no material adjustments. There were no other state income tax audits in progress. Unrecognized Benefits — The unrecognized tax benefit balance includes permanent tax positions, which if recognized would affect the annual ETR. In addition, the unrecognized tax benefit balance includes temporary tax positions for which the ultimate deductibility is highly certain but for which there is uncertainty about the timing of such deductibility. A change in the period of deductibility would not affect the ETR but would accelerate the payment of cash to the taxing authority to an earlier period. A reconciliation of the amount of unrecognized tax benefit is as follows: (Millions of Dollars) Sept. 30, 2018 Dec. 31, 2017 Unrecognized tax benefit — Permanent tax positions $ 2.0 $ 1.4 Unrecognized tax benefit — Temporary tax positions 0.9 1.0 Total unrecognized tax benefit $ 2.9 $ 2.4 The unrecognized tax benefit amounts were reduced by the tax benefits associated with NOL and tax credit carryforwards. The amounts of tax benefits associated with NOL and tax credit carryforwards are as follows: (Millions of Dollars) Sept. 30, 2018 Dec. 31, 2017 NOL and tax credit carryforwards $ (2.6 ) $ (1.9 ) It is reasonably possible that NSP-Wisconsin’s amount of unrecognized tax benefits could significantly change in the next 12 months as the IRS Appeals progresses and audit resumes, and state audits resume. As the IRS Appeals progresses and the IRS audit resumes, it is reasonably possible that the amount of unrecognized tax benefit could decrease up to approximately $2 million . Payables for interest related to unrecognized tax benefits were not material and no amounts were accrued for penalties related to unrecognized tax benefits as of Sept. 30, 2018 or Dec. 31, 2017.

Rate Matters Rate Matters

Rate Matters Rate Matters	9 Months Ended
	Sep. 30, 2018
Public Utilities, General Disclosures [Abstract]
Rate Matters	Rate Matters Except to the extent noted below, the circumstances set forth in Note 10 to the consolidated financial statements included in the NSP-Wisconsin Annual Report on Form 10-K for the year ended Dec. 31, 2017, as amended by the NSP-Wisconsin Amendment No. 1 to its Annual Report on Form 10-K/A, filed with the SEC on July 27, 2018, and in Note 5 to the consolidated financial statements to NSP-Wisconsin's Quarterly Report on Form 10-Q for the quarterly period ended March 31, 2018 and June 30, 2018, appropriately represent, in all material respects, the current status of other rate matters, and are incorporated herein by reference. Tax Reform — Regulatory Proceedings In May 2018, the Public Service Commission of Wisconsin (PSCW) issued its final order for the 2018 TCJA impact, which requires customer refunds of $27 million and defers approximately $5 million until NSP-Wisconsin’s next rate case proceeding. NSP-Wisconsin — Michigan — In May 2018, the Michigan Public Service Commission approved electric and natural gas tax reform settlement agreements. Most of the electric TCJA benefits were included in NSP-Wisconsin’s recently approved Michigan 2018 electric base rate case. The return of natural gas TCJA benefits is expected to be completed in 2019. Pending Regulatory Proceeding — Federal Energy Regulatory Commission (FERC) Midcontinent Independent System Operator, Inc. (MISO) Return on Equity (ROE) Complaints — In November 2013, a group of customers filed a complaint at the FERC against MISO transmission owners (TOs), including NSP-Minnesota and NSP-Wisconsin. The complaint argued for a reduction in the ROE in transmission formula rates in the MISO region from 12.38 percent to 9.15 percent , and the removal of ROE adders (including those for Regional Transmission Organization (RTO) membership), effective Nov. 12, 2013. In September 2016, the FERC approved an Administrative Law Judge (ALJ) recommendation that MISO TOs be granted a 10.32 percent base ROE using the methodology adopted by FERC in June 2014 (Opinion 531). This ROE would be applicable for the 15 -month refund period from Nov. 12, 2013 to Feb. 11, 2015, and prospectively from the date of the FERC order. The total prospective ROE would be 10.82 percent , including a 50 basis point adder for RTO membership. The requests are pending FERC action. In February 2015, a second complaint seeking to reduce the MISO ROE from 12.38 percent to 8.67 percent prior to any RTO adder was filed, resulting in a second period of potential refunds from Feb. 12, 2015 to May 11, 2016. In June 2016, an ALJ recommended a base ROE of 9.7 percent , applying the FERC Opinion 531 methodology. FERC action is pending. In April 2017, the United States Court of Appeals for the District of Columbia Circuit (D.C. Circuit) vacated and remanded Opinion 531. It is unclear how the D.C. Circuit’s opinion to vacate and remand Opinion 531 will affect the September 2016 FERC order or the timing and outcome of the second ROE complaint. NSP-Minnesota has recognized a current refund liability consistent with the best estimate of the final ROE.

Commitments and Contingencies

Commitments and Contingencies	9 Months Ended
	Sep. 30, 2018
Commitments and Contingencies Disclosure [Abstract]
Commitments and Contingencies	Commitments and Contingencies Except to the extent noted below and in Note 5 to the financial statements, the circumstances set forth in Notes 10 and 11 to the consolidated financial statements included in the NSP-Wisconsin Annual Report on Form 10-K for the year ended Dec. 31, 2017 , as amended by the NSP-Wisconsin Amendment No. 1 to its Annual Report on Form 10-K/A, filed with the SEC on July 27, 2018, and in Notes 5 and 6 to NSP-Wisconsin's Quarterly Reports on Form 10-Q for the quarterly periods ended March 31, 2018, and June 30, 2018, appropriately represent, in all material respects, the current status of commitments and contingent liabilities and are incorporated herein by reference. The following include commitments, contingencies and unresolved contingencies that are material to NSP-Wisconsin’s financial position. Guarantees NSP-Wisconsin provides a guarantee for payment of customer loans related to NSP-Wisconsin’s farm rewiring program. NSP-Wisconsin’s exposure under the guarantee is based upon the net liability under the agreement. The guarantee issued by NSP-Wisconsin has a stated maximum amount. The guarantee contains no recourse provisions and requires no collateral. These agreements have expiration dates through 2020 . The following table presents the guarantee issued and outstanding for NSP-Wisconsin: (Millions of Dollars) Sept. 30, 2018 Dec. 31, 2017 Guarantee issued and outstanding $ 1.0 $ 1.0 Current exposure under this guarantee — — Environmental Contingencies Ashland Manufactured Gas Plant (MGP) Site — NSP-Wisconsin was named a responsible party for contamination at a site in Ashland, Wis. The Ashland/Northern States Power Lakefront Superfund Site (the Site) includes NSP-Wisconsin property, previously operated as a MGP facility, an adjacent city lakeshore park area, and a sediment area of Lake Superior’s Chequamegon Bay. NSP-Wisconsin completed wet dredging at the Site in August of 2018 and anticipates completion of final site restoration activities in early 2019. Groundwater treatment activities at the Site will continue for many years. The current cost estimate for the remediation of the entire site is approximately $184 million , of which approximately $156 million has been spent. As of Sept. 30, 2018 and Dec. 31, 2017 , NSP-Wisconsin recorded a total liability of $28 million and $30 million , respectively, for the entire site. NSP-Wisconsin has deferred the unrecovered portion of the estimated Site remediation costs as a regulatory asset. The PSCW has authorized NSP-Wisconsin rate recovery for all remediation costs incurred at the Site. In 2012, the PSCW agreed to allow NSP-Wisconsin to pre-collect certain costs, to amortize costs over a ten -year period and to apply a three percent carrying cost to the unamortized regulatory asset. In December 2017, the PSCW approved an NSP-Wisconsin natural gas rate case, which included recovery of additional expenses associated with remediating the Site. The annual recovery of MGP clean-up costs increased from $12 million in 2017 to $18 million in 2018. Other MGP, Landfill or Disposal Sites — In addition to the Ashland MGP Site, NSP-Wisconsin is currently involved in investigating and/or remediating an MGP, landfill or other disposal site. NSP-Wisconsin has identified one site where investigation and/or remediation activities are currently underway. Other parties may have responsibility for some portion of the investigation and/or remediation activities. NSP-Wisconsin anticipates that these investigation or remediation activities will continue through at least 2019. NSP-Wisconsin accrued $0.1 million for this site as of Sept. 30, 2018 and Dec. 31, 2017 . Legal Contingencies NSP-Wisconsin 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-Wisconsin’s financial statements. Unless otherwise required by GAAP, legal fees are expensed as incurred. Employment, Tort and Commercial Litigation Gas Trading Litigation — e prime, inc. (e prime) is a wholly owned subsidiary of Xcel Energy. e prime was in the business of natural gas trading and marketing but has not engaged in natural gas trading or marketing activities since 2003. Thirteen lawsuits seeking monetary damages were commenced against e prime and Xcel Energy (and NSP-Wisconsin, in two instances) between 2003 and 2009 alleging fraud and anticompetitive activities in conspiring to restrain the trade of natural gas and manipulate natural gas prices. e prime, Xcel Energy Inc. and its other affiliates were sued along with several other gas marketing companies. These cases were all consolidated in the U.S. District Court in Nevada. Six of the cases remain active, which includes a multi-district litigation (MDL) matter consisting of a Colorado class (Breckenridge), a Wisconsin class (Arandell Corp.), a Missouri class, a Kansas class, and two other cases identified as “Sinclair Oil” and “Farmland.” In March 2017, summary judgment was granted by the MDL judge in favor of Xcel Energy and e prime in the Sinclair Oil and Farmland cases. In November 2017, the U.S. District Court in Nevada granted summary judgment against two plaintiffs in the Arandell Corp. case in favor of Xcel Energy and NSP-Wisconsin, leaving only three individual plaintiffs remaining in the litigation. In addition, the plaintiffs’ motions for class certification and remand back to originating courts in these cases were denied in March 2017. Plaintiffs appealed the summary judgment motions granted in the Farmland and Sinclair Oil cases and the denial of class certification and remand to the U.S. Court of Appeals for the Ninth Circuit (Ninth Circuit). In March 2018, the Ninth Circuit reversed and remanded the summary judgment in the Farmland case. The Farmland defendants subsequently filed a request for further review by the Ninth Circuit, which was denied. Upon Sinclair’s request, the Ninth Circuit reversed and remanded the summary judgment in the Sinclair case. Plaintiffs have asked the lower court to remand the cases back to the court where the actions were originally filed. The defendants have moved for the lower court to issue a renewed summary judgment in the Farmland case. Later in the summer of 2018 the Ninth Circuit also vacated, but did not reverse, the lower court’s denial of class certification. The defendants have drafted a proposal for a renewed denial for the lower court’s consideration. Xcel Energy, NSP-Wisconsin and e prime have concluded that a loss is remote.

Borrowings and Other Financing

Borrowings and Other Financing Instruments	9 Months Ended
	Sep. 30, 2018
Debt Disclosure [Abstract]
Borrowings and Other Financing Instruments	Borrowings and Other Financing Instruments Commercial Paper — NSP-Wisconsin meets its short-term liquidity requirements primarily through the issuance of commercial paper and borrowings under its credit facility. Commercial paper outstanding for NSP-Wisconsin was as follows: (Amounts in Millions, Except Interest Rates) Three Months Ended Sept. 30, 2018 Year Ended Dec. 31, 2017 Borrowing limit $ 150 $ 150 Amount outstanding at period end — 11 Average amount outstanding 24 52 Maximum amount outstanding 48 129 Weighted average interest rate, computed on a daily basis 2.22 % 1.23 % Weighted average interest rate at period end N/A 1.73 Letters of Credit — NSP-Wisconsin uses letters of credit, generally with terms of one year , to provide financial guarantees for certain operating obligations. At Sept. 30, 2018 and Dec. 31, 2017 , there were no letters of credit outstanding. Credit Facility — In order to use its commercial paper program to fulfill short-term funding needs, NSP-Wisconsin 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 aggregate 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. As of Sept. 30, 2018 , NSP-Wisconsin had the following committed credit facility available (in millions of dollars): Credit Facility (a) Drawn (b) Available $ 150 $ — $ 150 (a) This credit facility expires in June 2021 . (b) Includes outstanding commercial paper. All credit facility bank borrowings, outstanding letters of credit and outstanding commercial paper reduce the available capacity under the credit facility. NSP-Wisconsin had no direct advances on the credit facility outstanding at Sept. 30, 2018 and Dec. 31, 2017 . Other Short-Term Borrowings — The following table presents the notes payable of Clearwater Investments, Inc., a NSP-Wisconsin subsidiary, to Xcel Energy Inc.: (Amounts in Millions, Except Interest Rates) Sept. 30, 2018 Dec. 31, 2017 Notes payable to affiliates $ 0.5 $ 0.5 Weighted average interest rate at period end 2.61 % 1.73 % Long-Term Borrowings During the three months ended Sept. 30, 2018, NSP-Wisconsin issued $200 million of 4.20 percent first mortgage bonds due Sept. 1, 2048 .

Fair Value of Financial Assets

Fair Value of Financial Assets and Liabilities	9 Months Ended
	Sep. 30, 2018
Fair Value Disclosures [Abstract]
Fair Value of Financial Assets and Liabilities	Fair Value of Financial Assets and Liabilities Fair Value Measurements The accounting guidance for fair value measurements and disclosures provides a single definition of fair value and requires certain disclosures about assets and liabilities measured at fair value. A hierarchical framework for disclosing the observability of the inputs utilized in measuring assets and liabilities at fair value is established by this guidance. The three levels in the hierarchy are as follows: Level 1 — Quoted prices are available in active markets for identical assets or liabilities as of the reporting date. The types of assets and liabilities included in Level 1 are highly liquid and actively traded instruments with quoted prices. Level 2 — Pricing inputs are other than quoted prices in active markets, but are either directly or indirectly observable as of the reporting date. The types of assets and liabilities included in Level 2 are typically either comparable to actively traded securities or contracts or priced with models using highly observable inputs. Level 3 — Significant inputs to pricing have little or no observability as of the reporting date. The types of assets and liabilities included in Level 3 are those valued with models requiring significant management judgment or estimation. Specific valuation methods include the following: Cash equivalents — The fair values of cash equivalents are generally based on cost plus accrued interest; money market funds are measured using quoted net asset values. 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. Derivative Instruments Fair Value Measurements NSP-Wisconsin 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 and utility commodity prices. Interest Rate Derivatives — NSP-Wisconsin enters into various instruments that effectively fix the interest payments on certain floating rate debt obligations or effectively fix the yield or price on a specified benchmark interest rate for an anticipated debt issuance for a specific period. These derivative instruments are generally designated as cash flow hedges for accounting purposes. At Sept. 30, 2018 , accumulated other comprehensive losses related to interest rate derivatives included no net gains or 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. Commodity Derivatives — NSP-Wisconsin may enter 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 natural gas to generate electric energy and natural gas for resale. The following table details the gross notional amounts of commodity options at Sept. 30, 2018 and Dec. 31, 2017 : (Amounts in Thousands) (a)(b) Sept. 30, 2018 Dec. 31, 2017 Million British thermal units of natural gas 308 42 (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 Activities on Income and Accumulated Other Comprehensive Loss — There were immaterial pre-tax losses related to interest rate derivatives reclassified from accumulated other comprehensive loss into earnings during the three months ended Sept. 30, 2018 and 2017 . There were $0.1 million of net losses reclassified from accumulated other comprehensive loss into earnings during both the nine months ended Sept. 30, 2018 and 2017. During the three and nine months ended Sept. 30, 2018 , changes in the fair value of natural gas commodity derivatives resulted in immaterial net gains recognized as regulatory assets and liabilities. For the three and nine months ended Sept. 30, 2017, changes in the fair value of natural gas commodity derivatives resulted in $0.1 million and $0.2 million of net losses recognized as regulatory assets and liabilities, respectively. The classification as a regulatory asset or liability is based on commission approved regulatory recovery mechanisms. There were no natural gas commodity derivative settlement gains or losses recognized and $0.2 million of losses recognized for the nine months ended Sept. 30, 2018 and 2017 , respectively, and were subject to purchased natural gas cost recovery mechanisms, which result in reclassifications of derivative settlement gains and losses out of income to a regulatory asset or liability, as appropriate. There were no gains or losses recognized during the three months ended Sept. 30, 2018 and 2017. NSP-Wisconsin had no derivative instruments designated as fair value hedges during the three and nine months ended Sept. 30, 2018 and 2017 . Therefore, no gains or losses from fair value hedges or related hedged transactions were recognized for these periods. Consideration of Credit Risk and Concentrations — NSP-Wisconsin 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-Wisconsin’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-Wisconsin 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. Recurring Fair Value Measurements — The following table presents for each of the fair value hierarchy levels, NSP-Wisconsin’s derivative assets and liabilities measured at fair value on a recurring basis: Sept. 30, 2018 Fair Value Fair Value Total Counterparty Netting (a) Total (b) (Thousands of Dollars) Level 1 Level 2 Level 3 Current derivative assets Natural gas commodity $ — $ 142 $ — $ 142 $ — $ 142 Dec. 31, 2017 Fair Value Fair Value Total Counterparty Netting (a) Total (b) (Thousands of Dollars) Level 1 Level 2 Level 3 Current derivative assets Natural gas commodity $ — $ 14 $ — $ 14 $ — $ 14 (a) NSP-Wisconsin nets derivative instruments and related collateral in its consolidated balance sheet when supported by a legally enforceable master netting agreement, and all derivative instruments and related collateral amounts were subject to master netting agreements at Sept. 30, 2018 and Dec. 31, 2017 . The counterparty netting amounts presented exclude settlement receivables and payables and non-derivative amounts that may be subject to the same master netting agreements. (b) Included in prepayments and other current assets balance of $3.5 million at Sept. 30, 2018 and Dec. 31, 2017 in the consolidated balance sheets. Fair Value of Long-Term Debt As of Sept. 30, 2018 and Dec. 31, 2017 , other financial instruments for which the carrying amount did not equal fair value were as follows: Sept. 30, 2018 Dec. 31, 2017 (Thousands of Dollars) Carrying Amount Fair Value Carrying Amount Fair Value Long-term debt, including current portion $ 958,889 $ 1,000,793 $ 761,180 $ 856,106 The fair value of NSP-Wisconsin’s long-term debt is estimated based on recent trades and observable spreads from benchmark interest rates for similar securities. The fair value estimates are based on information available to management as of Sept. 30, 2018 and Dec. 31, 2017 , and given the observability of the inputs to these estimates, the fair values presented for long-term debt have been assigned a Level 2.

Other Income, Net

Other Income, Net	9 Months Ended
	Sep. 30, 2018
Other Income and Expenses [Abstract]
Other Income, Net	Other Expense, Net Other expense, net consisted of the following: Three Months Ended Sept. 30 Nine Months Ended Sept. 30 (Thousands of Dollars) 2018 2017 2018 2017 Interest income $ 271 $ 138 $ 412 $ 342 Other nonoperating income 8 8 56 238 Other nonoperating expense (4 ) (3 ) (11 ) (10 ) Insurance policy income (expense) 166 (48 ) 72 (146 ) Benefits non-service cost (1,223 ) (678 ) (2,180 ) (2,034 ) Other expense, net $ (782 ) $ (583 ) $ (1,651 ) $ (1,610 )

Segment Information

Segment Information	9 Months Ended
	Sep. 30, 2018
Segment Reporting [Abstract]
Segment Information	Segment Information Operating results from the regulated electric utility and regulated natural gas utility are each separately and regularly reviewed by NSP-Wisconsin’s chief operating decision maker. NSP-Wisconsin 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-Wisconsin has the following reportable segments: regulated electric utility, regulated natural gas utility and all other. • NSP-Wisconsin’s regulated electric utility segment generates, transmits and distributes electricity primarily in portions of Wisconsin and Michigan. • NSP-Wisconsin’s regulated natural gas utility segment purchases, transports, stores and distributes natural gas primarily in portions of Wisconsin and Michigan. • 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 investments in rental housing projects that qualify for low-income housing tax credits. Asset and capital expenditure information is not provided for NSP-Wisconsin’s reportable segments because as an integrated electric and natural gas utility, NSP-Wisconsin operates significant assets that are not dedicated to a specific business segment, and reporting assets and capital expenditures by business segment would require arbitrary and potentially misleading allocations which may not necessarily reflect the assets that would be required for the operation of the business segments on a stand-alone basis. To report income from operations for regulated electric and regulated natural gas utility segments, the majority of costs are directly assigned to each segment. However, some costs, such as common depreciation, common operating and maintenance (O&M) expenses and interest expense are allocated based on cost causation allocators. A general allocator is used for certain general and administrative expenses, including office supplies, rent, property insurance and general advertising. (Thousands of Dollars) Regulated Electric Regulated Natural Gas All Other Reconciling Eliminations Consolidated Total Three Months Ended Sept. 30, 2018 Operating revenues (a) $ 240,344 $ 15,355 $ 298 $ — $ 255,997 Intersegment revenues 112 (33 ) — (79 ) — Total revenues $ 240,456 $ 15,322 $ 298 $ (79 ) $ 255,997 Net income (loss) $ 34,256 $ (3,448 ) $ 200 $ — $ 31,008 (Thousands of Dollars) Regulated Electric Regulated Natural Gas All Other Reconciling Eliminations Consolidated Total Three Months Ended Sept. 30, 2017 Operating revenues (a) $ 232,802 $ 14,394 $ 315 $ — $ 247,511 Intersegment revenues 144 56 — (200 ) — Total revenues $ 232,946 $ 14,450 $ 315 $ (200 ) $ 247,511 Net income (loss) $ 24,594 $ (2,184 ) $ (85 ) $ — $ 22,325 (a) Operating revenues include $40 million and $46 million of affiliate electric revenue for the three months ended Sept. 30, 2018 and 2017 , respectively. (Thousands of Dollars) Regulated Regulated All Reconciling Consolidated Nine Months Ended Sept. 30, 2018 Operating revenues (a) $ 666,212 $ 93,836 $ 847 $ — $ 760,895 Intersegment revenues 327 242 — (569 ) — Total revenues $ 666,539 $ 94,078 $ 847 $ (569 ) $ 760,895 Net income $ 70,181 $ 6,802 $ 630 $ — $ 77,613 (Thousands of Dollars) Regulated Regulated All Reconciling Consolidated Nine Months Ended Sept. 30, 2017 Operating revenues (a) $ 659,853 $ 81,768 $ 847 $ — $ 742,468 Intersegment revenues 363 238 — (601 ) — Total revenues $ 660,216 $ 82,006 $ 847 $ (601 ) $ 742,468 Net income $ 54,580 $ 3,388 $ 1,017 $ — $ 58,985 (a) Operating revenues include $116 million and $131 million of affiliate electric revenue for the nine months ended Sept. 30, 2018 and 2017 , respectively.

Benefit Plans and Other Postret

Benefit Plans and Other Postretirement Benefits	9 Months Ended
	Sep. 30, 2018
Retirement Benefits [Abstract]
Benefit Plans and Other Postretirement Benefits	Benefit Plans and Other Postretirement Benefits Components of Net Periodic Benefit Cost Three Months Ended Sept. 30 2018 2017 2018 2017 (Thousands of Dollars) Pension Benefits Postretirement Health Care Benefits Service cost $ 1,194 $ 1,154 $ 9 $ 7 Interest cost (a) 1,361 1,554 142 147 Expected return on plan assets (a) (2,256 ) (2,295 ) (15 ) (7 ) Amortization of prior service (credit) cost (a) (8 ) 35 (88 ) (88 ) Amortization of net loss (a) 1,418 1,462 138 109 Settlement charge (b) 5,181 — — — Net periodic benefit cost 6,890 1,910 186 168 Costs not recognized due to the effects of regulation (3,053 ) — — — Net benefit cost recognized for financial reporting $ 3,837 $ 1,910 $ 186 $ 168 Nine Months Ended Sept. 30 2018 2017 2018 2017 (Thousands of Dollars) Pension Benefits Postretirement Health Care Benefits Service cost $ 3,583 $ 3,463 $ 27 $ 21 Interest cost (a) 4,081 4,663 426 443 Expected return on plan assets (a) (6,769 ) (6,885 ) (47 ) (23 ) Amortization of prior service (credit) cost (a) (22 ) 104 (264 ) (264 ) Amortization of net loss (a) 4,255 4,385 416 327 Settlement charge (b) 5,181 — — — Net periodic benefit cost 10,309 5,730 558 504 Costs not recognized due to the effects of regulation (2,832 ) — — — Net benefit cost recognized for financial reporting $ 7,477 $ 5,730 $ 558 $ 504 (a) The components of net periodic cost other than the service cost component are included in the line item “other expense, net” in the income statement or capitalized on the balance sheet as a regulatory asset. (b) A settlement charge is required when the amount of lump-sum distributions during the year is greater than the sum of the service and interest cost components of the annual net periodic pension cost. In the third quarter as a result of lump-sum distributions during the 2018 plan year, NSP-Wisconsin recorded a pension settlement charge of $5 million , the majority of which was not recognized due to the effects of regulation. A total of $1 million of that amount was recorded in other expense in the third quarter of 2018. In the fourth quarter of 2018 as a result of lump-sum distributions during the 2017 plan year, NSP-Wisconsin recorded a total pension settlement charge of $7 million , the majority of which was not recognized due to the effects of regulation. A total of $2 million of that amount was expensed in the fourth quarter of 2017. In January 2018, contributions of $150 million were made across four of Xcel Energy’s pension plans, of which $10.0 million was attributable to NSP-Wisconsin. Xcel Energy does not expect additional pension contributions during 2018.

Other Comprehensive Income

Other Comprehensive Income	9 Months Ended
	Sep. 30, 2018
Stockholders' Equity Note [Abstract]
Other Comprehensive Income	Other Comprehensive Income (Loss) Changes in accumulated other comprehensive loss, net of tax, for the three and nine months ended Sept. 30, 2018 and 2017 were as follows: Gains and Losses on Cash Flow Hedges (Thousands of Dollars) Three Months Ended Sept. 30, 2018 Three Months Ended Sept. 30, 2017 Accumulated other comprehensive loss at July 1 $ (23 ) $ (95 ) Losses reclassified from net accumulated other comprehensive loss 23 19 Net current period other comprehensive income 23 19 Accumulated other comprehensive loss at Sept. 30 $ — $ (76 ) Gains and Losses on Cash Flow Hedges (Thousands of Dollars) Nine Months Ended Sept. 30, 2018 Nine Months Ended Sept. 30, 2017 Accumulated other comprehensive loss at Jan. 1 $ (69 ) $ (133 ) Losses reclassified from net accumulated other comprehensive loss 69 57 Net current period other comprehensive income 69 57 Accumulated other comprehensive loss at Sept. 30 $ — $ (76 ) Reclassifications from accumulated other comprehensive loss for the three and nine months ended Sept. 30, 2018 and 2017 were as follows: Amounts Reclassified from Accumulated Other Comprehensive Loss (Thousands of Dollars) Three Months Ended Sept. 30, 2018 Three Months Ended Sept. 30, 2017 Losses on cash flow hedges: Interest rate derivatives $ 32 (a) $ 32 (a) Total, pre-tax 32 32 Tax benefit (9 ) (13 ) Total amounts reclassified, net of tax $ 23 $ 19 Amounts Reclassified from Accumulated Other Comprehensive Loss (Thousands of Dollars) Nine Months Ended Sept. 30, 2018 Nine Months Ended Sept. 30, 2017 Losses on cash flow hedges: Interest rate derivatives $ 95 (a) $ 94 (a) Total, pre-tax 95 94 Tax benefit (26 ) (37 ) Total amounts reclassified, net of tax $ 69 $ 57 (a) Included in interest charges.

Revenues

Revenues	9 Months Ended
	Sep. 30, 2018
Revenue from Contract with Customer [Abstract]
Revenues	Revenues NSP-Wisconsin principally generates revenue from the generation, transmission, distribution and sale of electricity and the transportation, distribution and sale of natural gas to retail customers. Performance obligations related to the sale of energy are satisfied as energy is delivered to customers. NSP-Wisconsin 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-Wisconsin does not recognize a separate financing component of its collections from customers. NSP-Wisconsin presents its revenues net of any excise or other fiduciary-type taxes or fees. NSP-Wisconsin 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. NSP-Wisconsin must submit a forward looking fuel cost plan annually for approval by the PSCW. The rules also allow for deferral of any under-recovery or over-recovery of fuel costs in excess of a two percent annual tolerance band, for future rate recovery or refund, subject to PSCW approval. Certain rate rider mechanisms qualify as alternative revenue programs under GAAP. These mechanisms arise from costs imposed upon the utility by action of a regulator or legislative body related to an environmental, public safety or other mandate. When certain criteria are met (including collection within 24 months), revenue is recognized equal to the revenue requirement, which may include return on rate base items and incentives. The mechanisms are revised periodically for differences between the total amount collected and the revenue recognized, which may increase or decrease the level of revenue collected from customers. Alternative revenue is recorded on a gross basis and is disclosed separate from revenue from contracts with customers in the period earned. In the following tables, revenue is classified by the type of goods/services rendered and market/customer type. The tables also reconcile revenue to the reportable segments. Three Months Ended Sept. 30, 2018 (Thousands of Dollars) Electric Natural Gas All Other Total Major revenue types Revenue from contracts with customers: Residential $ 67,919 $ 7,021 $ 12 $ 74,952 Commercial and industrial (C&I) 127,936 6,818 24 134,778 Other 1,325 — 262 1,587 Total retail 197,180 13,839 298 211,317 Interchange 39,993 — — 39,993 Other 363 940 — 1,303 Total revenue from contracts with customers 237,536 14,779 298 252,613 Alternative revenue and other 2,808 576 — 3,384 Total revenues $ 240,344 $ 15,355 $ 298 $ 255,997 Three Months Ended Sept. 30, 2017 (Thousands of Dollars) Electric Natural Gas All Other Total Major revenue types Revenue from contracts with customers: Residential $ 62,025 $ 6,428 $ 48 $ 68,501 C&I 119,495 6,441 23 125,959 Other 1,329 — 244 1,573 Total retail 182,849 12,869 315 196,033 Interchange 45,916 — — 45,916 Other 1,157 1,011 — 2,168 Total revenue from contracts with customers 229,922 13,880 315 244,117 Alternative revenue and other 2,880 514 — 3,394 Total revenues $ 232,802 $ 14,394 $ 315 $ 247,511 Nine Months Ended Sept. 30, 2018 (Thousands of Dollars) Electric Natural Gas All Other Total Major revenue types Revenue from contracts with customers: Residential $ 194,456 $ 48,259 $ 38 $ 242,753 Commercial and industrial (C&I) 339,120 40,775 74 379,969 Other 4,713 — 735 5,448 Total retail 538,289 89,034 847 628,170 Interchange 116,037 — — 116,037 Other 3,227 3,016 — 6,243 Total revenue from contracts with customers 657,553 92,050 847 750,450 Alternative revenue and other 8,659 1,786 — 10,445 Total revenues $ 666,212 $ 93,836 $ 847 $ 760,895 Nine Months Ended Sept. 30, 2017 (Thousands of Dollars) Electric Natural Gas All Other Total Major revenue types Revenue from contracts with customers: Residential $ 182,822 $ 42,071 $ 68 $ 224,961 C&I 329,774 35,276 74 365,124 Other 4,527 — 705 5,232 Total retail 517,123 77,347 847 595,317 Interchange 131,259 — — 131,259 Other 2,649 2,829 — 5,478 Total revenue from contracts with customers 651,031 80,176 847 732,054 Alternative revenue and other 8,822 1,592 — 10,414 Total revenues $ 659,853 $ 81,768 $ 847 $ 742,468

Selected Balance Sheet Data (Ta

Selected Balance Sheet Data (Tables)	9 Months Ended
	Sep. 30, 2018
Balance Sheet Related Disclosures [Abstract]
Accounts Receivable, Net	(Thousands of Dollars) Sept. 30, 2018 Dec. 31, 2017 Accounts receivable, net (a) Accounts receivable $ 56,498 $ 68,073 Less allowance for bad debts (5,074 ) (4,873 ) $ 51,424 $ 63,200
Inventories	(Thousands of Dollars) Sept. 30, 2018 Dec. 31, 2017 Inventories Materials and supplies $ 6,770 $ 6,916 Fuel 3,718 3,866 Natural gas 7,897 6,976 $ 18,385 $ 17,758
Property, Plant and Equipment, Net	(Thousands of Dollars) Sept. 30, 2018 Dec. 31, 2017 Property, plant and equipment, net Electric plant $ 2,653,711 $ 2,602,671 Natural gas plant 339,399 326,723 Common and other property 188,934 181,105 Construction work in progress 235,065 148,770 Total property, plant and equipment 3,417,109 3,259,269 Less accumulated depreciation (1,226,849 ) (1,170,541 ) $ 2,190,260 $ 2,088,728

Income Taxes (Tables)

Income Taxes (Tables)	9 Months Ended
	Sep. 30, 2018
Income Tax Disclosure [Abstract]
Schedule of Effective Income Tax Rate Reconciliation [Table Text Block]	Total income tax expense from operations differs from the amount computed by applying the statutory federal income tax rate to income before income tax expense. The following reconciles such differences: Nine Months Ended Sept. 30 2018 2017 Federal statutory rate 21.0 % 35.0 % State tax (net of federal tax effect) 6.2 5.1 Increases (decreases) in tax from: Regulatory differences - ARAM (a) (4.5 ) (0.1 ) Regulatory differences - ARAM deferral (b) 4.4 — Regulatory difference - reversal of prior quarters' ARAM deferral (b) (0.1 ) — Regulatory differences - other utility plant items (1.4 ) (1.3 ) Tax credits (net of federal income tax expense) (0.8 ) (1.0 ) Other (net) (0.1 ) (2.2 ) Effective income tax rate 24.7 % 35.5 % (a) The average rate assumption method (ARAM); a method to flow back excess deferred taxes to customers. (b) ARAM has been deferred when regulatory treatment has not been established. As NSP-Wisconsin received direction from its regulatory commissions regarding the return of excess deferred taxes to customers, the ARAM deferral was reversed. This resulted in a reduction to tax expense with a corresponding reduction to revenue.
Summary of Statute of Limitations Applicable to Open Tax Years [Table Text Block]	NSP-Wisconsin is a member of the Xcel Energy affiliated group that files a consolidated federal income tax return. The statute of limitations applicable to Xcel Energy’s federal income tax returns expire as follows: Tax Year(s) Expiration 2009 - 2014 October 2019 2015 September 2019 2016 September 2020
Reconciliation of Unrecognized Tax Benefits	A reconciliation of the amount of unrecognized tax benefit is as follows: (Millions of Dollars) Sept. 30, 2018 Dec. 31, 2017 Unrecognized tax benefit — Permanent tax positions $ 2.0 $ 1.4 Unrecognized tax benefit — Temporary tax positions 0.9 1.0 Total unrecognized tax benefit $ 2.9 $ 2.4
Tax Benefits Associated with NOL and Tax Credit Carryforwards	The unrecognized tax benefit amounts were reduced by the tax benefits associated with NOL and tax credit carryforwards. The amounts of tax benefits associated with NOL and tax credit carryforwards are as follows: (Millions of Dollars) Sept. 30, 2018 Dec. 31, 2017 NOL and tax credit carryforwards $ (2.6 ) $ (1.9 )

Commitments and Contingencies (

Commitments and Contingencies (Tables)	9 Months Ended
	Sep. 30, 2018
Commitments and Contingencies Disclosure [Abstract]
Guarantees Issued and Outstanding	The following table presents the guarantee issued and outstanding for NSP-Wisconsin: (Millions of Dollars) Sept. 30, 2018 Dec. 31, 2017 Guarantee issued and outstanding $ 1.0 $ 1.0 Current exposure under this guarantee — —

Borrowings and Other Financin_2

Borrowings and Other Financing Instruments (Tables)	9 Months Ended
	Sep. 30, 2018
Borrowings and Other Financing Instruments [Abstract]
Credit Facilities	As of Sept. 30, 2018 , NSP-Wisconsin had the following committed credit facility available (in millions of dollars): Credit Facility (a) Drawn (b) Available $ 150 $ — $ 150 (a) This credit facility expires in June 2021 . (b) Includes outstanding commercial paper.
Commercial Paper
Borrowings and Other Financing Instruments [Abstract]
Short-Term Borrowings	Commercial paper outstanding for NSP-Wisconsin was as follows: (Amounts in Millions, Except Interest Rates) Three Months Ended Sept. 30, 2018 Year Ended Dec. 31, 2017 Borrowing limit $ 150 $ 150 Amount outstanding at period end — 11 Average amount outstanding 24 52 Maximum amount outstanding 48 129 Weighted average interest rate, computed on a daily basis 2.22 % 1.23 % Weighted average interest rate at period end N/A 1.73
Notes Payable To Affiliates
Borrowings and Other Financing Instruments [Abstract]
Short-Term Borrowings	Other Short-Term Borrowings — The following table presents the notes payable of Clearwater Investments, Inc., a NSP-Wisconsin subsidiary, to Xcel Energy Inc.: (Amounts in Millions, Except Interest Rates) Sept. 30, 2018 Dec. 31, 2017 Notes payable to affiliates $ 0.5 $ 0.5 Weighted average interest rate at period end 2.61 % 1.73 %

Fair Value of Financial Asset_2

Fair Value of Financial Assets and Liabilities (Tables)	9 Months Ended
	Sep. 30, 2018
Fair Value Disclosures [Abstract]
Gross Notional Amounts of Commodity Forwards and Options	The following table details the gross notional amounts of commodity options at Sept. 30, 2018 and Dec. 31, 2017 : (Amounts in Thousands) (a)(b) Sept. 30, 2018 Dec. 31, 2017 Million British thermal units of natural gas 308 42 (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.
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-Wisconsin’s derivative assets and liabilities measured at fair value on a recurring basis: Sept. 30, 2018 Fair Value Fair Value Total Counterparty Netting (a) Total (b) (Thousands of Dollars) Level 1 Level 2 Level 3 Current derivative assets Natural gas commodity $ — $ 142 $ — $ 142 $ — $ 142 Dec. 31, 2017 Fair Value Fair Value Total Counterparty Netting (a) Total (b) (Thousands of Dollars) Level 1 Level 2 Level 3 Current derivative assets Natural gas commodity $ — $ 14 $ — $ 14 $ — $ 14 (a) NSP-Wisconsin nets derivative instruments and related collateral in its consolidated balance sheet when supported by a legally enforceable master netting agreement, and all derivative instruments and related collateral amounts were subject to master netting agreements at Sept. 30, 2018 and Dec. 31, 2017 . The counterparty netting amounts presented exclude settlement receivables and payables and non-derivative amounts that may be subject to the same master netting agreements. (b) Included in prepayments and other current assets balance of $3.5 million at Sept. 30, 2018 and Dec. 31, 2017 in the consolidated balance sheets.
Carrying Amount and Fair Value of Long-term Debt	As of Sept. 30, 2018 and Dec. 31, 2017 , other financial instruments for which the carrying amount did not equal fair value were as follows: Sept. 30, 2018 Dec. 31, 2017 (Thousands of Dollars) Carrying Amount Fair Value Carrying Amount Fair Value Long-term debt, including current portion $ 958,889 $ 1,000,793 $ 761,180 $ 856,106

Other Income, Net (Tables)

Other Income, Net (Tables)	9 Months Ended
	Sep. 30, 2018
Other Income and Expenses [Abstract]
Other Income, Net	Other expense, net consisted of the following: Three Months Ended Sept. 30 Nine Months Ended Sept. 30 (Thousands of Dollars) 2018 2017 2018 2017 Interest income $ 271 $ 138 $ 412 $ 342 Other nonoperating income 8 8 56 238 Other nonoperating expense (4 ) (3 ) (11 ) (10 ) Insurance policy income (expense) 166 (48 ) 72 (146 ) Benefits non-service cost (1,223 ) (678 ) (2,180 ) (2,034 ) Other expense, net $ (782 ) $ (583 ) $ (1,651 ) $ (1,610 )

Segment Information (Tables)

Segment Information (Tables)	9 Months Ended
	Sep. 30, 2018
Segment Reporting [Abstract]
Results from Operations by Reportable Segment	(Thousands of Dollars) Regulated Electric Regulated Natural Gas All Other Reconciling Eliminations Consolidated Total Three Months Ended Sept. 30, 2018 Operating revenues (a) $ 240,344 $ 15,355 $ 298 $ — $ 255,997 Intersegment revenues 112 (33 ) — (79 ) — Total revenues $ 240,456 $ 15,322 $ 298 $ (79 ) $ 255,997 Net income (loss) $ 34,256 $ (3,448 ) $ 200 $ — $ 31,008 (Thousands of Dollars) Regulated Electric Regulated Natural Gas All Other Reconciling Eliminations Consolidated Total Three Months Ended Sept. 30, 2017 Operating revenues (a) $ 232,802 $ 14,394 $ 315 $ — $ 247,511 Intersegment revenues 144 56 — (200 ) — Total revenues $ 232,946 $ 14,450 $ 315 $ (200 ) $ 247,511 Net income (loss) $ 24,594 $ (2,184 ) $ (85 ) $ — $ 22,325 (a) Operating revenues include $40 million and $46 million of affiliate electric revenue for the three months ended Sept. 30, 2018 and 2017 , respectively. (Thousands of Dollars) Regulated Regulated All Reconciling Consolidated Nine Months Ended Sept. 30, 2018 Operating revenues (a) $ 666,212 $ 93,836 $ 847 $ — $ 760,895 Intersegment revenues 327 242 — (569 ) — Total revenues $ 666,539 $ 94,078 $ 847 $ (569 ) $ 760,895 Net income $ 70,181 $ 6,802 $ 630 $ — $ 77,613 (Thousands of Dollars) Regulated Regulated All Reconciling Consolidated Nine Months Ended Sept. 30, 2017 Operating revenues (a) $ 659,853 $ 81,768 $ 847 $ — $ 742,468 Intersegment revenues 363 238 — (601 ) — Total revenues $ 660,216 $ 82,006 $ 847 $ (601 ) $ 742,468 Net income $ 54,580 $ 3,388 $ 1,017 $ — $ 58,985 (a) Operating revenues include $116 million and $131 million of affiliate electric revenue for the nine months ended Sept. 30, 2018 and 2017 , respectively.

Benefit Plans and Other Postr_2

Benefit Plans and Other Postretirement Benefits (Tables)	9 Months Ended
	Sep. 30, 2018
Retirement Benefits [Abstract]
Components of Net Periodic Benefit Cost	Components of Net Periodic Benefit Cost Three Months Ended Sept. 30 2018 2017 2018 2017 (Thousands of Dollars) Pension Benefits Postretirement Health Care Benefits Service cost $ 1,194 $ 1,154 $ 9 $ 7 Interest cost (a) 1,361 1,554 142 147 Expected return on plan assets (a) (2,256 ) (2,295 ) (15 ) (7 ) Amortization of prior service (credit) cost (a) (8 ) 35 (88 ) (88 ) Amortization of net loss (a) 1,418 1,462 138 109 Settlement charge (b) 5,181 — — — Net periodic benefit cost 6,890 1,910 186 168 Costs not recognized due to the effects of regulation (3,053 ) — — — Net benefit cost recognized for financial reporting $ 3,837 $ 1,910 $ 186 $ 168 Nine Months Ended Sept. 30 2018 2017 2018 2017 (Thousands of Dollars) Pension Benefits Postretirement Health Care Benefits Service cost $ 3,583 $ 3,463 $ 27 $ 21 Interest cost (a) 4,081 4,663 426 443 Expected return on plan assets (a) (6,769 ) (6,885 ) (47 ) (23 ) Amortization of prior service (credit) cost (a) (22 ) 104 (264 ) (264 ) Amortization of net loss (a) 4,255 4,385 416 327 Settlement charge (b) 5,181 — — — Net periodic benefit cost 10,309 5,730 558 504 Costs not recognized due to the effects of regulation (2,832 ) — — — Net benefit cost recognized for financial reporting $ 7,477 $ 5,730 $ 558 $ 504 (a) The components of net periodic cost other than the service cost component are included in the line item “other expense, net” in the income statement or capitalized on the balance sheet as a regulatory asset. (b) A settlement charge is required when the amount of lump-sum distributions during the year is greater than the sum of the service and interest cost components of the annual net periodic pension cost. In the third quarter as a result of lump-sum distributions during the 2018 plan year, NSP-Wisconsin recorded a pension settlement charge of $5 million , the majority of which was not recognized due to the effects of regulation. A total of $1 million of that amount was recorded in other expense in the third quarter of 2018. In the fourth quarter of 2018 as a result of lump-sum distributions during the 2017 plan year, NSP-Wisconsin recorded a total pension settlement charge of $7 million , the majority of which was not recognized due to the effects of regulation. A total of $2 million of that amount was expensed in the fourth quarter of 2017.

Other Comprehensive Income (Tab

Other Comprehensive Income (Tables)	9 Months Ended
	Sep. 30, 2018
Stockholders' Equity Note [Abstract]
Changes in Accumulated Other Comprehensive Loss, Net of Tax	Changes in accumulated other comprehensive loss, net of tax, for the three and nine months ended Sept. 30, 2018 and 2017 were as follows: Gains and Losses on Cash Flow Hedges (Thousands of Dollars) Three Months Ended Sept. 30, 2018 Three Months Ended Sept. 30, 2017 Accumulated other comprehensive loss at July 1 $ (23 ) $ (95 ) Losses reclassified from net accumulated other comprehensive loss 23 19 Net current period other comprehensive income 23 19 Accumulated other comprehensive loss at Sept. 30 $ — $ (76 ) Gains and Losses on Cash Flow Hedges (Thousands of Dollars) Nine Months Ended Sept. 30, 2018 Nine Months Ended Sept. 30, 2017 Accumulated other comprehensive loss at Jan. 1 $ (69 ) $ (133 ) Losses reclassified from net accumulated other comprehensive loss 69 57 Net current period other comprehensive income 69 57 Accumulated other comprehensive loss at Sept. 30 $ — $ (76 )
Reclassifications out of Accumulated Other Comprehensive Loss	Reclassifications from accumulated other comprehensive loss for the three and nine months ended Sept. 30, 2018 and 2017 were as follows: Amounts Reclassified from Accumulated Other Comprehensive Loss (Thousands of Dollars) Three Months Ended Sept. 30, 2018 Three Months Ended Sept. 30, 2017 Losses on cash flow hedges: Interest rate derivatives $ 32 (a) $ 32 (a) Total, pre-tax 32 32 Tax benefit (9 ) (13 ) Total amounts reclassified, net of tax $ 23 $ 19 Amounts Reclassified from Accumulated Other Comprehensive Loss (Thousands of Dollars) Nine Months Ended Sept. 30, 2018 Nine Months Ended Sept. 30, 2017 Losses on cash flow hedges: Interest rate derivatives $ 95 (a) $ 94 (a) Total, pre-tax 95 94 Tax benefit (26 ) (37 ) Total amounts reclassified, net of tax $ 69 $ 57 (a) Included in interest charges.

Revenues (Tables)

Revenues (Tables)	9 Months Ended
	Sep. 30, 2018
Revenue from Contract with Customer [Abstract]
Disaggregation of Revenue	In the following tables, revenue is classified by the type of goods/services rendered and market/customer type. The tables also reconcile revenue to the reportable segments. Three Months Ended Sept. 30, 2018 (Thousands of Dollars) Electric Natural Gas All Other Total Major revenue types Revenue from contracts with customers: Residential $ 67,919 $ 7,021 $ 12 $ 74,952 Commercial and industrial (C&I) 127,936 6,818 24 134,778 Other 1,325 — 262 1,587 Total retail 197,180 13,839 298 211,317 Interchange 39,993 — — 39,993 Other 363 940 — 1,303 Total revenue from contracts with customers 237,536 14,779 298 252,613 Alternative revenue and other 2,808 576 — 3,384 Total revenues $ 240,344 $ 15,355 $ 298 $ 255,997 Three Months Ended Sept. 30, 2017 (Thousands of Dollars) Electric Natural Gas All Other Total Major revenue types Revenue from contracts with customers: Residential $ 62,025 $ 6,428 $ 48 $ 68,501 C&I 119,495 6,441 23 125,959 Other 1,329 — 244 1,573 Total retail 182,849 12,869 315 196,033 Interchange 45,916 — — 45,916 Other 1,157 1,011 — 2,168 Total revenue from contracts with customers 229,922 13,880 315 244,117 Alternative revenue and other 2,880 514 — 3,394 Total revenues $ 232,802 $ 14,394 $ 315 $ 247,511 Nine Months Ended Sept. 30, 2018 (Thousands of Dollars) Electric Natural Gas All Other Total Major revenue types Revenue from contracts with customers: Residential $ 194,456 $ 48,259 $ 38 $ 242,753 Commercial and industrial (C&I) 339,120 40,775 74 379,969 Other 4,713 — 735 5,448 Total retail 538,289 89,034 847 628,170 Interchange 116,037 — — 116,037 Other 3,227 3,016 — 6,243 Total revenue from contracts with customers 657,553 92,050 847 750,450 Alternative revenue and other 8,659 1,786 — 10,445 Total revenues $ 666,212 $ 93,836 $ 847 $ 760,895 Nine Months Ended Sept. 30, 2017 (Thousands of Dollars) Electric Natural Gas All Other Total Major revenue types Revenue from contracts with customers: Residential $ 182,822 $ 42,071 $ 68 $ 224,961 C&I 329,774 35,276 74 365,124 Other 4,527 — 705 5,232 Total retail 517,123 77,347 847 595,317 Interchange 131,259 — — 131,259 Other 2,649 2,829 — 5,478 Total revenue from contracts with customers 651,031 80,176 847 732,054 Alternative revenue and other 8,822 1,592 — 10,414 Total revenues $ 659,853 $ 81,768 $ 847 $ 742,468

Accounting Pronouncements Accou

Accounting Pronouncements Accounting Pronouncements (Details) - Accounting Standards Update 2017-07 - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
	Sep. 30, 2017	Sep. 30, 2017
Operating and Maintenance Expense
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Net periodic benefit cost (credit)	$ 700	$ 2,000
Other Income
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Net periodic benefit cost (credit)	$ (700)	$ (2,000)

Selected Balance Sheet Data, Ac

Selected Balance Sheet Data, Accounts Receivable (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Balance Sheet Related Disclosures [Abstract]
Due from Affiliate, Current	$ 300	$ 3,400
Accounts Receivable, Net
Accounts receivable	56,498	68,073
Less allowance for bad debts	(5,074)	(4,873)
Accounts receivable, net	$ 51,424	$ 63,200

Selected Balance Sheet Data, In

Selected Balance Sheet Data, Inventory (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Public Utilities, Inventory [Line Items]
Inventories	$ 18,385	$ 17,758
Materials and supplies
Public Utilities, Inventory [Line Items]
Inventories	6,770	6,916
Fuel
Public Utilities, Inventory [Line Items]
Inventories	3,718	3,866
Natural gas
Public Utilities, Inventory [Line Items]
Inventories	$ 7,897	$ 6,976

Selected Balance Sheet Data, Pr

Selected Balance Sheet Data, Property, Plant and Equipment (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Public Utility, Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Gross	$ 3,417,109	$ 3,259,269
Less accumulated depreciation	(1,226,849)	(1,170,541)
Property, plant and equipment, net	2,190,260	2,088,728
Electric plant
Public Utility, Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Gross	2,653,711	2,602,671
Natural gas plant
Public Utility, Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Gross	339,399	326,723
Common and other property
Public Utility, Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Gross	188,934	181,105
Construction work in progress
Public Utility, Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Gross	$ 235,065	$ 148,770

Income Taxes (Details)

Income Taxes (Details) - USD ($)		9 Months Ended			12 Months Ended
		Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2015	Dec. 31, 2016	Dec. 31, 2012	Dec. 31, 2017
Income Tax Examination [Line Items]
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		6.20%	5.10%
Regulatory differences - ARAM (a)	[1]	(4.50%)	(0.10%)
Regulatory differences - ARAM deferral (b)	[2]	4.40%	0.00%
Regulatory difference - reversal of prior quarters' ARAM deferral (b)	[2]	(0.10%)	0.00%
Effective Income Tax Rate Reconciliation, Other Regulatory Items, Percent		(1.40%)	(1.30%)
Effective Income Tax Rate Reconciliation Regulatory Differences Utility Plant Items		(0.80%)	(1.00%)
Effective Income Tax Rate Reconciliation, Other Adjustments, Percent		(0.10%)	(2.20%)
Effective Income Tax Rate Reconciliation, Percent		24.70%	35.50%
Unrecognized Tax Benefits [Abstract]
Unrecognized tax benefit — Permanent tax positions		$ 2,000,000					$ 1,400,000
Unrecognized tax benefit — Temporary tax positions		900,000					1,000,000
Total unrecognized tax benefit		2,900,000					2,400,000
NOL and tax credit carryforwards		(2,600,000)					(1,900,000)
Upper bound of decrease in unrecognized tax benefit that is reasonably possible		2,000,000
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 (Wisconsin)
Tax Audits [Abstract]
Year(s) under examination					2012 and 2013
Earliest year subject to examination		2,012
[1]	The average rate assumption method (ARAM); a method to flow back excess deferred taxes to customers.
[2]	ARAM has been deferred when regulatory treatment has not been established. As NSP-Wisconsin received direction from its regulatory commissions regarding the return of excess deferred taxes to customers, the ARAM deferral was reversed. This resulted in a reduction to tax expense with a corresponding reduction to revenue.

Rate Matters Rate Matters (Deta

Rate Matters Rate Matters (Details) - USD ($) $ in Millions	1 Months Ended
	May 31, 2018	Sep. 30, 2016	Jun. 30, 2016	Feb. 28, 2015	Nov. 30, 2013
Public Service Commission of Wisconsin (PSCW) [Member] | PSCW Proceeding - Tax Cuts and Jobs Act of 2017 [Member]
Public Utilities, General Disclosures [Line Items]
Tax Cuts and Jobs Act of 2017, Approved Refund of 2018 TCJA Benefits	$ 27
Tax Cuts and Jobs Act of 2017, Approved Deferral of 2018 TCJA Benefits	$ 5
NSP-Minnesota | FERC Proceeding, MISO ROE Complaint
Public Utilities, General Disclosures [Line Items]
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 Regional Transmission Operator's Region, Recommended By Third Parties				8.67%	9.15%
NSP-Minnesota | Federal Energy Regulatory Commission (FERC) | FERC Proceeding, MISO ROE Complaint
Public Utilities, General Disclosures [Line Items]
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
NSP-Minnesota | Administrative Law Judge | FERC Proceeding, MISO ROE Complaint
Public Utilities, General Disclosures [Line Items]
Public Utilities, ROE Applicable To Transmission Formula Rates In The Regional Transmission Operator's Region, Recommended By Third Parties			9.70%

Commitments and Contingencies,

Commitments and Contingencies, Guarantees and Indemnifications (Details) - USD ($)	9 Months Ended
	Sep. 30, 2018	Dec. 31, 2017
Guarantor Obligations [Line Items]
Assets Held As Collateral	$ 0	$ 0
Payment or Performance Guarantee | Customer Loans for Farm Rewiring Program
Guarantor Obligations [Line Items]
Payment Guarantee Expiration (year)	2,020
Guarantee issued and outstanding	$ 1,000,000	1,000,000
Current exposure under this guarantee	$ 0	$ 0

Commitments and Contingencies_2

Commitments and Contingencies, Environmental Contingencies - Site Contingencies (Details) $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2018USD ($)	Dec. 31, 2017USD ($)
Ashland MGP Site
Ashland Manufactured Gas Plant (MGP) Site [Abstract]
Current Cost Estimate for Site Remediation	$ 184
Estimated amount spent on cleanup	156
Accrual for Environmental Loss Contingencies, Gross	$ 28	$ 30
Approved amortization period for recovery of remediation costs in natural gas rates (in years)	10 years
Carrying cost percentage to be applied to unamortized regulatory asset	3.00%
Other MGP Sites [Member]
Ashland Manufactured Gas Plant (MGP) Site [Abstract]
Accrual for Environmental Loss Contingencies, Gross	$ 0.1	0.1
Number of identified MGP sites under current investigation and/or remediation in addition to those separately disclosed	1
PSCW Proceeding - Gas Rate Case 2017 - Gas Rates 2017 [Member] | Ashland MGP Site
Ashland Manufactured Gas Plant (MGP) Site [Abstract]
Public Utilities, Approved annual recovery collected through base rates		$ 12
PSCW Proceeding - Gas Rate Case 2017 - Gas Rates 2018 [Member] | Ashland MGP Site
Ashland Manufactured Gas Plant (MGP) Site [Abstract]
Public Utilities, Requested annual recovery collected through base rates	$ 18

Commitments and Contingencies C

Commitments and Contingencies Commitments and Contingencies - Legal Contingencies (Details) - Gas Trading Litigation	Sep. 30, 2018	Dec. 31, 2009
Loss Contingencies [Line Items]
Loss Contingency, Pending Claims, Number	6	13
Loss Contingency, Subset of Cases within Multi-District Litigation, Number	2
NSP-Wisconsin
Loss Contingencies [Line Items]
Loss Contingency, Pending Claims, Number		2

Borrowings and Other Financin_3

Borrowings and Other Financing Instruments, Commercial Paper (Details) - USD ($)	3 Months Ended	12 Months Ended
	Sep. 30, 2018	Dec. 31, 2017
Short-term Debt [Line Items]
Amount outstanding at period end	$ 0	$ 11,000,000
Commercial Paper
Short-term Debt [Line Items]
Borrowing limit	150,000,000	150,000,000
Amount outstanding at period end	0	11,000,000
Average amount outstanding	24,000,000	52,000,000
Maximum amount outstanding	$ 48,000,000	$ 129,000,000
Weighted average interest rate, computed on a daily basis (percentage)	2.22%	1.23%
Weighted average interest rate at period end (percentage)		1.73%

Borrowings and Other Financin_4

Borrowings and Other Financing Instruments, Letters of Credit (Details) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2018	Dec. 31, 2017
Line of Credit Facility [Line Items]
Amount outstanding at period end	$ 0	$ 11,000
Letter of Credit
Line of Credit Facility [Line Items]
Amount outstanding at period end	$ 0	$ 0
Letter of Credit | Letter of Credit
Line of Credit Facility [Line Items]
Term of letters of credit (in years)	1 year

Borrowings and Other Financin_5

Borrowings and Other Financing Instruments, Credit Facility (Details) - Credit Facility - USD ($)		9 Months Ended
		Sep. 30, 2018	Dec. 31, 2017
Line of Credit Facility [Line Items]
Credit Facility	[1]	$ 150,000,000
Drawn	[2]	0
Available		$ 150,000,000
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 commercial paper.

Borrowings and Other Financin_6

Borrowings and Other Financing Instruments, Intercompany Borrowing Arrangement and Other Short-Term Borrowings (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Short-term Debt [Line Items]
Notes payable to affiliates	$ 500	$ 500
Notes Payable To Affiliates
Short-term Debt [Line Items]
Notes payable to affiliates	$ 500	$ 500
Weighted average interest rate at period end (percentage)	2.61%	1.73%

Borrowings and Other Financin_7

Borrowings and Other Financing Instruments Borrowings and Other Long Term Financing Instruments (Details) - NSP-Wisconsin - Bonds [Member] - Series Due September 1, 2048 [Member]	9 Months Ended
	Sep. 30, 2018USD ($)
Debt Instrument [Line Items]
Debt Instrument, Face Amount	$ 200,000,000
Debt Instrument, Interest Rate, Stated Percentage	4.20%
Maturity Date	Sep. 1, 2048

Fair Value of Financial Asset_3

Fair Value of Financial Assets and Liabilities, Derivative Instruments (Details) MMBTU in Thousands, $ in Millions		Sep. 30, 2018USD ($)MMBTU	Dec. 31, 2017MMBTU
Interest Rate Swap
Derivative [Line Items]
Interest Rate Cash Flow Hedge Gain (Loss) to be Reclassified During Next 12 Months, Net | $		$ 0
Natural Gas Commodity (in million British thermal units)
Gross Notional Amounts of Commodity Options [Abstract]
Derivative, Nonmonetary Notional amount | MMBTU	[1],[2]	308	42
[1]	Amounts are not reflective of net positions in the underlying commodities.
[2]	Notional amounts for options are included on a gross basis, but are weighted for the probability of exercise.

Fair Value of Financial Asset_4

Fair Value of Financial Assets and Liabilities, Impact of Derivative Activity (Details) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Financial Impact of Qualifying Fair Value Hedges on Earnings [Abstract]
Derivative instruments designated as fair value hedges	$ 0	$ 0	$ 0	$ 0
Recognized gains (losses) from fair value hedges or related hedged transactions	0	0	0	0
Other Derivative Instruments | Natural Gas Commodity
Derivative Instruments, Gain (Loss) [Line Items]
Pre-tax fair value gains (losses) recognized during the period in regulatory (assets) and liabilities		(100,000)		(200,000)
Pre-tax (gains) losses reclassified into income during the period from regulatory assets and (liabilities)	$ 0	$ 0	0	200,000
Cash Flow Hedges | Designated as Hedging Instrument | Interest Rate Contract [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Interest Rate Cash Flow Hedge Gain (Loss) Reclassified to Earnings, Net			$ (100,000)	$ (100,000)

Fair Value of Financial Asset_5

Fair Value of Financial Assets and Liabilities, Derivative Assets and Liabilities at Fair Value (Details) - USD ($) $ in Thousands		Sep. 30, 2018	Dec. 31, 2017
Derivatives, Fair Value [Line Items]
Prepayments and other		$ 3,474	$ 3,450
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	[1]	142	14
Derivative Asset, Fair Value, Gross Liability and Obligation to Return Cash, Offset	[2]	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 2 | Other Current Assets | Other Derivative Instruments | Natural Gas Commodity
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset		142	14
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
Estimate of Fair Value Measurement [Member] | Fair Value Measured on a Recurring Basis
Derivatives, Fair Value [Line Items]
Prepayments and other		3,500	3,500
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		$ 142	$ 14
[1]	Included in prepayments and other current assets balance of $3.5 million at Sept. 30, 2018 and Dec. 31, 2017 in the consolidated balance sheets.
[2]	NSP-Wisconsin nets derivative instruments and related collateral in its consolidated balance sheet when supported by a legally enforceable master netting agreement, and all derivative instruments and related collateral amounts were subject to master netting agreements at Sept. 30, 2018 and Dec. 31, 2017. The counterparty netting amounts presented exclude settlement receivables and payables and non-derivative amounts that may be subject to the same master netting agreements.

Fair Value of Financial Asset_6

Fair Value of Financial Assets and Liabilities, Fair Value of Long-Term Debt (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Carrying Amount
Financial Liabilities, Balance Sheet Groupings [Abstract]
Long-term debt, including current portion	$ 958,889	$ 761,180
Fair Value
Financial Liabilities, Balance Sheet Groupings [Abstract]
Long-term debt, including current portion	$ 1,000,793	$ 856,106

Other Income, Net (Details)

Other Income, Net (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Other Income and Expenses [Abstract]
Interest income	$ 271	$ 138	$ 412	$ 342
Other nonoperating income	8	8	56	238
Other nonoperating expense	(4)	(3)	(11)	(10)
Insurance policy income (expense)	166	(48)	72	(146)
Defined Benefit Plan, Non-service Costs	(1,223)	(678)	(2,180)	(2,034)
Other expense, net	$ (782)	$ (583)	$ (1,651)	$ (1,610)

Segment Information (Details)

Segment Information (Details) - USD ($) $ in Thousands		3 Months Ended		9 Months Ended
		Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Segment Reporting Information [Line Items]
Operating revenues		$ 255,997	$ 247,511	$ 760,895	$ 742,468
Net income (loss)		31,008	22,325	77,613	58,985
Affiliate electric revenue		40,000	46,000	116,000	131,000
Regulated Electric
Segment Reporting Information [Line Items]
Operating revenues		240,456	232,946	666,539	660,216
Net income (loss)		34,256	24,594	70,181	54,580
Regulated Natural Gas
Segment Reporting Information [Line Items]
Operating revenues		15,322	14,450	94,078	82,006
Net income (loss)		(3,448)	(2,184)	6,802	3,388
All Other
Segment Reporting Information [Line Items]
Operating revenues		298	315	847	847
Net income (loss)		200	(85)	630	1,017
Total revenues
Segment Reporting Information [Line Items]
Operating revenues	[1]	255,997	247,511	760,895	742,468
Total revenues | Regulated Electric
Segment Reporting Information [Line Items]
Operating revenues	[1]	240,344	232,802	666,212	659,853
Total revenues | Regulated Natural Gas
Segment Reporting Information [Line Items]
Operating revenues		15,355	14,394	93,836	81,768
Total revenues | All Other
Segment Reporting Information [Line Items]
Operating revenues		298	315	847	847
Intersegment Eliminations
Segment Reporting Information [Line Items]
Operating revenues		(79)	(200)	(569)	(601)
Net income (loss)		0	0	0	0
Intersegment Eliminations | Regulated Electric
Segment Reporting Information [Line Items]
Operating revenues		112	144	327	363
Intersegment Eliminations | Regulated Natural Gas
Segment Reporting Information [Line Items]
Operating revenues		(33)	56	242	238
Intersegment Eliminations | All Other
Segment Reporting Information [Line Items]
Operating revenues		$ 0	$ 0	$ 0	$ 0
[1]	(a) Operating revenues include $116 million and $131 million of affiliate electric revenue for the nine months ended Sept. 30, 2018 and 2017, respectively.

Benefit Plans and Other Postr_3

Benefit Plans and Other Postretirement Benefits (Details) $ in Thousands		1 Months Ended	3 Months Ended				9 Months Ended				12 Months Ended
		Jan. 31, 2018USD ($)Plan	Sep. 30, 2018USD ($)		Sep. 30, 2017USD ($)		Sep. 30, 2018USD ($)		Sep. 30, 2017USD ($)		Dec. 31, 2017USD ($)
Components of Net Periodic Benefit Cost [Abstract]
Operating and maintenance expenses			$ 50,057		$ 51,130		$ 149,335		$ 148,846
Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Net Periodic Benefit Cost (Credit), Gain (Loss) Due to Settlement			5,181	[1]	0	[1]	5,181	[1]	0	[1]	$ 2,000
Components of Net Periodic Benefit Cost [Abstract]
Service cost			1,194		1,154		3,583		3,463
Interest cost	[2]		1,361		1,554		4,081		4,663
Expected return on plan assets	[2]		(2,256)		(2,295)		(6,769)		(6,885)
Amortization of prior service cost (credit)	[2]		(8)		35		(22)		104
Amortization of net loss	[2]		1,418		1,462		4,255		4,385
Defined Benefit Plan Net Benefit Cost (Credit) Recognized Before Regulatory Adjustments			6,890		1,910		10,309		5,730
Defined Benefit Plan Credits Not Recognized Due To Effects of Regulation			(3,053)		0		(2,832)		0
Defined Benefit Plan, Net Periodic Benefit Cost (Credit)			3,837		1,910		7,477		5,730
Operating and maintenance expenses							1,000				$ 7,000
Total contributions to the pension plans during the period		$ 10,000
Other Postretirement Benefits Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Net Periodic Benefit Cost (Credit), Gain (Loss) Due to Settlement	[1]		0		0		0		0
Components of Net Periodic Benefit Cost [Abstract]
Service cost			9		7		27		21
Interest cost	[2]		142		147		426		443
Expected return on plan assets	[2]		(15)		(7)		(47)		(23)
Amortization of prior service cost (credit)	[2]		(88)		(88)		(264)		(264)
Amortization of net loss	[2]		138		109		416		327
Defined Benefit Plan Net Benefit Cost (Credit) Recognized Before Regulatory Adjustments			186		168		558		504
Defined Benefit Plan Credits Not Recognized Due To Effects of Regulation			0		0		0		0
Defined Benefit Plan, Net Periodic Benefit Cost (Credit)			$ 186		$ 168		$ 558		$ 504
Xcel Energy Inc. | Pension Plan [Member]
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
[1]	A settlement charge is required when the amount of lump-sum distributions during the year is greater than the sum of the service and interest cost components of the annual net periodic pension cost. In the third quarter as a result of lump-sum distributions during the 2018 plan year, NSP-Wisconsin recorded a pension settlement charge of $5 million, the majority of which was not recognized due to the effects of regulation. A total of $1 million of that amount was recorded in other expense in the third quarter of 2018. In the fourth quarter of 2018 as a result of lump-sum distributions during the 2017 plan year, NSP-Wisconsin recorded a total pension settlement charge of $7 million, the majority of which was not recognized due to the effects of regulation. A total of $2 million of that amount was expensed in the fourth quarter of 2017.
[2]	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 (Det

Other Comprehensive Income (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Accumulated other comprehensive loss at beginning of period			$ 876,589
Accumulated other comprehensive loss at end of period	$ 907,506		907,506
Gains and Losses on Cash Flow Hedges
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Accumulated other comprehensive loss at beginning of period	(23)	$ (95)	(69)	$ (133)
Losses reclassified from net accumulated other comprehensive loss	23	19	69	57
Net current period other comprehensive income	23	19	69	57
Accumulated other comprehensive loss at end of period	$ 0	$ (76)	$ 0	$ (76)

Other Comprehensive Income (Rec

Other Comprehensive Income (Reclassification from AOCI) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total, pre-tax	$ (40,922)	$ (32,271)	$ (103,107)	$ (91,441)
Tax benefit	9,914	9,946	25,494	32,456
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	32	32	95	94
Tax benefit	(9)	(13)	(26)	(37)
Total, net of tax	23	19	69	57
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	$ 32	$ 32	$ 95	$ 94

Revenues (Details)

Revenues (Details) - USD ($) $ in Thousands		3 Months Ended		9 Months Ended
		Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Disaggregation of Revenue [Line Items]
Total operating revenues		$ 255,997	$ 247,511	$ 760,895	$ 742,468
Maximum number of months following end of annual period in which revenues are earned to be included in incentive programs				24 months
Tolerance band				2
Total revenue from contracts with customers
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers		252,613	244,117	$ 750,450	732,054
Retail
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers		211,317	196,033	628,170	595,317
Retail | Residential
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers		74,952	68,501	242,753	224,961
Retail | Commercial and industrial (C&I)
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers		134,778	125,959	379,969	365,124
Retail | Other
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers		1,587	1,573	5,448	5,232
Interchange [Member]
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers		39,993	45,916	116,037	131,259
Other
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers		1,303	2,168	6,243	5,478
Alternative revenue and other
Disaggregation of Revenue [Line Items]
Total operating revenues		3,384	3,394	10,445	10,414
Regulated Electric
Disaggregation of Revenue [Line Items]
Total operating revenues		240,456	232,946	666,539	660,216
Regulated Electric | Total revenue from contracts with customers
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers		237,536	229,922	657,553	651,031
Regulated Electric | Retail
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers		197,180	182,849	538,289	517,123
Regulated Electric | Retail | Residential
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers		67,919	62,025	194,456	182,822
Regulated Electric | Retail | Commercial and industrial (C&I)
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers		127,936	119,495	339,120	329,774
Regulated Electric | Retail | Other
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers		1,325	1,329	4,713	4,527
Regulated Electric | Interchange [Member]
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers		39,993	45,916	116,037	131,259
Regulated Electric | Other
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers		363	1,157	3,227	2,649
Regulated Electric | Alternative revenue and other
Disaggregation of Revenue [Line Items]
Total operating revenues		2,808	2,880	8,659	8,822
Regulated Natural Gas
Disaggregation of Revenue [Line Items]
Total operating revenues		15,322	14,450	94,078	82,006
Regulated Natural Gas | Total revenue from contracts with customers
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers		14,779	13,880	92,050	80,176
Regulated Natural Gas | Retail
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers		13,839	12,869	89,034	77,347
Regulated Natural Gas | Retail | Residential
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers		7,021	6,428	48,259	42,071
Regulated Natural Gas | Retail | Commercial and industrial (C&I)
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers		6,818	6,441	40,775	35,276
Regulated Natural Gas | Retail | Other
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers		0	0	0	0
Regulated Natural Gas | Interchange [Member]
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		940	1,011	3,016	2,829
Regulated Natural Gas | Alternative revenue and other
Disaggregation of Revenue [Line Items]
Total operating revenues		576	514	1,786	1,592
All Other
Disaggregation of Revenue [Line Items]
Total operating revenues		298	315	847	847
All Other | Total revenue from contracts with customers
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers		298	315	847	847
All Other | Retail
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers		298	315	847	847
All Other | Retail | Residential
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers		12	48	38	68
All Other | Retail | Commercial and industrial (C&I)
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers		24	23	74	74
All Other | Retail | Other
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers		262	244	735	705
All Other | Interchange [Member]
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers		0	0	0	0
All Other | Other
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers		0	0	0	0
All Other | Alternative revenue and other
Disaggregation of Revenue [Line Items]
Total operating revenues		0	0	0	0
Total revenues
Disaggregation of Revenue [Line Items]
Total operating revenues	[1]	255,997	247,511	760,895	742,468
Total revenues | Regulated Electric
Disaggregation of Revenue [Line Items]
Total operating revenues	[1]	240,344	232,802	666,212	659,853
Total revenues | Regulated Natural Gas
Disaggregation of Revenue [Line Items]
Total operating revenues		15,355	14,394	93,836	81,768
Total revenues | All Other
Disaggregation of Revenue [Line Items]
Total operating revenues		$ 298	$ 315	$ 847	$ 847
[1]	(a) Operating revenues include $116 million and $131 million of affiliate electric revenue for the nine months ended Sept. 30, 2018 and 2017, respectively.