Southwestern Public Service 10-Q27 Jul 182018 Q2 Quarterly reportFinancial data

Company Profile

Document and Entity Information

Document and Entity Information - shares	6 Months Ended
Document and Entity Information - shares	Jun. 30, 2018	Jul. 27, 2018
Document and Entity Information [Abstract]
Entity Registrant Name	SOUTHWESTERN PUBLIC SERVICE CO
Entity Central Index Key	92,521
Current Fiscal Year End Date	--12-31
Entity Well-known Seasoned Issuer	No
Entity Voluntary Filers	No
Entity Current Reporting Status	Yes
Entity Filer Category	Non-accelerated Filer
Document Type	10-Q
Document Period End Date	Jun. 30, 2018
Document Fiscal Year Focus	2,018
Document Fiscal Period Focus	Q2
Amendment Flag	false
Entity Common Stock, Shares Outstanding		100

STATEMENTS OF INCOME (UNAUDITED

STATEMENTS OF INCOME (UNAUDITED) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
STATEMENTS OF INCOME (UNAUDITED) - USD ($) $ in Thousands	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Income Statement [Abstract]
Operating revenues	$ 481,338	$ 479,796	$ 928,570	$ 939,868
Operating expenses
Electric fuel and purchased power	257,642	267,942	511,586	521,627
Operating and maintenance expenses	66,148	69,421	132,216	145,561
Demand side management expenses	4,779	3,691	8,937	7,566
Depreciation and amortization	49,579	46,815	97,995	97,233
Taxes (other than income taxes)	15,629	16,689	33,219	33,479
Total operating expenses	393,777	404,558	783,953	805,466
Operating income	87,561	75,238	144,617	134,402
Other expense, net	(782)	(613)	(1,486)	(1,331)
Allowance for funds used during construction — equity	3,201	1,869	6,618	4,004
Interest charges and financing costs
Interest charges — includes other financing costs of $702, $581, $1,396, and $1,156, respectively	20,621	21,946	40,776	44,684
Allowance for funds used during construction — debt	(1,532)	(1,128)	(3,303)	(2,467)
Total interest charges and financing costs	19,089	20,818	37,473	42,217
Income before income taxes	70,891	55,676	112,276	94,858
Income taxes	12,440	20,314	20,726	34,441
Net income	$ 58,451	$ 35,362	$ 91,550	$ 60,417

STATEMENTS OF INCOME (UNAUDITE3

STATEMENTS OF INCOME (UNAUDITED) (Parenthetical) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Interest charges and financing costs
Other financing costs	$ 702	$ 581	$ 1,396	$ 1,156

STATEMENTS OF COMPREHENSIVE INC

STATEMENTS OF COMPREHENSIVE INCOME (UNAUDITED) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Comprehensive income:
Net income	$ 58,451	$ 35,362	$ 91,550	$ 60,417
Pension and retiree medical benefits:
Amortization of losses included in net periodic benefit cost, net of tax of $5, $9, $10 and $18, respectively	18	15	37	30
Derivative instruments:
Reclassification of losses to net income, net of tax of $4, $6, $7 and $12, respectively	12	10	24	19
Other comprehensive income	30	25	61	49
Comprehensive income	$ 58,481	$ 35,387	$ 91,611	$ 60,466

STATEMENTS OF COMPREHENSIVE IN5

STATEMENTS OF COMPREHENSIVE INCOME (UNAUDITED) (Parenthetical) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Derivative instruments:
Reclassification of losses to net income, tax	$ 4	$ 6	$ 7	$ 12
Other Comprehensive (Income) Loss, Defined Benefit Plan, Prior Service Cost (Credit), Reclassification Adjustment from AOCI, Tax	$ 5	$ 9	$ 10	$ 18

STATEMENTS OF CASH FLOWS (UNAUD

STATEMENTS OF CASH FLOWS (UNAUDITED) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017
Operating activities
Net income	$ 91,550	$ 60,417
Adjustments to reconcile net income to cash provided by operating activities:
Depreciation and amortization	98,128	97,126
Demand side management program amortization	837	837
Deferred income taxes	(2,306)	48,099
Amortization of investment tax credits	(26)	(66)
Allowance for equity funds used during construction	(6,618)	(4,004)
Net derivative losses	31	31
Other, net	(5)	10
Changes in operating assets and liabilities:
Accounts receivable	(25,351)	(12,210)
Accrued unbilled revenues	2,329	(16,668)
Inventories	7,915	5,411
Prepayments and other	671	7,028
Accounts payable	640	16,799
Net regulatory assets and liabilities	46,163	(3,477)
Other current liabilities	13,937	(2,896)
Pension and other employee benefit obligations	(7,885)	(21,946)
Change in other noncurrent assets	4,397	(373)
Change in other noncurrent liabilities	(458)	(2,351)
Net cash provided by operating activities	223,949	171,767
Investing activities
Utility capital/construction expenditures	(478,352)	(279,918)
Allowance for equity funds used during construction	6,618	4,004
Investments in utility money pool arrangement	46,000	0
Repayments from utility money pool arrangement	111,000	0
Payments for (Proceeds from) Other Investing Activities	0	(493)
Net cash used in investing activities	(406,734)	(276,407)
Financing activities
Proceeds from short-term borrowings, net	132,000	56,000
Borrowings under utility money pool arrangement	180,000	572,000
Repayments under utility money pool arrangement	(80,000)	(511,000)
Capital contributions from parent	360	45,000
Repayment of long-term debt	0	(18)
Dividends paid to parent	(60,008)	(57,585)
Proceeds from (Payments for) Other Financing Activities	(31)	0
Net cash provided by financing activities	172,321	104,397
Net change in cash and cash equivalents	(10,464)	(243)
Cash and cash equivalents at beginning of period	10,871	844
Cash and cash equivalents at end of period	407	601
Supplemental disclosure of cash flow information:
Cash paid for interest (net of amounts capitalized)	(36,680)	(40,450)
Cash (paid) received for income taxes, net	(7,560)	17,213
Supplemental disclosure of non-cash investing transactions:
Property, plant and equipment additions in accounts payable	$ 43,286	$ 34,529

BALANCE SHEETS (UNAUDITED)

BALANCE SHEETS (UNAUDITED) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Current assets
Cash and cash equivalents	$ 407	$ 10,871
Accounts receivable, net	102,379	79,581
Accounts receivable from affiliates	4,764	1,297
Investments Receivable in Utility Money Pool Arrangement	0	65,000
Accrued unbilled revenues	127,475	129,804
Inventories	32,518	40,433
Regulatory assets	26,093	31,538
Derivative instruments	38,549	15,882
Prepaid taxes	15,710	15,025
Prepayments and other	10,219	10,341
Total current assets	358,114	399,772
Property, plant and equipment, net	5,434,187	5,095,609
Other assets
Regulatory assets	360,902	362,943
Derivative instruments	17,374	18,954
Other	3,710	11,266
Total other assets	381,986	393,163
Total assets	6,174,287	5,888,544
Current liabilities
Short-term debt	132,000	0
Borrowings Payable Under Utility Money Pool Arrangement	100,000	0
Accounts payable	184,825	211,756
Accounts payable to affiliates	15,036	22,577
Regulatory liabilities	123,303	68,835
Taxes accrued	51,965	35,243
Accrued interest	23,413	23,275
Dividends payable	30,697	26,753
Derivative instruments	3,565	3,565
Other	26,019	29,641
Total current liabilities	690,823	421,645
Deferred credits and other liabilities
Deferred income taxes	577,492	574,906
Regulatory liabilities	781,917	784,564
Asset retirement obligations	29,279	28,524
Derivative instruments	18,166	19,949
Pension and employee benefit obligations	82,326	90,266
Other	4,630	8,386
Total deferred credits and other liabilities	1,493,810	1,506,595
Commitments and contingencies
Capitalization
Long-term debt	1,830,508	1,829,941
Common stock — 200 shares authorized of $1.00 par value; 100 shares outstanding at June 30, 2018 and Dec. 31, 2017, respectively	0	0
Additional paid in capital	1,591,366	1,590,242
Retained earnings	569,186	541,588
Accumulated other comprehensive loss	(1,406)	(1,467)
Total common stockholder’s equity	2,159,146	2,130,363
Total liabilities and equity	$ 6,174,287	$ 5,888,544

BALANCE SHEETS (UNAUDITED) (Par

BALANCE SHEETS (UNAUDITED) (Parenthetical) - $ / shares	Jun. 30, 2018	Dec. 31, 2017
Capitalization, Long-term Debt and Equity [Abstract]
Common stock, shares authorized (in shares)	200	200
Common stock, par value (in dollars per share)	$ 1	$ 1
Common stock, shares outstanding (in shares)	100	100

Management's Opinion

Management's Opinion	6 Months Ended
Management's Opinion	Jun. 30, 2018
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Management's Opinion	In the opinion of management, the accompanying unaudited 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 SPS as of June 30, 2018 and Dec. 31, 2017 ; the results of its operations, including the components of net income and comprehensive income, for the three and six months ended June 30, 2018 and 2017 ; and its cash flows for the six months ended June 30, 2018 and 2017 . All adjustments are of a normal, recurring nature, except as otherwise disclosed. Management has also evaluated the impact of events occurring after June 30, 2018 up to the date of issuance of these 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 financial statements included in the SPS Annual Report on Form 10-K for the year ended Dec. 31, 2017 . These notes to the 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 financial statements and notes thereto, included in the SPS Annual Report on Form 10-K for the year ended Dec. 31, 2017 , filed with the SEC on Feb. 23, 2018. Due to the seasonality of SPS’ electric sales, interim results are not necessarily an appropriate base from which to project annual results.

Summary of Significant Accounti

Summary of Significant Accounting Policies	6 Months Ended
Summary of Significant Accounting Policies	Jun. 30, 2018
Accounting Policies [Abstract]
Summary of Significant Accounting Policies	Summary of Significant Accounting Policies The significant accounting policies set forth in Note 1 to the financial statements in the SPS Annual Report on Form 10-K for the year ended Dec. 31, 2017 , appropriately represent, in all material respects, the current status of accounting policies and are incorporated herein by reference.

Accounting Pronouncements

Accounting Pronouncements	6 Months Ended
Accounting Pronouncements	Jun. 30, 2018
New Accounting Pronouncements and Changes in Accounting Principles [Abstract]
Accounting Pronouncements	Accounting Pronouncements Recently Issued Leases — I n February 2016, the Financial Accounting Standards Board (FASB) issued Leases, Topic 842 (Accounting Standards Update (ASU) No. 2016-02) , which for lessees requires balance sheet recognition of right-of-use assets and lease liabilities for most leases. This guidance will be effective for interim and annual reporting periods beginning after Dec. 15, 2018. SPS has not yet fully determined the impacts of implementation. However, adoption is expected to occur on Jan. 1, 2019 utilizing the practical expedients provided by the standard and proposed in Targeted Improvements, Topic 842 (Proposed ASU 2018-200) . On Jan. 1, 2019 agreements considered leases for the use of office space, equipment and natural gas storage assets, as well as certain purchased power agreements (PPAs) for fossil-fueled generating facilities are expected to be recognized on the balance sheet. Recently Adopted Revenue Recognition — In May 2014, the FASB issued Revenue from Contracts with Customers, Topic 606 (ASU No. 2014-09) , which provides a new framework for the recognition of revenue. SPS implemented the guidance on a modified retrospective basis on Jan. 1, 2018. Results for reporting periods beginning after Dec. 31, 2017 are presented in accordance with Topic 606, while prior period results have not been adjusted and continue to be reported in accordance with prior accounting guidance. Other than increased disclosures regarding revenues related to contracts with customers, the implementation did not have a significant impact on SPS’ financial statements. For related disclosures, see Note 12 to the 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. SPS implemented the guidance on Jan. 1, 2018 and the implementation did not have a material impact on its 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 statement of income. SPS implemented the new guidance on Jan. 1, 2018, and as a result, $1.5 million of pension costs were retrospectively reclassified from operating and maintenance expenses to other income, net on the income statement for the six months ended June 30, 2017. Under a practical expedient permitted by the standard, SPS used benefit cost amounts disclosed for prior periods as the basis for retrospective application.

Selected Balance Sheet Data

Selected Balance Sheet Data	6 Months Ended
Selected Balance Sheet Data	Jun. 30, 2018
Balance Sheet Related Disclosures [Abstract]
Selected Balance Sheet Data	Selected Balance Sheet Data (Thousands of Dollars) June 30, 2018 Dec. 31, 2017 Accounts receivable, net Accounts receivable $ 107,990 $ 85,929 Less allowance for bad debts (5,611 ) (6,348 ) $ 102,379 $ 79,581 (Thousands of Dollars) June 30, 2018 Dec. 31, 2017 Inventories Materials and supplies $ 25,416 $ 26,218 Fuel 7,102 14,215 $ 32,518 $ 40,433 (Thousands of Dollars) June 30, 2018 Dec. 31, 2017 Property, plant and equipment, net Electric plant $ 7,059,344 $ 6,765,371 Construction work in progress 447,945 351,875 Total property, plant and equipment 7,507,289 7,117,246 Less accumulated depreciation (2,073,102 ) (2,021,637 ) $ 5,434,187 $ 5,095,609

Income Taxes

Income Taxes	6 Months Ended
Income Taxes	Jun. 30, 2018
Income Tax Disclosure [Abstract]
Income Taxes	Income Taxes Except to the extent noted below, Note 6 to the financial statements included in SPS’ Annual Report on Form 10-K for the year ended Dec. 31, 2017 appropriately represents, in all material respects, the current status of other income tax matters, and are incorporated herein by reference. Total income tax expense from operations differs from the amount computed by applying the statutory federal income tax rate to income before income tax expense. The following reconciles such differences: Six Months Ended June 30 2018 2017 Federal statutory rate 21.0 % 35.0 % State tax, net of federal tax effect 2.4 2.2 Increases (decreases) in tax from: Regulatory differences - ARAM (a) (4.2 ) — Regulatory differences - ARAM (b) 1.3 — Regulatory differences - other utility plant items (1.4 ) (0.8 ) Tax credits, net of federal income tax expense (0.7 ) (0.5 ) Other, net 0.1 0.4 Effective income tax rate 18.5 % 36.3 % (a) The average rate assumption method (ARAM); a method to flow back excess deferred taxes to customers. (b) As we receive direction from our regulatory commissions regarding the return of excess deferred taxes (to our customers resulting from the Tax Cuts and Jobs Act (TCJA)), the ARAM deferral may decrease during the year, which would result in a reduction to tax expense with a corresponding reduction to revenue. Federal Audits — SPS is a member of the Xcel Energy affiliated group that files a consolidated federal income tax return. The statutes of limitations applicable to Xcel Energy’s federal income tax returns expire as follows: Tax Year(s) Expiration 2009 - 2011 December 2018 2012 - 2014 October 2019 2015 September 2019 2016 September 2020 In 2012, the Internal Revenue Service (IRS) commenced an examination of tax years 2010 and 2011 , including the 2009 carryback claim. The IRS proposed an adjustment to the federal tax loss carryback claims and in 2015 the IRS forwarded the issue to the Office of Appeals (Appeals). In 2017 Xcel Energy and Appeals reached an agreement and the benefit related to the agreed upon portions was recognized. SPS 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). After evaluating the proposed adjustment Xcel Energy filed a protest with the IRS. As of June 30, 2018 the case has been forwarded to Appeals and Xcel Energy has recognized its best estimate of income tax expense that will result from a final resolution of this issue; however, the outcome and timing of a resolution is unknown. State Audits — SPS is a member of the Xcel Energy affiliated group that files consolidated state income tax returns. As of June 30, 2018 , SPS’ earliest open tax year that is subject to examination by state taxing authorities under applicable statutes of limitations is 2009 . There are currently no 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) June 30, 2018 Dec. 31, 2017 Unrecognized tax benefit — Permanent tax positions $ 2.5 $ 2.3 Unrecognized tax benefit — Temporary tax positions 1.6 2.0 Total unrecognized tax benefit $ 4.1 $ 4.3 The unrecognized tax benefit amounts were reduced by the tax benefits associated with NOL and tax credit carryforwards. The amounts of tax benefits associated with NOL and tax credit carryforwards are as follows: (Millions of Dollars) June 30, 2018 Dec. 31, 2017 NOL and tax credit carryforwards $ (6.6 ) $ (5.9 ) It is reasonably possible that SPS’ 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 $3 million . The payable for interest related to unrecognized tax benefits is partially offset by the interest benefit associated with NOL and tax credit carryforwards. The payables for interest related to unrecognized tax benefits at June 30, 2018 and Dec. 31, 2017 were not material. No amounts were accrued for penalties related to unrecognized tax benefits as of June 30, 2018 or Dec. 31, 2017 .

Rate Matters

Rate Matters	6 Months Ended
Rate Matters	Jun. 30, 2018
Public Utilities, General Disclosures [Abstract]
Rate Matters	Rate Matters Except to the extent noted below, the circumstances set forth in Note 10 to the financial statements included in SPS’ Annual Report on Form 10-K for the year ended Dec. 31, 2017 and in Note 5 to the financial statements included in to SPS’ Quarterly Report on Form 10-Q for the quarterly period ended March 31, 2018, appropriately represent, in all material respects, the current status of other rate matters, and are incorporated herein by reference. Tax Reform — Regulatory Proceedings The specific impacts of the TCJA on customer rates are subject to regulatory approval. Each of the states in Xcel Energy’s service areas, including Texas and New Mexico, have opened dockets to address the impacts of the TCJA. Texas — In June 2018, SPS, the Public Utility Commission of Texas (PUCT) Staff and various intervenors reached a settlement in the Texas electric rate case which included the impacts of the TCJA. The settlement reflects no change in customer rates or refunds and SPS’ actual capital structure, which SPS has informed the parties it intends to be a 57 percent equity ratio to offset the negative impacts on its credit metrics and potentially its credit ratings. New Mexico — In February 2018, SPS indicated that the TCJA would reduce revenue requirements by approximately $11 million and recommended increasing its equity ratio to 58 percent to offset the negative impact of the TCJA on its credit metrics and potentially its credit ratings. The impact of the TCJA is expected to be addressed as part of SPS’ pending New Mexico electric rate case. Pending Regulatory Proceedings — PUCT Texas 2017 Electric Rate Case — In 2017, SPS filed a $54 million , or 5.8 percent , retail electric, non-fuel base rate increase case in Texas with each of its Texas municipalities and the PUCT. The request was based on a historic test year (HTY) ended June 30, 2017, a requested return on equity (ROE) of 10.25 percent , an electric rate base of approximately $1.9 billion and an equity ratio of 53.97 percent . The request also reflects the acceleration of depreciation lives for the two generating units at the Tolk Generating Station from 2042 and 2045 to 2032. In May 2018, SPS filed rebuttal testimony and revised its request to an overall increase in the annual base rate revenue of approximately $32 million , or 5.9 percent , net of the TCJA (approximately $32 million after adjusting for a 58 percent equity ratio) and other adjustments. This request would be equivalent to approximately $17 million after adjusting for the Transmission Cost Recovery Factor (TCRF) rider. In June 2018, SPS, the PUCT Staff and various intervenors reached a settlement, which results in no overall change to SPS’ revenues after adjusting for the impact of the TCJA and the lower costs of long-term debt. The following are key terms: • The ability to use an equity ratio that reflects SPS' actual capital structure, which SPS has informed the parties it intends to be 57 percent to mitigate the impact of TCJA on credit metrics; • A 9.5 percent ROE for the calculation of allowance for funds used during construction (AFUDC); • TCRF rider will remain in effect; • SPS will accelerate depreciation rates for the Tolk Generating Station Units 1 and 2 by 50 percent of the original request; and • SPS agrees that it will file its next base rate case no later than Dec. 31, 2019. A reconciliation of the settlement is as follows: (Millions of Dollars) Original base rate request $ 69 Base rate revenue to be recovered through TCRF (15 ) Net revenue request 54 Adjustment for TCJA and other items (37 ) Requested incremental revenue 17 Unspecified settlement adjustments (13 ) Accelerated depreciation (Tolk plant) (4 ) SPS' net revenue change $ — Under the terms of the settlement, the final rates would not change from the current rates. However, SPS would be permitted to surcharge customers for unrecovered TCRF charges that were not billed during the period of Jan. 23, 2018 through June 10, 2018. A PUCT decision is expected in the third quarter of 2018. Appeal of the Texas 2015 Electric Rate Case Decision — In 2014, SPS had requested an overall retail electric revenue rate increase of $42 million . In 2015, the PUCT approved an overall rate decrease of approximately $4 million , net of rate case expenses. In April 2016, SPS filed an appeal with the Texas State District Court (District Court) challenging the PUCT’s order. In 2017, the District Court denied SPS’ appeal, and SPS appealed the District Court’s decision to the state Court of Appeals for the 7th Circuit. In 2018, the Court of Appeals upheld the District Court’s decision on the PUCT’s order, rejecting SPS’ appeal. As part of the settlement of the 2017 Texas rate case, SPS has agreed to end its appeal. Pending Regulatory Proceeding — New Mexico Public Regulation Commission (NMPRC) New Mexico 2017 Electric Rate Case — In October 2017, SPS filed an electric rate case with the NMPRC seeking an increase in base rates of approximately $43 million . The request was based on a HTY ended June 30, 2017, a ROE of 10.25 percent , an equity ratio of 53.97 percent , a 35 percent federal income tax rate and a rate base of approximately $885 million , including rate base additions through Nov. 30, 2017. In May 2018, SPS reduced its request to $27 million , net of the TCJA (approximately $11 million ) and other adjustments, based on a requested ROE of 10.25 percent and an equity ratio of 58.0 percent . In June 2018, the New Mexico Hearing Examiner issued a recommended decision proposing an increase of $12 million based on a ROE of 9.4 percent and an equity ratio of 53.97 percent . She also denied SPS' requests to shorten depreciation lives related to Tolk Units 1 and 2 and Cunningham Unit 1. The Hearing Examiner rejected intervenor proposals to refund the impacts of the TCJA back to Jan. 1, 2018. The following table summarizes certain parties’ proposed modifications to SPS’ request, SPS’ revised request, and the Hearing Examiner’s recommendation: (Millions of Dollars) NMPRC Staff Testimony NMAG Testimony SPS Rebuttal Testimony Hearing Examiner's Recommendation SPS request $ 43 $ 43 $ 43 $ 43 Reduction to request for the impact of the TCJA (11 ) (11 ) (11 ) (11 ) SPS request, including the impact of the TCJA 32 32 32 32 ROE (4 ) (6 ) — (5 ) Capital structure (7 ) (3 ) — (3 ) Depreciation lives (Tolk and Cunningham plants) (3 ) (3 ) — (3 ) Disallow rate case expenses (2 ) (3 ) (1 ) — Regional transmission revenue and expense (adjustment for the impact of the TCJA): Impact of the TCJA — (3 ) — (1 ) Aligning costs with transmission plant in rate base — — — (1 ) Post test year plant (updated to actual) (1 ) (2 ) (3 ) — Excess generation adjustment — (1 ) — (1 ) Other, net (4 ) (4 ) (1 ) (6 ) Recommended rate increase $ 11 $ 7 $ 27 $ 12 ROE 9.0 % 9.21 % 10.25 % 9.4 % Equity ratio 52.0 % 53.97 % 58.0 % 53.97 % SPS anticipates a decision and implementation of final rates in the third quarter of 2018. Appeal of the New Mexico 2016 Electric Rate Case Dismissal — In November 2016, SPS filed an electric rate case with the NMPRC seeking an increase in base rates of approximately $41 million , representing a total revenue increase of approximately 10.9 percent . The rate filing was based on a requested ROE of 10.1 percent , an equity ratio of 53.97 percent , an electric rate base of approximately $832 million and a future test year ending June 30, 2018. In 2017, the NMPRC dismissed SPS’ rate case. SPS filed a notice of appeal in the New Mexico Supreme Court. A decision is not expected until the second half of 2019. Pending Regulatory Proceeding — Federal Energy Regulatory Commission (FERC) Southwest Power Pool, Inc. (SPP) Open Access Transmission Tariff (OATT) Upgrade Costs — Under the SPP OATT, costs of participant funded, or “sponsored,” transmission upgrades may be recovered from other SPP customers whose transmission service depends on capacity enabled by the upgrade. The SPP OATT has allowed SPP to charge for these upgrades since 2008, but SPP had not been charging its customers for these upgrades. In 2016, the FERC granted SPP’s request to recover the charges not billed since 2008. SPP subsequently billed SPS approximately $13 million for these charges. SPP is also billing SPS ongoing charges of approximately $0.5 million per month. In November 2017, the FERC denied an SPS request for rehearing. In January 2018, SPS appealed the FERC request to the United States Court of Appeals for the District of Columbia Circuit (D.C. Circuit). SPS has filed to recover the SPP charges as part of the appeal. The appeal is currently pending. In October 2017, SPS filed a complaint against SPP regarding the amounts billed asserting that SPP has assessed upgrade charges to SPS in violation of the SPP OATT. In March 2018, the FERC denied SPS’ complaint. SPS sought rehearing in April 2018, and the FERC approved the rehearing request for further consideration on May 7, 2018. If SPS’ complaint results in additional charges or refunds, SPS will seek to recover or refund the differential in future rate proceedings.

Commitments and Contingencies

Commitments and Contingencies	6 Months Ended
Commitments and Contingencies	Jun. 30, 2018
Commitments and Contingencies Disclosure [Abstract]
Commitments and Contingencies	Commitments and Contingencies Except to the extent noted below and in Note 5 above, the circumstances set forth in Notes 10 and 11 to the financial statements included in SPS’ Annual Report on Form 10-K for the year ended Dec. 31, 2017 and in Notes 5 and 6 to the financial statements included in SPS’ Quarterly Report on Form 10-Q for the quarterly period ended March 31, 2018, appropriately represent, in all material respects, the current status of commitments and contingent liabilities and are incorporated herein by reference. The following include commitments, contingencies and unresolved contingencies that are material to SPS’ financial position. PPAs SPS purchases power from independent power producing entities that own natural gas fueled power plants for which SPS is required to reimburse natural gas fuel costs, or to participate in tolling arrangements under which SPS procures the natural gas required to produce the energy that it purchases. These specific PPAs create a variable interest in the associated independent power producing entity. SPS had approximately 897 Megawatts (MW) of capacity under long-term PPAs as of June 30, 2018 and Dec. 31, 2017 , with entities that have been determined to be variable interest entities. SPS has concluded that these entities are not required to be consolidated in its financial statements because it does not have the power to direct the activities that most significantly impact the entities’ economic performance. These agreements have various expiration dates through 2041 . Environmental Contingencies Manufactured Gas Plant (MGP), Landfill or Disposal Sites — SPS is currently involved in investigating and/or remediating an MGP, landfill or other disposal site. SPS has identified one site where contamination is present and where investigation and/or remediation activities are currently underway. Other parties may have responsibility for some portion of the investigation and/or remediation activities. SPS anticipates that the investigation or remediation activities will continue through at least 2018. SPS accrued $0.1 million for the site as of June 30, 2018 and Dec. 31, 2017 , respectively. There may be insurance recovery and/or recovery from other potentially responsible parties that will offset any costs incurred. SPS anticipates that any amounts spent will be fully recovered from customers. Legal Contingencies SPS 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 SPS’ financial statements. Unless otherwise required by GAAP, legal fees are expensed as incurred.

Borrowings and Other Financing

Borrowings and Other Financing Instruments	6 Months Ended
Borrowings and Other Financing Instruments	Jun. 30, 2018
Debt Disclosure [Abstract]
Borrowings and Other Financing Instruments	Borrowings and Other Financing Instruments Short-Term Borrowings Money Pool — Xcel Energy Inc. and its utility subsidiaries have established a money pool arrangement that allows for short-term investments in and borrowings between the utility subsidiaries. Xcel Energy Inc. may make investments in the utility subsidiaries at market-based interest rates; however, the money pool arrangement does not allow the utility subsidiaries to make investments in Xcel Energy Inc. Money pool borrowings for SPS were as follows: (Amounts in Millions, Except Interest Rates) Three Months Ended June 30, 2018 Year Ended Dec. 31, 2017 Borrowing limit $ 100 $ 100 Amount outstanding at period end 100 — Average amount outstanding 24 13 Maximum amount outstanding 100 100 Weighted average interest rate, computed on a daily basis 1.84 % 1.12 % Weighted average interest rate at period end 1.85 N/A Commercial Paper — SPS meets its short-term liquidity requirements primarily through the issuance of commercial paper and borrowings under its credit facility and the money pool. Commercial paper outstanding for SPS was as follows: (Amounts in Millions, Except Interest Rates) Three Months Ended June 30, 2018 Year Ended Dec. 31, 2017 Borrowing limit $ 400 $ 400 Amount outstanding at period end 132 — Average amount outstanding 32 69 Maximum amount outstanding 140 176 Weighted average interest rate, computed on a daily basis 2.25 % 1.13 % Weighted average interest rate at period end 2.29 N/A Letters of Credit — SPS uses letters of credit, generally with terms of one year , to provide financial guarantees for certain operating obligations. As of June 30, 2018 and Dec. 31, 2017 , there were $2 million and $3 million , respectively, of letters of credit outstanding under the credit facility. The contract amounts of these letters of credit approximate their fair value and are subject to fees. Credit Facility — In order to use its commercial paper program to fulfill short-term funding needs, SPS 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 June 30, 2018 , SPS had the following committed credit facility available (in millions of dollars): Credit Facility (a) Drawn (b) Available $ 400 $ 134 $ 266 (a) This credit facility expires in June 2021 . (b) Includes outstanding commercial paper and letters of credit. All credit facility bank borrowings, outstanding letters of credit and outstanding commercial paper reduce the available capacity under the credit facility. SPS had no direct advances on the credit facility outstanding as of June 30, 2018 and Dec. 31, 2017 .

Fair Value of Financial Assets

Fair Value of Financial Assets and Liabilities	6 Months Ended
Fair Value of Financial Assets and Liabilities	Jun. 30, 2018
Fair Value Disclosures [Abstract]
Fair Value of Financial Assets and Liabilities	Fair Value of Financial Assets and Liabilities Fair Value Measurements The accounting guidance for fair value measurements and disclosures provides a single definition of fair value and requires certain disclosures about assets and liabilities measured at fair value. A hierarchical framework for disclosing the observability of the inputs utilized in measuring assets and liabilities at fair value is established by this guidance. The three levels in the hierarchy are as follows: Level 1 — Quoted prices are available in active markets for identical assets or liabilities as of the reporting date. The types of assets and liabilities included in Level 1 are highly liquid and actively traded instruments with quoted prices. Level 2 — Pricing inputs are other than quoted prices in active markets, but are either directly or indirectly observable as of the reporting date. The types of assets and liabilities included in Level 2 are typically either comparable to actively traded securities or contracts, or priced with models using highly observable inputs. Level 3 — Significant inputs to pricing have little or no observability as of the reporting date. The types of assets and liabilities included in Level 3 are those valued with models requiring significant management judgment or estimation. Specific valuation methods include the following: Cash equivalents — The fair values of cash equivalents are generally based on cost plus accrued interest; money market funds are measured using quoted net asset value. Interest rate derivatives — The fair values of interest rate derivatives are based on broker quotes that utilize current market interest rate forecasts. Commodity derivatives — The methods used to measure the fair value of commodity derivative forwards and options utilize forward prices and volatilities, as well as pricing adjustments for specific delivery locations, and are generally assigned a Level 2 classification. When contractual settlements extend to periods beyond those readily observable on active exchanges or quoted by brokers, the significance of the use of less observable forecasts of long-term forward prices and volatilities on a valuation is evaluated and may result in Level 3 classification. Electric commodity derivatives held by SPS include transmission congestion instruments, generally referred to as financial transmission rights (FTRs), purchased from SPP. FTRs purchased from a regional transmission organization (RTO) are financial instruments that entitle or obligate the holder to monthly revenues or charges based on transmission congestion across a given transmission path. The value of an FTR is derived from, and designed to offset, the cost of transmission congestion. In addition to overall transmission load, congestion is also influenced by the operating schedules of power plants and the consumption of electricity pertinent to a given transmission path. Unplanned plant outages, scheduled plant maintenance, changes in the relative costs of fuels used in generation, weather and overall changes in demand for electricity can each impact the operating schedules of the power plants on the transmission grid and the value of an FTR. The valuation process for FTRs utilizes the cleared prices for each FTR for the most recent auction. If forecasted costs of electric transmission congestion increase or decrease for a given FTR path, the value of that particular FTR instrument will likewise increase or decrease. Given the limited transparency in the auction process, fair value measurements for FTRs have been assigned a Level 3. Non-trading monthly FTR settlements are expected to be recovered through fuel and purchased energy cost recovery mechanisms, and therefore changes in the fair value of the yet to be settled portions of FTRs are deferred as a regulatory asset or liability. Given this regulatory treatment and the limited magnitude of FTRs, the limited transparency associated with the valuation of FTRs is insignificant to the financial statements of SPS. Derivative Instruments Fair Value Measurements SPS enters into derivative instruments, including forward contracts, for trading purposes and to manage risk in connection with changes in interest rates and electric utility commodity prices. Interest Rate Derivatives — SPS may enter 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. As of June 30, 2018 , accumulated other comprehensive losses related to interest rate derivatives included immaterial net losses expected to be reclassified into earnings during the next 12 months as the related hedged interest rate transactions impact earnings, including forecasted amounts for unsettled hedges, as applicable. Wholesale and Commodity Trading Risk — SPS conducts various wholesale and commodity trading activities, including the purchase and sale of electric capacity, energy and energy-related instruments, including derivatives. SPS’ risk management policy allows management to conduct these activities within guidelines and limitations as approved by its risk management committee, which is made up of management personnel not directly involved in the activities governed by this policy. Commodity Derivatives — SPS enters into derivative instruments to manage variability of future cash flows from changes in commodity prices in its electric utility operations. This could include the purchase or sale of energy or energy-related products and FTRs. The following table details the gross notional amounts of commodity FTRs as of June 30, 2018 and Dec. 31, 2017 : (Amounts in Thousands) (a) June 30, 2018 Dec. 31, 2017 Megawatt hours of electricity 12,941 4,251 (a) Amounts are not reflective of net positions in the underlying commodities. Impact of Derivative Activities on Income and Accumulated Other Comprehensive Loss — Pre-tax losses related to interest rate derivatives reclassified from accumulated other comprehensive loss into earnings were immaterial for each of the three and six months ended June 30, 2018 and 2017. During the three and six months ended June 30, 2018 , changes in the fair value of FTRs resulted in pre-tax net gains of $13.0 million and $13.4 million , respectively, and were recognized as regulatory assets and liabilities. For the three and six months ended June 30, 2017, changes in the fair value of FTRs resulted in pre-tax net gains of $0.2 million and $2.3 million , respectively, and were recognized as regulatory assets and liabilities. The classification as a regulatory asset or liability is based on expected recovery of FTR settlements through fuel and purchased energy cost recovery mechanisms. FTR settlement gains of $3.9 million and $3.4 million were recognized for the three and six months ended June 30, 2018 , respectively, and were recorded to electric fuel and purchased power. For the three and six months ended June 30, 2017, FTR settlement gains of $1.2 million and $2.4 million , respectively, were recognized and recorded to electric fuel and purchased power. These derivative settlement gains and losses are shared with electric customers through fuel and purchased energy cost-recovery mechanisms, and reclassified out of income as regulatory assets or liabilities, as appropriate. SPS had no derivative instruments designated as fair value hedges during the three and six months ended June 30, 2018 and 2017 . Therefore, no gains or losses from fair value hedges or related hedged transactions were recognized for these periods. Consideration of Credit Risk and Concentrations — SPS 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 SPS’ 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 balance sheets. SPS 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. SPS’ most significant concentrations of credit risk with particular entities or industries are contracts with counterparties to its wholesale, trading and non-trading commodity activities. As of June 30, 2018, six of SPS’ most significant counterparties, comprising $25.7 million or 50 percent of this credit exposure, were not rated by Standard & Poor’s, Moody’s or Fitch Ratings, but based on SPS’ internal analysis, had credit quality consistent with investment grade. The one remaining significant counterparty, comprising $0.2 million or less than 1 percent of this credit exposure, had credit quality less than investment grade, based on ratings from internal analysis. All seven of these significant counterparties are municipal or cooperative electric entities or other utilities. Recurring Fair Value Measurements — The following table presents for each of the fair value hierarchy levels, SPS’ derivative assets and liabilities measured at fair value on a recurring basis as of June 30, 2018 : June 30, 2018 Fair Value Fair Value Total Counterparty Netting (b) (Thousands of Dollars) Level 1 Level 2 Level 3 Total Current derivative assets Other derivative instruments: Electric commodity $ — $ — $ 35,897 $ 35,897 $ (508 ) $ 35,389 Total current derivative assets $ — $ — $ 35,897 $ 35,897 $ (508 ) 35,389 PPAs (a) 3,160 Current derivative instruments $ 38,549 Noncurrent derivative assets PPAs (a) $ 17,374 Noncurrent derivative instruments $ 17,374 Current derivative liabilities Other derivative instruments: Electric commodity $ — $ — $ 508 $ 508 $ (508 ) $ — Total current derivative liabilities $ — $ — $ 508 $ 508 $ (508 ) — PPAs (a) 3,565 Current derivative instruments $ 3,565 Noncurrent derivative liabilities PPAs (a) $ 18,166 Noncurrent derivative instruments $ 18,166 (a) During 2006, SPS qualified these contracts under the normal purchase exception. Based on this qualification, the contracts are no longer adjusted to fair value and the previous carrying value of these contracts is being amortized over the remaining contract lives along with the offsetting regulatory assets and liabilities. (b) SPS nets derivative instruments and related collateral in its balance sheet when supported by a legally enforceable master netting agreement, and all derivative instruments and related collateral amounts were subject to master netting agreements at June 30, 2018 . At June 30, 2018 , derivative assets and liabilities include no obligations to return cash collateral or rights to reclaim cash collateral. The counterparty netting amounts presented exclude settlement receivables and payables and non-derivative amounts that may be subject to the same master netting agreements. The following table presents for each of the fair value hierarchy levels, SPS’ derivative assets and liabilities measured at fair value on a recurring basis as of Dec. 31, 2017: Dec. 31, 2017 Fair Value Fair Value Total Counterparty Netting (b) (Thousands of Dollars) Level 1 Level 2 Level 3 Total Current derivative assets Other derivative instruments: Electric commodity $ — $ — $ 14,717 $ 14,717 $ (1,994 ) $ 12,723 Total current derivative assets $ — $ — $ 14,717 $ 14,717 $ (1,994 ) 12,723 PPAs (a) 3,159 Current derivative instruments $ 15,882 Noncurrent derivative assets PPAs (a) $ 18,954 Noncurrent derivative instruments $ 18,954 Current derivative liabilities Other derivative instruments: Electric commodity $ — $ — $ 1,994 $ 1,994 $ (1,994 ) $ — Total current derivative liabilities $ — $ — $ 1,994 $ 1,994 $ (1,994 ) — PPAs (a) 3,565 Current derivative instruments $ 3,565 Noncurrent derivative liabilities PPAs (a) $ 19,949 Noncurrent derivative instruments $ 19,949 (a) During 2006, SPS qualified these contracts under the normal purchase exception. Based on this qualification, the contracts are no longer adjusted to fair value and the previous carrying value of these contracts is being amortized over the remaining contract lives along with the offsetting regulatory assets and liabilities. (b) SPS nets derivative instruments and related collateral in its balance sheet when supported by a legally enforceable master netting agreement, and all derivative instruments and related collateral amounts were subject to master netting agreements at Dec. 31, 2017. At Dec. 31, 2017, derivative assets and liabilities include no obligations to return cash collateral or rights to reclaim cash collateral. The counterparty netting amounts presented exclude settlement receivables and payables and non-derivative amounts that may be subject to the same master netting agreements. The following table presents the changes in Level 3 commodity derivatives for the three and six months ended June 30, 2018 and 2017 : Three Months Ended June 30, (Thousands of Dollars) 2018 2017 Balance at April 1 $ 5,343 $ 1,192 Purchases 18,668 35,822 Settlements (14,798 ) (14,098 ) Net transactions recorded during the period: Net gains recognized as regulatory assets and liabilities 26,176 5,749 Balance at June 30 $ 35,389 $ 28,665 Six Months Ended June 30 (Thousands of Dollars) 2018 2017 Balance at Jan. 1 $ 12,723 $ 1,955 Purchases 19,348 39,333 Settlements (25,237 ) (30,498 ) Net transactions recorded during the period: Net gains recognized as regulatory assets and liabilities 28,555 17,875 Balance at June 30 $ 35,389 $ 28,665 SPS recognizes transfers between levels as of the beginning of each period. There were no transfers of amounts between levels for derivative instruments for the three and six months ended June 30, 2018 and 2017 . Fair Value of Long-Term Debt As of June 30, 2018 and Dec. 31, 2017 , other financial instruments for which the carrying amount did not equal fair value were as follows: June 30, 2018 Dec. 31, 2017 (Thousands of Dollars) Carrying Amount Fair Value Carrying Amount Fair Value Long-term debt, including current portion $ 1,830,508 $ 1,858,497 $ 1,829,941 $ 2,001,992 The fair value of SPS’ long-term debt is estimated based on recent trades and observable spreads from benchmark interest rates for similar securities. The fair value estimates are based on information available to management as of June 30, 2018 and Dec. 31, 2017 , and given the observability of the inputs to these estimates, the fair values presented for long-term debt have been assigned a Level 2.

Other Income, Net

Other Income, Net	6 Months Ended
Other Income, Net	Jun. 30, 2018
Other Income and Expenses [Abstract]
Other Income, Net	Other Expense, Net Other expense, net consisted of the following: Three Months Ended June 30 Six Months Ended June 30 (Thousands of Dollars) 2018 2017 2018 2017 Interest income $ 356 $ 147 $ 298 $ 192 Other nonoperating income 1 1 3 — Other nonoperating expense — — — (1 ) Insurance policy expense (12 ) (12 ) (24 ) (24 ) Benefits non-service cost (1,127 ) (749 ) (1,763 ) (1,498 ) Other expense, net $ (782 ) $ (613 ) $ (1,486 ) $ (1,331 )

Benefit Plans and Other Postret

Benefit Plans and Other Postretirement Benefits	6 Months Ended
Benefit Plans and Other Postretirement Benefits	Jun. 30, 2018
Retirement Benefits [Abstract]
Benefit Plans and Other Postretirement Benefits	Benefit Plans and Other Postretirement Benefits Components of Net Periodic Benefit Cost (Credit) Three Months Ended June 30 2018 2017 2018 2017 (Thousands of Dollars) Pension Benefits Postretirement Health Care Benefits Service cost $ 2,430 $ 2,440 $ 280 $ 219 Interest cost (a) 4,602 4,927 411 415 Expected return on plan assets (a) (7,082 ) (6,971 ) (616 ) (589 ) Amortization of prior service credit (a) (34 ) — (100 ) (100 ) Amortization of net loss (gain) (a) 3,517 3,245 (113 ) (155 ) Net periodic benefit cost (credit) 3,433 3,641 (138 ) (210 ) Credits not recognized due to the effects of regulation 761 574 — — Net benefit cost (credit) recognized for financial reporting $ 4,194 $ 4,215 $ (138 ) $ (210 ) Six Months Ended June 30 2018 2017 2018 2017 (Thousands of Dollars) Pension Benefits Postretirement Health Service cost $ 4,860 $ 4,880 $ 559 $ 438 Interest cost (a) 9,205 9,855 821 830 Expected return on plan assets (a) (14,164 ) (13,942 ) (1,231 ) (1,178 ) Amortization of prior service credit (a) (69 ) — (201 ) (200 ) Amortization of net loss (gain) (a) 7,034 6,490 (226 ) (310 ) Net periodic benefit cost (credit) 6,866 7,283 (278 ) (420 ) Credits not recognized due to the effects of regulation 1,735 722 — — Net benefit cost (credit) recognized for financial reporting $ 8,601 $ 8,005 $ (278 ) $ (420 ) (a) The components of net periodic cost other than the service cost component are included in the line item “other expense, net” in the income statement or capitalized on the balance sheet as a regulatory asset. In January 2018, contributions of $150 million were made across four of Xcel Energy’s pension plans, of which $8.0 million was attributable to SPS. Xcel Energy does not expect additional pension contributions during 2018.

Other Comprehensive Income

Other Comprehensive Income	6 Months Ended
Other Comprehensive Income	Jun. 30, 2018
Stockholders' Equity Note [Abstract]
Other Comprehensive Income	Other Comprehensive Income (Loss) Changes in accumulated other comprehensive loss, net of tax, for the three and six months ended June 30, 2018 and 2017 were as follows: Three Months Ended June 30, 2018 (Thousands of Dollars) Gains and Losses on Cash Flow Hedges Defined Benefit and Postretirement Items Total Accumulated other comprehensive loss at April 1 $ (764 ) $ (672 ) $ (1,436 ) Losses reclassified from net accumulated other comprehensive loss 12 18 30 Net current period other comprehensive income 12 18 30 Accumulated other comprehensive loss at June 30 $ (752 ) $ (654 ) $ (1,406 ) Three Months Ended June 30, 2017 (Thousands of Dollars) Gains and Losses on Cash Flow Hedges Defined Benefit and Postretirement Items Total Accumulated other comprehensive loss at April 1 $ (669 ) $ (597 ) $ (1,266 ) Losses reclassified from net accumulated other comprehensive loss 10 15 25 Net current period other comprehensive income 10 15 25 Accumulated other comprehensive loss at June 30 $ (659 ) $ (582 ) $ (1,241 ) Six Months Ended June 30, 2018 (Thousands of Dollars) Gains and Losses on Cash Flow Hedges Defined Benefit and Postretirement Items Total Accumulated other comprehensive loss at Jan. 1 $ (776 ) $ (691 ) $ (1,467 ) Losses reclassified from net accumulated other comprehensive loss 24 37 61 Net current period other comprehensive income 24 37 61 Accumulated other comprehensive loss at June 30 $ (752 ) $ (654 ) $ (1,406 ) Six Months Ended June 30, 2017 (Thousands of Dollars) Gains and Losses on Cash Flow Hedges Defined Benefit and Postretirement Items Total Accumulated other comprehensive loss at Jan. 1 $ (678 ) $ (612 ) $ (1,290 ) Losses reclassified from net accumulated other comprehensive loss 19 30 49 Net current period other comprehensive income 19 30 49 Accumulated other comprehensive loss at June 30 $ (659 ) $ (582 ) $ (1,241 ) Reclassifications from accumulated other comprehensive loss for the three and six months ended June 30, 2018 and 2017 were as follows: Amounts Reclassified from Accumulated Other Comprehensive Loss (Thousands of Dollars) Three Months Ended June 30, 2018 Three Months Ended June 30, 2017 Losses on cash flow hedges: Interest rate derivatives $ 16 (a) $ 16 (a) Total, pre-tax 16 16 Tax benefit (4 ) (6 ) Total, net of tax 12 10 Defined benefit pension and postretirement losses: Amortization of net loss 23 (b) 24 (b) Total, pre-tax 23 24 Tax benefit (5 ) (9 ) Total, net of tax 18 15 Total amounts reclassified, net of tax $ 30 $ 25 Amounts Reclassified from Accumulated Other Comprehensive Loss (Thousands of Dollars) Six Months Ended June 30, 2018 Six Months Ended June 30, 2017 Losses on cash flow hedges: Interest rate derivatives $ 31 (a) $ 31 (a) Total, pre-tax 31 31 Tax benefit (7 ) (12 ) Total, net of tax 24 19 Defined benefit pension and postretirement losses: Amortization of net loss 47 (b) 48 (b) Total, pre-tax 47 48 Tax benefit (10 ) (18 ) Total, net of tax 37 30 Total amounts reclassified, net of tax $ 61 $ 49 (a) Included in interest charges. (b) Included in the computation of net periodic pension and postretirement benefit costs. See Note 10 to the financial statements for details regarding these benefit plans.

Revenues

Revenues	6 Months Ended
Revenues	Jun. 30, 2018
Revenue from Contract with Customer [Abstract]
Revenue from Contract with Customer [Text Block]	Revenues SPS principally generates revenue from the generation, transmission, distribution and sale of electricity to wholesale and retail customers. Performance obligations related to the sale of energy are satisfied as energy is delivered to customers. SPS 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 SPS does not recognize a separate financing component of its collections from customers. SPS presents its revenues net of any excise or other fiduciary-type taxes or fees. SPS participates in SPP. SPS recognizes sales to both native load and other end use customers on a gross basis in electric revenues and cost of sales. Revenues and charges for short term wholesale sales of excess energy transacted through RTOs are recorded on a gross basis. Other revenues and charges related to participating and transacting in RTOs are also recorded on a net basis in cost of sales. SPS has various rate-adjustment mechanisms in place that provide for the recovery of natural gas, electric fuel and purchased energy costs. These cost-adjustment tariffs may increase or decrease the level of revenue collected from customers and are revised periodically for differences between the total amount collected under the clauses and the costs incurred. When applicable, under governing regulatory commission rate orders, fuel cost over-recoveries (the excess of fuel revenue billed to customers over fuel costs incurred) are deferred as regulatory liabilities and under-recoveries (the excess of fuel costs incurred over fuel revenues billed to customers) are deferred as regulatory assets. Certain rate rider mechanisms qualify as alternative revenue programs under GAAP. These mechanisms arise from costs imposed upon the utility by action of a regulator or legislative body related to an environmental, public safety or other mandate. When certain criteria are met (including collection within 24 months), revenue is recognized equal to the revenue requirement, which may include return on rate base items and incentives. The mechanisms are revised periodically for differences between the total amount collected and the revenue recognized, which may increase or decrease the level of revenue collected from customers. Alternative revenue is recorded on a gross basis and is disclosed separate from revenue from contracts with customers in the period earned. In the following tables, regulated electric revenue is classified by the type of goods/services rendered and market/customer type. Three Months Ended (Thousands of Dollars) June 30, 2018 June 30, 2017 Major product lines Revenue from contracts with customers: Residential $ 85,107 $ 84,188 Commercial and industrial (C&I) 200,760 215,805 Other 11,363 12,242 Total retail 297,230 312,235 Wholesale 115,629 101,893 Transmission 58,970 56,394 Other 2,858 2,065 Total revenue from contracts with customers 474,687 472,587 Alternative revenue and other 6,651 7,209 Total revenues $ 481,338 $ 479,796 Six Months Ended (Thousands of Dollars) June 30, 2018 June 30, 2017 Major product lines Revenue from contracts with customers: Residential $ 165,156 $ 163,789 C&I 396,531 416,762 Other 21,027 21,854 Total retail 582,714 602,405 Wholesale 208,861 193,034 Transmission 114,616 110,572 Other 10,389 4,010 Total revenue from contracts with customers 916,580 910,021 Alternative revenue and other 11,990 29,847 Total revenues $ 928,570 $ 939,868

Selected Balance Sheet Data (Ta

Selected Balance Sheet Data (Tables)	6 Months Ended
Selected Balance Sheet Data (Tables)	Jun. 30, 2018
Balance Sheet Related Disclosures [Abstract]
Accounts Receivable, Net	(Thousands of Dollars) June 30, 2018 Dec. 31, 2017 Accounts receivable, net Accounts receivable $ 107,990 $ 85,929 Less allowance for bad debts (5,611 ) (6,348 ) $ 102,379 $ 79,581
Inventories	(Thousands of Dollars) June 30, 2018 Dec. 31, 2017 Inventories Materials and supplies $ 25,416 $ 26,218 Fuel 7,102 14,215 $ 32,518 $ 40,433
Property, Plant and Equipment, Net	(Thousands of Dollars) June 30, 2018 Dec. 31, 2017 Property, plant and equipment, net Electric plant $ 7,059,344 $ 6,765,371 Construction work in progress 447,945 351,875 Total property, plant and equipment 7,507,289 7,117,246 Less accumulated depreciation (2,073,102 ) (2,021,637 ) $ 5,434,187 $ 5,095,609

Income Taxes (Tables)

Income Taxes (Tables)	6 Months Ended
Income Taxes (Tables)	Jun. 30, 2018
Income Tax Disclosure [Abstract]
Schedule of Effective Income Tax Rate Reconciliation [Table Text Block]	Total income tax expense from operations differs from the amount computed by applying the statutory federal income tax rate to income before income tax expense. The following reconciles such differences: Six Months Ended June 30 2018 2017 Federal statutory rate 21.0 % 35.0 % State tax, net of federal tax effect 2.4 2.2 Increases (decreases) in tax from: Regulatory differences - ARAM (a) (4.2 ) — Regulatory differences - ARAM (b) 1.3 — Regulatory differences - other utility plant items (1.4 ) (0.8 ) Tax credits, net of federal income tax expense (0.7 ) (0.5 ) Other, net 0.1 0.4 Effective income tax rate 18.5 % 36.3 % (a) The average rate assumption method (ARAM); a method to flow back excess deferred taxes to customers. (b) As we receive direction from our regulatory commissions regarding the return of excess deferred taxes (to our customers resulting from the Tax Cuts and Jobs Act (TCJA)), the ARAM deferral may decrease during the year, which would result in a reduction to tax expense with a corresponding reduction to revenue.
Summary of Statute of Limitations Applicable to Open Tax Years [Table Text Block]	SPS is a member of the Xcel Energy affiliated group that files a consolidated federal income tax return. The statutes of limitations applicable to Xcel Energy’s federal income tax returns expire as follows: Tax Year(s) Expiration 2009 - 2011 December 2018 2012 - 2014 October 2019 2015 September 2019 2016 September 2020
Reconciliation of Unrecognized Tax Benefits	A reconciliation of the amount of unrecognized tax benefit is as follows: (Millions of Dollars) June 30, 2018 Dec. 31, 2017 Unrecognized tax benefit — Permanent tax positions $ 2.5 $ 2.3 Unrecognized tax benefit — Temporary tax positions 1.6 2.0 Total unrecognized tax benefit $ 4.1 $ 4.3
Tax Benefits Associated with NOL and Tax Credit Carryforwards	The unrecognized tax benefit amounts were reduced by the tax benefits associated with NOL and tax credit carryforwards. The amounts of tax benefits associated with NOL and tax credit carryforwards are as follows: (Millions of Dollars) June 30, 2018 Dec. 31, 2017 NOL and tax credit carryforwards $ (6.6 ) $ (5.9 )

Rate Matters Rate Matters (Tabl

Rate Matters Rate Matters (Tables)	6 Months Ended
Rate Matters Rate Matters (Tables)	Jun. 30, 2018
Public Utilities, General Disclosure [Abstract]
Texas 2017 Electric Rate Case - Settlement Reconciliation [Table Text Block]	A reconciliation of the settlement is as follows: (Millions of Dollars) Original base rate request $ 69 Base rate revenue to be recovered through TCRF (15 ) Net revenue request 54 Adjustment for TCJA and other items (37 ) Requested incremental revenue 17 Unspecified settlement adjustments (13 ) Accelerated depreciation (Tolk plant) (4 ) SPS' net revenue change $ —
New Mexico 2017 Electric Rate Case [Table Text Block]	The following table summarizes certain parties’ proposed modifications to SPS’ request, SPS’ revised request, and the Hearing Examiner’s recommendation: (Millions of Dollars) NMPRC Staff Testimony NMAG Testimony SPS Rebuttal Testimony Hearing Examiner's Recommendation SPS request $ 43 $ 43 $ 43 $ 43 Reduction to request for the impact of the TCJA (11 ) (11 ) (11 ) (11 ) SPS request, including the impact of the TCJA 32 32 32 32 ROE (4 ) (6 ) — (5 ) Capital structure (7 ) (3 ) — (3 ) Depreciation lives (Tolk and Cunningham plants) (3 ) (3 ) — (3 ) Disallow rate case expenses (2 ) (3 ) (1 ) — Regional transmission revenue and expense (adjustment for the impact of the TCJA): Impact of the TCJA — (3 ) — (1 ) Aligning costs with transmission plant in rate base — — — (1 ) Post test year plant (updated to actual) (1 ) (2 ) (3 ) — Excess generation adjustment — (1 ) — (1 ) Other, net (4 ) (4 ) (1 ) (6 ) Recommended rate increase $ 11 $ 7 $ 27 $ 12 ROE 9.0 % 9.21 % 10.25 % 9.4 % Equity ratio 52.0 % 53.97 % 58.0 % 53.97 %

Borrowings and Other Financin25

Borrowings and Other Financing Instruments (Tables)	6 Months Ended
Borrowings and Other Financing Instruments (Tables)	Jun. 30, 2018
Borrowings and Other Financing Instruments [Abstract]
Credit Facilities	As of June 30, 2018 , SPS had the following committed credit facility available (in millions of dollars): Credit Facility (a) Drawn (b) Available $ 400 $ 134 $ 266 (a) This credit facility expires in June 2021 . (b) Includes outstanding commercial paper and letters of credit.
Money Pool
Borrowings and Other Financing Instruments [Abstract]
Short-Term Borrowings	Money pool borrowings for SPS were as follows: (Amounts in Millions, Except Interest Rates) Three Months Ended June 30, 2018 Year Ended Dec. 31, 2017 Borrowing limit $ 100 $ 100 Amount outstanding at period end 100 — Average amount outstanding 24 13 Maximum amount outstanding 100 100 Weighted average interest rate, computed on a daily basis 1.84 % 1.12 % Weighted average interest rate at period end 1.85 N/A
Commercial Paper
Borrowings and Other Financing Instruments [Abstract]
Short-Term Borrowings	Commercial paper outstanding for SPS was as follows: (Amounts in Millions, Except Interest Rates) Three Months Ended June 30, 2018 Year Ended Dec. 31, 2017 Borrowing limit $ 400 $ 400 Amount outstanding at period end 132 — Average amount outstanding 32 69 Maximum amount outstanding 140 176 Weighted average interest rate, computed on a daily basis 2.25 % 1.13 % Weighted average interest rate at period end 2.29 N/A

Fair Value of Financial Asset26

Fair Value of Financial Assets and Liabilities (Tables)	6 Months Ended
Fair Value of Financial Assets and Liabilities (Tables)	Jun. 30, 2018
Fair Value Disclosures [Abstract]
Gross Notional Amounts of Commodity FTRs	The following table details the gross notional amounts of commodity FTRs as of June 30, 2018 and Dec. 31, 2017 : (Amounts in Thousands) (a) June 30, 2018 Dec. 31, 2017 Megawatt hours of electricity 12,941 4,251 (a) Amounts are not reflective of net positions in the underlying commodities.
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, SPS’ derivative assets and liabilities measured at fair value on a recurring basis as of June 30, 2018 : June 30, 2018 Fair Value Fair Value Total Counterparty Netting (b) (Thousands of Dollars) Level 1 Level 2 Level 3 Total Current derivative assets Other derivative instruments: Electric commodity $ — $ — $ 35,897 $ 35,897 $ (508 ) $ 35,389 Total current derivative assets $ — $ — $ 35,897 $ 35,897 $ (508 ) 35,389 PPAs (a) 3,160 Current derivative instruments $ 38,549 Noncurrent derivative assets PPAs (a) $ 17,374 Noncurrent derivative instruments $ 17,374 Current derivative liabilities Other derivative instruments: Electric commodity $ — $ — $ 508 $ 508 $ (508 ) $ — Total current derivative liabilities $ — $ — $ 508 $ 508 $ (508 ) — PPAs (a) 3,565 Current derivative instruments $ 3,565 Noncurrent derivative liabilities PPAs (a) $ 18,166 Noncurrent derivative instruments $ 18,166 (a) During 2006, SPS qualified these contracts under the normal purchase exception. Based on this qualification, the contracts are no longer adjusted to fair value and the previous carrying value of these contracts is being amortized over the remaining contract lives along with the offsetting regulatory assets and liabilities. (b) SPS nets derivative instruments and related collateral in its balance sheet when supported by a legally enforceable master netting agreement, and all derivative instruments and related collateral amounts were subject to master netting agreements at June 30, 2018 . At June 30, 2018 , derivative assets and liabilities include no obligations to return cash collateral or rights to reclaim cash collateral. The counterparty netting amounts presented exclude settlement receivables and payables and non-derivative amounts that may be subject to the same master netting agreements. The following table presents for each of the fair value hierarchy levels, SPS’ derivative assets and liabilities measured at fair value on a recurring basis as of Dec. 31, 2017: Dec. 31, 2017 Fair Value Fair Value Total Counterparty Netting (b) (Thousands of Dollars) Level 1 Level 2 Level 3 Total Current derivative assets Other derivative instruments: Electric commodity $ — $ — $ 14,717 $ 14,717 $ (1,994 ) $ 12,723 Total current derivative assets $ — $ — $ 14,717 $ 14,717 $ (1,994 ) 12,723 PPAs (a) 3,159 Current derivative instruments $ 15,882 Noncurrent derivative assets PPAs (a) $ 18,954 Noncurrent derivative instruments $ 18,954 Current derivative liabilities Other derivative instruments: Electric commodity $ — $ — $ 1,994 $ 1,994 $ (1,994 ) $ — Total current derivative liabilities $ — $ — $ 1,994 $ 1,994 $ (1,994 ) — PPAs (a) 3,565 Current derivative instruments $ 3,565 Noncurrent derivative liabilities PPAs (a) $ 19,949 Noncurrent derivative instruments $ 19,949 (a) During 2006, SPS qualified these contracts under the normal purchase exception. Based on this qualification, the contracts are no longer adjusted to fair value and the previous carrying value of these contracts is being amortized over the remaining contract lives along with the offsetting regulatory assets and liabilities. (b) SPS nets derivative instruments and related collateral in its balance sheet when supported by a legally enforceable master netting agreement, and all derivative instruments and related collateral amounts were subject to master netting agreements at Dec. 31, 2017. At Dec. 31, 2017, derivative assets and liabilities include no obligations to return cash collateral or rights to reclaim cash collateral. The counterparty netting amounts presented exclude settlement receivables and payables and non-derivative amounts that may be subject to the same master netting agreements.
Changes in Level 3 Commodity Derivatives	The following table presents the changes in Level 3 commodity derivatives for the three and six months ended June 30, 2018 and 2017 : Three Months Ended June 30, (Thousands of Dollars) 2018 2017 Balance at April 1 $ 5,343 $ 1,192 Purchases 18,668 35,822 Settlements (14,798 ) (14,098 ) Net transactions recorded during the period: Net gains recognized as regulatory assets and liabilities 26,176 5,749 Balance at June 30 $ 35,389 $ 28,665 Six Months Ended June 30 (Thousands of Dollars) 2018 2017 Balance at Jan. 1 $ 12,723 $ 1,955 Purchases 19,348 39,333 Settlements (25,237 ) (30,498 ) Net transactions recorded during the period: Net gains recognized as regulatory assets and liabilities 28,555 17,875 Balance at June 30 $ 35,389 $ 28,665
Carrying Amount and Fair Value of Long-term Debt	As of June 30, 2018 and Dec. 31, 2017 , other financial instruments for which the carrying amount did not equal fair value were as follows: June 30, 2018 Dec. 31, 2017 (Thousands of Dollars) Carrying Amount Fair Value Carrying Amount Fair Value Long-term debt, including current portion $ 1,830,508 $ 1,858,497 $ 1,829,941 $ 2,001,992

Other Income, Net (Tables)

Other Income, Net (Tables)	6 Months Ended
Other Income, Net (Tables)	Jun. 30, 2018
Other Income and Expenses [Abstract]
Other Income, Net	Other expense, net consisted of the following: Three Months Ended June 30 Six Months Ended June 30 (Thousands of Dollars) 2018 2017 2018 2017 Interest income $ 356 $ 147 $ 298 $ 192 Other nonoperating income 1 1 3 — Other nonoperating expense — — — (1 ) Insurance policy expense (12 ) (12 ) (24 ) (24 ) Benefits non-service cost (1,127 ) (749 ) (1,763 ) (1,498 ) Other expense, net $ (782 ) $ (613 ) $ (1,486 ) $ (1,331 )

Benefit Plans and Other Postr28

Benefit Plans and Other Postretirement Benefits (Tables)	6 Months Ended
Benefit Plans and Other Postretirement Benefits (Tables)	Jun. 30, 2018
Retirement Benefits [Abstract]
Components of Net Periodic Benefit Cost (Credit)	Components of Net Periodic Benefit Cost (Credit) Three Months Ended June 30 2018 2017 2018 2017 (Thousands of Dollars) Pension Benefits Postretirement Health Care Benefits Service cost $ 2,430 $ 2,440 $ 280 $ 219 Interest cost (a) 4,602 4,927 411 415 Expected return on plan assets (a) (7,082 ) (6,971 ) (616 ) (589 ) Amortization of prior service credit (a) (34 ) — (100 ) (100 ) Amortization of net loss (gain) (a) 3,517 3,245 (113 ) (155 ) Net periodic benefit cost (credit) 3,433 3,641 (138 ) (210 ) Credits not recognized due to the effects of regulation 761 574 — — Net benefit cost (credit) recognized for financial reporting $ 4,194 $ 4,215 $ (138 ) $ (210 ) Six Months Ended June 30 2018 2017 2018 2017 (Thousands of Dollars) Pension Benefits Postretirement Health Service cost $ 4,860 $ 4,880 $ 559 $ 438 Interest cost (a) 9,205 9,855 821 830 Expected return on plan assets (a) (14,164 ) (13,942 ) (1,231 ) (1,178 ) Amortization of prior service credit (a) (69 ) — (201 ) (200 ) Amortization of net loss (gain) (a) 7,034 6,490 (226 ) (310 ) Net periodic benefit cost (credit) 6,866 7,283 (278 ) (420 ) Credits not recognized due to the effects of regulation 1,735 722 — — Net benefit cost (credit) recognized for financial reporting $ 8,601 $ 8,005 $ (278 ) $ (420 ) (a) The components of net periodic cost other than the service cost component are included in the line item “other expense, net” in the income statement or capitalized on the balance sheet as a regulatory asset.

Other Comprehensive Income (Tab

Other Comprehensive Income (Tables)	6 Months Ended
Other Comprehensive Income (Tables)	Jun. 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 six months ended June 30, 2018 and 2017 were as follows: Three Months Ended June 30, 2018 (Thousands of Dollars) Gains and Losses on Cash Flow Hedges Defined Benefit and Postretirement Items Total Accumulated other comprehensive loss at April 1 $ (764 ) $ (672 ) $ (1,436 ) Losses reclassified from net accumulated other comprehensive loss 12 18 30 Net current period other comprehensive income 12 18 30 Accumulated other comprehensive loss at June 30 $ (752 ) $ (654 ) $ (1,406 ) Three Months Ended June 30, 2017 (Thousands of Dollars) Gains and Losses on Cash Flow Hedges Defined Benefit and Postretirement Items Total Accumulated other comprehensive loss at April 1 $ (669 ) $ (597 ) $ (1,266 ) Losses reclassified from net accumulated other comprehensive loss 10 15 25 Net current period other comprehensive income 10 15 25 Accumulated other comprehensive loss at June 30 $ (659 ) $ (582 ) $ (1,241 ) Six Months Ended June 30, 2018 (Thousands of Dollars) Gains and Losses on Cash Flow Hedges Defined Benefit and Postretirement Items Total Accumulated other comprehensive loss at Jan. 1 $ (776 ) $ (691 ) $ (1,467 ) Losses reclassified from net accumulated other comprehensive loss 24 37 61 Net current period other comprehensive income 24 37 61 Accumulated other comprehensive loss at June 30 $ (752 ) $ (654 ) $ (1,406 ) Six Months Ended June 30, 2017 (Thousands of Dollars) Gains and Losses on Cash Flow Hedges Defined Benefit and Postretirement Items Total Accumulated other comprehensive loss at Jan. 1 $ (678 ) $ (612 ) $ (1,290 ) Losses reclassified from net accumulated other comprehensive loss 19 30 49 Net current period other comprehensive income 19 30 49 Accumulated other comprehensive loss at June 30 $ (659 ) $ (582 ) $ (1,241 )
Reclassifications out of Accumulated Other Comprehensive Loss	Reclassifications from accumulated other comprehensive loss for the three and six months ended June 30, 2018 and 2017 were as follows: Amounts Reclassified from Accumulated Other Comprehensive Loss (Thousands of Dollars) Three Months Ended June 30, 2018 Three Months Ended June 30, 2017 Losses on cash flow hedges: Interest rate derivatives $ 16 (a) $ 16 (a) Total, pre-tax 16 16 Tax benefit (4 ) (6 ) Total, net of tax 12 10 Defined benefit pension and postretirement losses: Amortization of net loss 23 (b) 24 (b) Total, pre-tax 23 24 Tax benefit (5 ) (9 ) Total, net of tax 18 15 Total amounts reclassified, net of tax $ 30 $ 25 Amounts Reclassified from Accumulated Other Comprehensive Loss (Thousands of Dollars) Six Months Ended June 30, 2018 Six Months Ended June 30, 2017 Losses on cash flow hedges: Interest rate derivatives $ 31 (a) $ 31 (a) Total, pre-tax 31 31 Tax benefit (7 ) (12 ) Total, net of tax 24 19 Defined benefit pension and postretirement losses: Amortization of net loss 47 (b) 48 (b) Total, pre-tax 47 48 Tax benefit (10 ) (18 ) Total, net of tax 37 30 Total amounts reclassified, net of tax $ 61 $ 49 (a) Included in interest charges. (b) Included in the computation of net periodic pension and postretirement benefit costs. See Note 10 to the financial statements for details regarding these benefit plans.

Revenues (Tables)

Revenues (Tables)	6 Months Ended
Revenues (Tables)	Jun. 30, 2018
Revenue from Contract with Customer [Abstract]
Disaggregation of Revenue [Table Text Block]	In the following tables, regulated electric revenue is classified by the type of goods/services rendered and market/customer type. Three Months Ended (Thousands of Dollars) June 30, 2018 June 30, 2017 Major product lines Revenue from contracts with customers: Residential $ 85,107 $ 84,188 Commercial and industrial (C&I) 200,760 215,805 Other 11,363 12,242 Total retail 297,230 312,235 Wholesale 115,629 101,893 Transmission 58,970 56,394 Other 2,858 2,065 Total revenue from contracts with customers 474,687 472,587 Alternative revenue and other 6,651 7,209 Total revenues $ 481,338 $ 479,796 Six Months Ended (Thousands of Dollars) June 30, 2018 June 30, 2017 Major product lines Revenue from contracts with customers: Residential $ 165,156 $ 163,789 C&I 396,531 416,762 Other 21,027 21,854 Total retail 582,714 602,405 Wholesale 208,861 193,034 Transmission 114,616 110,572 Other 10,389 4,010 Total revenue from contracts with customers 916,580 910,021 Alternative revenue and other 11,990 29,847 Total revenues $ 928,570 $ 939,868

Accounting Pronouncements (Deta

Accounting Pronouncements (Details) - Accounting Standards Update 2017-07 $ in Millions	6 Months Ended
	Jun. 30, 2017USD ($)
Operating and Maintenance Expense
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Net benefit cost (credit) recognized for financial reporting	$ (1.5)
Other Income
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Net benefit cost (credit) recognized for financial reporting	$ 1.5

Selected Balance Sheet Data (De

Selected Balance Sheet Data (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Accounts receivable, net
Accounts receivable	$ 107,990	$ 85,929
Less allowance for bad debts	(5,611)	(6,348)
Accounts receivable, net	$ 102,379	$ 79,581

Selected Balance Sheet Data Bal

Selected Balance Sheet Data Balance Sheet Related Disclosures, Inventories (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Public Utilities, Inventory [Line Items]
Inventories	$ 32,518	$ 40,433
Materials and supplies
Public Utilities, Inventory [Line Items]
Inventories	25,416	26,218
Fuel
Public Utilities, Inventory [Line Items]
Inventories	$ 7,102	$ 14,215

Selected Balance Sheet Data B34

Selected Balance Sheet Data Balance Sheet Related Disclosures, Property, Plant and Equipment (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Public Utility, Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	$ 7,507,289	$ 7,117,246
Less accumulated depreciation	(2,073,102)	(2,021,637)
Property, plant and equipment, net	5,434,187	5,095,609
Electric plant
Public Utility, Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	7,059,344	6,765,371
Construction work in progress
Public Utility, Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	$ 447,945	$ 351,875

Income Taxes (Details)

Income Taxes (Details) - USD ($)	6 Months Ended		9 Months Ended	12 Months Ended
Income Taxes (Details) - USD ($)	Jun. 30, 2018	Jun. 30, 2017	Sep. 30, 2015	Dec. 31, 2012	Dec. 31, 2017
Tax Audits [Abstract]
Effective Income Tax Rate Reconciliation, at Federal Statutory Income Tax Rate, Percent	21.00%	35.00%
Effective Income Tax Rate Reconciliation, State and Local Income Taxes, Percent	2.40%	2.20%
Effective Income Tax Rate Reconciliation, Change in Enacted Tax Rate, Percent	(4.20%)	0.00%
Effective Income Tax Rate Reconciliation, Tax Cuts and Jobs Act of 2017, Change in Tax Rate, Percent	1.30%	0.00%
Effective Income Tax Rate Reconciliation, Other Regulatory Items, Percent	(1.40%)	(0.80%)
Effective Income Tax Rate Reconciliation Regulatory Differences Utility Plant Items	(0.70%)	(0.50%)
Effective Income Tax Rate Reconciliation, Other Adjustments, Percent	0.10%	0.40%
Effective Income Tax Rate Reconciliation, Percent	18.50%	36.30%
Unrecognized Tax Benefits [Abstract]
Unrecognized tax benefit - Permanent tax positions	$ 2,500,000				$ 2,300,000
Unrecognized tax benefit - Temporary tax positions	1,600,000				2,000,000
Total unrecognized tax benefit	4,100,000				4,300,000
NOL and tax credit carryforwards	(6,600,000)				(5,900,000)
Upper bound of decrease in unrecognized tax benefit that is reasonably possible	3,000,000
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 (Texas)
Tax Audits [Abstract]
Earliest year subject to examination	2,009

Rate Matters (Details)

Rate Matters (Details) - USD ($) $ in Millions	1 Months Ended					6 Months Ended	12 Months Ended
Rate Matters (Details) - USD ($) $ in Millions	Jun. 30, 2018	May 31, 2018	Feb. 28, 2018	Oct. 31, 2017	Nov. 30, 2016	Jun. 30, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Hearing Examiner [Member] \| New Mexico 2017 Electric Rate Case [Member]
Public Utilities, General Disclosures [Line Items]
Public Utilities, Recommended ROE, Percentage	9.40%
Public Utilities, Recommended Equity Capital Structure, Percentage	53.97%
Public Utilities, Increase (Decrease) of Recommended ROE	$ (5)
Public Utilities, Increase (Decrease) of Recommended Capital Structure and Cost of Debt	(3)
Public Utilities, Increase (Decrease) from Recommended Accelerated Depreciation for Tolk Plant	(3)
Public Utilities, Increase (Decrease) Rate Case Expense Disallowance	0
Public Utilities, Increase (Decrease) from Regional Transmission Revenue TCJA Adjustment	(1)
Public Utilities, Increase (Decrease) from Post Test Year Plant Adjustment	0
Public Utilities, Increase (Decrease) Other, net	(6)
Public Utilities, Total Recommended Rate Increase (Decrease)	12
Public Utilities, Increase (Decrease) from Regional Transmission Revenue TCJA Adjustment Aligning Costs With Plant In Rate Base	(1)
Public Utilities, Requested Increase (Decrease) from Excess Generation Adjustment	$ (1)
NMPRC Staff [Member] \| New Mexico 2017 Electric Rate Case [Member]
Public Utilities, General Disclosures [Line Items]
Public Utilities, Recommended ROE, Percentage						9.00%
Public Utilities, Recommended Equity Capital Structure, Percentage						52.00%
Public Utilities, Increase (Decrease) of Recommended ROE						$ (4)
Public Utilities, Increase (Decrease) of Recommended Capital Structure and Cost of Debt						(7)
Public Utilities, Increase (Decrease) from Recommended Accelerated Depreciation for Tolk Plant						(3)
Public Utilities, Increase (Decrease) Rate Case Expense Disallowance						(2)
Public Utilities, Increase (Decrease) from Regional Transmission Revenue TCJA Adjustment						0
Public Utilities, Increase (Decrease) from Post Test Year Plant Adjustment						(1)
Public Utilities, Increase (Decrease) Other, net						(4)
Public Utilities, Total Recommended Rate Increase (Decrease)						11
Public Utilities, Increase (Decrease) from Regional Transmission Revenue TCJA Adjustment Aligning Costs With Plant In Rate Base						0
Public Utilities, Requested Increase (Decrease) from Excess Generation Adjustment						$ 0
New Mexico Attorney General (NMAG) [Member] \| New Mexico 2017 Electric Rate Case [Member]
Public Utilities, General Disclosures [Line Items]
Public Utilities, Recommended ROE, Percentage						9.21%
Public Utilities, Recommended Equity Capital Structure, Percentage						53.97%
Public Utilities, Increase (Decrease) of Recommended ROE						$ (6)
Public Utilities, Increase (Decrease) of Recommended Capital Structure and Cost of Debt						(3)
Public Utilities, Increase (Decrease) from Recommended Accelerated Depreciation for Tolk Plant						(3)
Public Utilities, Increase (Decrease) Rate Case Expense Disallowance						(3)
Public Utilities, Increase (Decrease) from Regional Transmission Revenue TCJA Adjustment						(3)
Public Utilities, Increase (Decrease) from Post Test Year Plant Adjustment						(2)
Public Utilities, Increase (Decrease) Other, net						(4)
Public Utilities, Total Recommended Rate Increase (Decrease)						7
Public Utilities, Increase (Decrease) from Regional Transmission Revenue TCJA Adjustment Aligning Costs With Plant In Rate Base						0
Public Utilities, Requested Increase (Decrease) from Excess Generation Adjustment						(1)
SPS \| PUCT Proceeding - Texas 2017 Electric Rate Case
Public Utilities, General Disclosures [Line Items]
Public Utilities, Incremental revenue request							$ 69
Public Utilities, Intended Equity Capital Structure, Percentage	57.00%
Public Utilities, Requested Rate Increase (Decrease), Amount							$ 54
Public Utilities, Requested Rate Increase (Decrease), Percentage							5.80%
Public Utilities, Requested Return on Equity, Percentage							10.25%
Public Utilities, Requested Rate Base, Amount							$ 1,900
Public Utilities, Requested Equity Capital Structure, Percentage							53.97%
Public Utilities, Revised Requested Rate Increase		$ 32
Public Utilities, Revised Requested Rate Increase, Percentage		5.90%
Public Utilities, Revised requested rate increase after equity ratio adjustment		$ 32
Public Utilities, Revised Requested Capital Structure, Percentage		58.00%
Public Utilities, Revised requested rate increase after TCRF rider adjustment		$ 17
SPS \| PUCT Proceeding - Appeal of the Texas 2015 Electric Rate Case Decision
Public Utilities, General Disclosures [Line Items]
Public Utilities, Requested Rate Increase (Decrease), Amount										$ 42
SPS \| New Mexico 2017 Electric Rate Case [Member]
Public Utilities, General Disclosures [Line Items]
Tax Cuts and Jobs Act of 2017, Estimated Electric Rate Reduction		$ 11	$ 11	$ 11
Public Utilities, Revised Requested Equity Capital Structure, Percentage			58.00%
Public Utilities, Requested Rate Increase (Decrease), Amount				$ 43
Public Utilities, Requested Return on Equity, Percentage		10.25%		10.25%
Public Utilities, Requested Rate Base, Amount				$ 885
Public Utilities, Requested Equity Capital Structure, Percentage		58.00%		53.97%
Public Utilities, Requested Rate Increase (Decrease) Including TCJA Impact, Amount		$ 27		$ 32
Public Utilities, Increase (Decrease) of Recommended ROE		0
Public Utilities, Increase (Decrease) of Recommended Capital Structure and Cost of Debt		0
Public Utilities, Increase (Decrease) from Recommended Accelerated Depreciation for Tolk Plant		0
Public Utilities, Increase (Decrease) Rate Case Expense Disallowance		(1)
Public Utilities, Increase (Decrease) from Regional Transmission Revenue TCJA Adjustment		0
Public Utilities, Increase (Decrease) from Post Test Year Plant Adjustment		(3)
Public Utilities, Increase (Decrease) Other, net		(1)
Public Utilities, Total Recommended Rate Increase (Decrease)		27
Public Utilities, Increase (Decrease) from Regional Transmission Revenue TCJA Adjustment Aligning Costs With Plant In Rate Base		0
Public Utilities, Requested Increase (Decrease) from Excess Generation Adjustment		$ 0
SPS \| NMPRC Proceeding - New Mexico 2016 Electric Rate Case [Member]
Public Utilities, General Disclosures [Line Items]
Public Utilities, Requested Rate Increase (Decrease), Amount					$ 41
Public Utilities, Requested Rate Increase (Decrease), Percentage					10.90%
Public Utilities, Requested Return on Equity, Percentage					10.10%
Public Utilities, Requested Rate Base, Amount					$ 832
Public Utilities, Requested Equity Capital Structure, Percentage					53.97%
Public Utiilities, Federal Income Tax Rate Used in Rate Request				35.00%
SPS \| Settlement Group [Member] \| PUCT Proceeding - Texas 2017 Electric Rate Case
Public Utilities, General Disclosures [Line Items]
Public Utilities, Incremental revenue request	$ 17
Public Utilities, Requested Return on Equity, Percentage to Calculate AFUDC	9.50%
Public Utilities, Increase (Decrease) from Recommended Accelerated Depreciation for Tolk Plant	$ (4)
Public Utilities, Increase (Decrease) Related to Tax Cuts and Jobs Act and Other Items	(37)
Public Utilities, Increase (Decrease) Related to Unspecified Settlement Adjustments	(13)
Public Utilities, Rate Increase Under the Settlement	$ 0
Public Utilities, Decrease in Requested Accelerated Depreciation for Tolk Plant, Percent	50.00%
SPS \| Public Utility Commission of Texas (PUCT) \| PUCT Proceeding - Appeal of the Texas 2015 Electric Rate Case Decision
Public Utilities, General Disclosures [Line Items]
Public Utilities, Approved Rate Decrease, Net of Rate Case Expenses									$ 4
SPS \| Southwest Power Pool (SPP) \| SPP Open Access Transmission Tariff Upgrade Costs
Public Utilities, General Disclosures [Line Items]
Public Utilities, Billed Charges For Transmission Service Upgrades								$ 13
Public Utilities, Monthly Billed Charges For Transmission Service Upgrades						$ 0.5
SPS \| Transmission Cost Recovery Factory (TCRF) Rider \| PUCT Proceeding - Texas 2017 Electric Rate Case
Public Utilities, General Disclosures [Line Items]
Public Utilities, Rider Conversion to Base Rates							$ (15)

Commitments and Contingencies,

Commitments and Contingencies, Purchased Power Agreements (Details) - Independent Power Producing Entities - MW	6 Months Ended
	Jun. 30, 2018	Dec. 31, 2017
Purchased Power Agreements [Abstract]
Generating capacity (in MW)	897	897
Purchase Power Agreement Expiration (year)	2,041

Commitments and Contingencies C

Commitments and Contingencies Commitments and Contingencies, Environmental Contingencies - Site Contingencies (Details) - Other MGP, Landfill, or Disposal Sites [Domain] $ in Millions	Jun. 30, 2018USD ($)Site	Dec. 31, 2017USD ($)
Site Contingency [Line Items]
Number of identified MGP, landfill, or disposal sites under current investigation and/or remediation \| Site	1
Accrual for Environmental Loss Contingencies \| $	$ 0.1	$ 0.1

Borrowings and Other Financin39

Borrowings and Other Financing Instruments, Short-Term Borrowings (Details) - USD ($)	3 Months Ended	12 Months Ended
	Jun. 30, 2018	Dec. 31, 2017
Short-term Debt [Line Items]
Amount outstanding at period end	$ 132,000,000	$ 0
Money Pool
Short-term Debt [Line Items]
Borrowing limit	100,000,000	100,000,000
Amount outstanding at period end	100,000,000	0
Average amount outstanding	24,000,000	13,000,000
Maximum amount outstanding	$ 100,000,000	$ 100,000,000
Weighted average interest rate, computed on a daily basis (percentage)	1.84%	1.12%
Weighted average interest rate at period end (percentage)	1.85%
Commercial Paper
Short-term Debt [Line Items]
Borrowing limit	$ 400,000,000	$ 400,000,000
Amount outstanding at period end	132,000,000	0
Average amount outstanding	32,000,000	69,000,000
Maximum amount outstanding	$ 140,000,000	$ 176,000,000
Weighted average interest rate, computed on a daily basis (percentage)	2.25%	1.13%
Weighted average interest rate at period end (percentage)	2.29%

Borrowings and Other Financin40

Borrowings and Other Financing Instruments, Letters of Credit (Details) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2018	Dec. 31, 2017
Line of Credit Facility [Line Items]
Amount outstanding at period end	$ 132,000	$ 0
Letter of Credit
Line of Credit Facility [Line Items]
Amount outstanding at period end	$ 2,000	$ 3,000
Letter of Credit \| Letter of Credit
Line of Credit Facility [Line Items]
Term of letters of credit (in years)	1 year

Borrowings and Other Financin41

Borrowings and Other Financing Instruments, Credit Facility (Details) - Credit Facility - USD ($)		6 Months Ended
		Jun. 30, 2018	Dec. 31, 2017
Line of Credit Facility [Line Items]
Credit Facility	[1]	$ 400,000,000
Drawn	[2]	134,000,000
Available		$ 266,000,000
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 and letters of credit.

Fair Value of Financial Asset42

Fair Value of Financial Assets and Liabilities, Derivative Instruments (Details) MWh in Thousands, $ in Millions		Jun. 30, 2018USD ($)MWh	Dec. 31, 2017MWh
Credit Concentration Risk \| Municipal or Cooperative Entities or Other Utilities [Member]
Consideration of Credit Risk and Concentrations [Abstract]
Number of most significant counterparties for wholesale, trading and non-trading commodity activities with credit exposure		7
Credit Concentration Risk \| No Investment Grade Ratings from External Credit Rating Agencies [Member]
Consideration of Credit Risk and Concentrations [Abstract]
Number of most significant counterparties for wholesale, trading and non-trading commodity activities with credit exposure		6
Wholesale, trading and non-trading commodity credit exposure for the most significant counterparties		$ 25.7
Percentage of wholesale, trading and non-trading commodity credit exposure for the most significant counterparties (in hundredths)		50.00%
Credit Concentration Risk \| External Credit Rating, Non Investment Grade [Member]
Consideration of Credit Risk and Concentrations [Abstract]
Number of most significant counterparties for wholesale, trading and non-trading commodity activities with credit exposure		1
Wholesale, trading and non-trading commodity credit exposure for the most significant counterparties		$ 0.2
Percentage of wholesale, trading and non-trading commodity credit exposure for the most significant counterparties (in hundredths)		1.00%
Electric Commodity (in megawatt hours)
Gross Notional Amounts of Commodity FTRs [Abstract]
Derivative, Nonmonetary Notional amount \| MWh	[1]	12,941	4,251

[1]	(a) Amounts are not reflective of net positions in the underlying commodities.

Fair Value of Financial Asset43

Fair Value of Financial Assets and Liabilities, Impact of Derivative Activity (Details) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Financial Impact of Qualifying Fair Value Hedges on Earnings [Abstract]
Derivative instruments designated as fair value hedges	$ 0	$ 0	$ 0	$ 0
Recognized gains (losses) from fair value hedges or related hedged transactions	0	0	0	0
Other Derivative Instruments \| Electric Commodity
Impact of Derivative Activity on Accumulated Other Comprehensive Loss, Regulatory Assets and Liabilities, and Income [Abstract]
Pre-tax fair value gains (losses) recognized during the period in regulatory (assets) and liabilities	13,000,000	200,000	13,400,000	2,300,000
Pre-tax (gains) losses reclassified into income during the period from regulatory assets and (liabilities)	$ (3,900,000)	$ (1,200,000)	$ (3,400,000)	$ (2,400,000)

Fair Value of Financial Asset44

Fair Value of Financial Assets and Liabilities, Fair Value Measurements (Details) - USD ($)	Jun. 30, 2018		Dec. 31, 2017
Derivatives, Fair Value [Line Items]
Derivative Asset, Collateral, Obligation to Return Cash, Offset	$ 0		$ 0
Derivative Liability, Collateral, Right to Reclaim Cash, Offset	0		0
Other Current Assets
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	38,549,000		15,882,000
Other Noncurrent Assets
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	17,374,000		18,954,000
Other Current Liabilities
Derivatives, Fair Value [Line Items]
Derivative Liability, Fair Value, Gross Liability	3,565,000		3,565,000
Other Noncurrent Liabilities
Derivatives, Fair Value [Line Items]
Derivative Liability, Fair Value, Gross Liability	18,166,000		19,949,000
Fair Value Measured on a Recurring Basis \| Other Current Assets
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	35,389,000		12,723,000
Derivative Asset, Fair Value, Gross Liability and Obligation to Return Cash, Offset	(508,000)	[1]	(1,994,000)	[2]
Fair Value Measured on a Recurring Basis \| Other Current Assets \| Other Derivative Instruments \| Electric Commodity
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	35,389,000		12,723,000
Derivative Asset, Fair Value, Gross Liability and Obligation to Return Cash, Offset	(508,000)	[1]	(1,994,000)	[2]
Fair Value Measured on a Recurring Basis \| Other Current Liabilities
Derivatives, Fair Value [Line Items]
Derivative Liability, Fair Value, Gross Liability	0		0
Derivative Liability, Fair Value, Gross Asset and Right to Reclaim Cash, Offset	(508,000)	[1]	(1,994,000)	[2]
Fair Value Measured on a Recurring Basis \| Other Current Liabilities \| Other Derivative Instruments \| Electric Commodity
Derivatives, Fair Value [Line Items]
Derivative Liability, Fair Value, Gross Liability	0		0
Derivative Liability, Fair Value, Gross Asset and Right to Reclaim Cash, Offset	(508,000)	[1]	(1,994,000)	[2]
Fair Value Measured on a Recurring Basis \| Level 1 \| Other Current Assets
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	0		0
Fair Value Measured on a Recurring Basis \| Level 1 \| Other Current Assets \| Other Derivative Instruments \| Electric Commodity
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	0		0
Fair Value Measured on a Recurring Basis \| Level 1 \| Other Current Liabilities
Derivatives, Fair Value [Line Items]
Derivative Liability, Fair Value, Gross Liability	0		0
Fair Value Measured on a Recurring Basis \| Level 1 \| Other Current Liabilities \| Other Derivative Instruments \| Electric Commodity
Derivatives, Fair Value [Line Items]
Derivative Liability, Fair Value, Gross Liability	0		0
Fair Value Measured on a Recurring Basis \| Level 2 \| Other Current Assets
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 \| Electric Commodity
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	0		0
Fair Value Measured on a Recurring Basis \| Level 2 \| Other Current Liabilities
Derivatives, Fair Value [Line Items]
Derivative Liability, Fair Value, Gross Liability	0		0
Fair Value Measured on a Recurring Basis \| Level 2 \| Other Current Liabilities \| Other Derivative Instruments \| Electric Commodity
Derivatives, Fair Value [Line Items]
Derivative Liability, Fair Value, Gross Liability	0		0
Fair Value Measured on a Recurring Basis \| Level 3 \| Other Current Assets
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	35,897,000		14,717,000
Fair Value Measured on a Recurring Basis \| Level 3 \| Other Current Assets \| Other Derivative Instruments \| Electric Commodity
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	35,897,000		14,717,000
Fair Value Measured on a Recurring Basis \| Level 3 \| Other Current Liabilities
Derivatives, Fair Value [Line Items]
Derivative Liability, Fair Value, Gross Liability	508,000		1,994,000
Fair Value Measured on a Recurring Basis \| Level 3 \| Other Current Liabilities \| Other Derivative Instruments \| Electric Commodity
Derivatives, Fair Value [Line Items]
Derivative Liability, Fair Value, Gross Liability	508,000		1,994,000
Fair Value, Measurements, Nonrecurring \| Other Current Assets \| Purchased Power Agreements
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	3,160,000	[3]	3,159,000	[4]
Fair Value, Measurements, Nonrecurring \| Other Noncurrent Assets \| Purchased Power Agreements
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	17,374,000	[3]	18,954,000	[4]
Fair Value, Measurements, Nonrecurring \| Other Current Liabilities \| Purchased Power Agreements
Derivatives, Fair Value [Line Items]
Derivative Liability, Fair Value, Gross Liability	3,565,000	[3]	3,565,000	[4]
Fair Value, Measurements, Nonrecurring \| Other Noncurrent Liabilities \| Purchased Power Agreements
Derivatives, Fair Value [Line Items]
Derivative Liability, Fair Value, Gross Liability	18,166,000	[3]	19,949,000	[4]
Estimate of Fair Value Measurement [Member] \| Fair Value Measured on a Recurring Basis \| Other Current Assets
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	35,897,000		14,717,000
Estimate of Fair Value Measurement [Member] \| Fair Value Measured on a Recurring Basis \| Other Current Assets \| Other Derivative Instruments \| Electric Commodity
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	35,897,000		14,717,000
Estimate of Fair Value Measurement [Member] \| Fair Value Measured on a Recurring Basis \| Other Current Liabilities
Derivatives, Fair Value [Line Items]
Derivative Liability, Fair Value, Gross Liability	508,000		1,994,000
Estimate of Fair Value Measurement [Member] \| Fair Value Measured on a Recurring Basis \| Other Current Liabilities \| Other Derivative Instruments \| Electric Commodity
Derivatives, Fair Value [Line Items]
Derivative Liability, Fair Value, Gross Liability	$ 508,000		$ 1,994,000

[1]	SPS nets derivative instruments and related collateral in its balance sheet when supported by a legally enforceable master netting agreement, and all derivative instruments and related collateral amounts were subject to master netting agreements at June 30, 2018. At June 30, 2018, derivative assets and liabilities include no obligations to return cash collateral or rights to reclaim cash collateral. The counterparty netting amounts presented exclude settlement receivables and payables and non-derivative amounts that may be subject to the same master netting agreements.
[2]	SPS nets derivative instruments and related collateral in its balance sheet when supported by a legally enforceable master netting agreement, and all derivative instruments and related collateral amounts were subject to master netting agreements at Dec. 31, 2017. At Dec. 31, 2017, derivative assets and liabilities include no obligations to return cash collateral or rights to reclaim cash collateral. The counterparty netting amounts presented exclude settlement receivables and payables and non-derivative amounts that may be subject to the same master netting agreements.
[3]	During 2006, SPS qualified these contracts under the normal purchase exception. Based on this qualification, the contracts are no longer adjusted to fair value and the previous carrying value of these contracts is being amortized over the remaining contract lives along with the offsetting regulatory assets and liabilities.
[4]	During 2006, SPS qualified these contracts under the normal purchase exception. Based on this qualification, the contracts are no longer adjusted to fair value and the previous carrying value of these contracts is being amortized over the remaining contract lives along with the offsetting regulatory assets and liabilities.

Fair Value of Financial Asset45

Fair Value of Financial Assets and Liabilities Fair Value of Financial Assets and Liabilities, Changes in Level 3 Commodity Derivatives (Details) - Commodity Contract - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Balance at beginning of period	$ 5,343,000	$ 1,192,000	$ 12,723,000	$ 1,955,000
Purchases	18,668,000	35,822,000	19,348,000	39,333,000
Settlements	(14,798,000)	(14,098,000)	(25,237,000)	(30,498,000)
Net gains recognized as regulatory assets and liabilities	26,176,000	5,749,000	28,555,000	17,875,000
Balance at end of period	35,389,000	28,665,000	35,389,000	28,665,000
Transfers into Level 3	0	0	0	0
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Inputs Reconciliation, Transfers out of Level 3	$ 0	$ 0	$ 0	$ 0

Fair Value of Financial Asset46

Fair Value of Financial Assets and Liabilities, Fair Value of Long-Term Debt (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Carrying Amount
Financial Liabilities, Balance Sheet Groupings [Abstract]
Long-term debt, including current portion	$ 1,830,508	$ 1,829,941
Fair Value
Financial Liabilities, Balance Sheet Groupings [Abstract]
Long-term debt, including current portion	$ 1,858,497	$ 2,001,992

Other Income, Net (Details)

Other Income, Net (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
Other Income, Net (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Other Income and Expenses [Abstract]
Interest Income (Expense), Nonoperating, Net	$ 356	$ 147	$ 298	$ 192
Other Nonoperating Income	1	1	3	0
Other Nonoperating Expense	0	0	0	(1)
Insurance policy expense	(12)	(12)	(24)	(24)
Defined Benefit Plan, Non-service Costs	(1,127)	(749)	(1,763)	(1,498)
Other expense, net	$ (782)	$ (613)	$ (1,486)	$ (1,331)

Benefit Plans and Other Postr48

Benefit Plans and Other Postretirement Benefits (Details) $ in Thousands	1 Months Ended	3 Months Ended		6 Months Ended
	Jan. 31, 2018USD ($)Plan	Jun. 30, 2018USD ($)	Jun. 30, 2017USD ($)	Jun. 30, 2018USD ($)	Jun. 30, 2017USD ($)
Pension Plan [Member]
Components of Net Periodic Benefit Cost (Credit) [Abstract]
Service cost		$ 2,430	$ 2,440	$ 4,860	$ 4,880
Interest cost		4,602	4,927	9,205	9,855
Expected return on plan assets		(7,082)	(6,971)	(14,164)	(13,942)
Amortization of prior service cost (credit)		(34)	0	(69)	0
Amortization of net loss (gain)		3,517	3,245	7,034	6,490
Net periodic benefit cost (credit)		3,433	3,641	6,866	7,283
Defined Benefit Plan Credits Not Recognized Due To Effects of Regulation		761	574	1,735	722
Net benefit cost (credit) recognized for financial reporting		4,194	4,215	8,601	8,005
Total contributions to the pension plans during the period	$ 8,000
Pension Plan [Member] \| Xcel Energy Inc.
Components of Net Periodic Benefit Cost (Credit) [Abstract]
Total contributions to the pension plans during the period	$ 150,000
Number of Xcel Energy's pension plans to which contributions were made \| Plan	4
Other Postretirement Benefits Plan [Member]
Components of Net Periodic Benefit Cost (Credit) [Abstract]
Service cost		280	219	559	438
Interest cost		411	415	821	830
Expected return on plan assets		(616)	(589)	(1,231)	(1,178)
Amortization of prior service cost (credit)		(100)	(100)	(201)	(200)
Amortization of net loss (gain)		(113)	(155)	(226)	(310)
Net periodic benefit cost (credit)		(138)	(210)	(278)	(420)
Defined Benefit Plan Credits Not Recognized Due To Effects of Regulation		0	0	0	0
Net benefit cost (credit) recognized for financial reporting		$ (138)	$ (210)	$ (278)	$ (420)

Other Comprehensive Income (Det

Other Comprehensive Income (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Accumulated other comprehensive loss at beginning of period			$ 2,130,363
Losses reclassified from net accumulated other comprehensive loss	$ 30	$ 25	61	$ 49
Accumulated other comprehensive loss at end of period	2,159,146		2,159,146
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	(764)	(669)	(776)	(678)
Losses reclassified from net accumulated other comprehensive loss	12	10	24	19
Net current period other comprehensive income	12	10	24	19
Accumulated other comprehensive loss at end of period	(752)	(659)	(752)	(659)
Defined Benefit Pension and Postretirement Items
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Accumulated other comprehensive loss at beginning of period	(672)	(597)	(691)	(612)
Losses reclassified from net accumulated other comprehensive loss	18	15	37	30
Net current period other comprehensive income	18	15	37	30
Accumulated other comprehensive loss at end of period	(654)	(582)	(654)	(582)
Total
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Accumulated other comprehensive loss at beginning of period	(1,436)	(1,266)	(1,467)	(1,290)
Losses reclassified from net accumulated other comprehensive loss	30	25	61	49
Net current period other comprehensive income	30	25	61	49
Accumulated other comprehensive loss at end of period	$ (1,406)	$ (1,241)	$ (1,406)	$ (1,241)

Other Comprehensive Income (Rec

Other Comprehensive Income (Reclassifications from AOCI) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018		Jun. 30, 2017
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total, pre-tax	$ (70,891)	$ (55,676)	$ (112,276)		$ (94,858)
Tax benefit	12,440	20,314	20,726		34,441
Total amounts reclassified, net of tax	30	25	61		49
Gains and Losses on Cash Flow Hedges
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total amounts reclassified, net of tax	12	10	24		19
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	16	16	31		31
Tax benefit	(4)	(6)	(7)		(12)
Total, net of tax	12	10	24		19
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	16	16	31		31	[1]
Amortization of net loss \| Amounts Reclassified from Accumulated Other Comprehensive Loss
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total, pre-tax	23	24	47	[2]	48	[2]
Defined Benefit Pension and Postretirement Items
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total amounts reclassified, net of tax	18	15	37		30
Defined Benefit Pension and Postretirement Items \| Amounts Reclassified from Accumulated Other Comprehensive Loss
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total, pre-tax	23	24	47		48
Tax benefit	(5)	(9)	(10)		(18)
Total amounts reclassified, net of tax	$ 18	$ 15	$ 37		$ 30

[1]	Included in interest charges.
[2]	Included in the computation of net periodic pension and postretirement benefit costs. See Note 10 to the financial statements for details regarding these benefit plans.

Revenues (Details)

Revenues (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
Revenues (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	$ 474,687	$ 472,587	$ 916,580	$ 910,021
Alternative revenue and other	6,651	7,209	11,990	29,847
Total operating revenues	481,338	479,796	$ 928,570	939,868
Maximum number of months following end of annual period in which revenues are earned to be included in incentive programs			24 months
Retail
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	297,230	312,235	$ 582,714	602,405
Retail \| Residential
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	85,107	84,188	165,156	163,789
Retail \| Commercial and industrial (C&I)
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	200,760	215,805	396,531	416,762
Retail \| Other
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	11,363	12,242	21,027	21,854
Wholesale
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	115,629	101,893	208,861	193,034
Transmission
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	58,970	56,394	114,616	110,572
Other
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	$ 2,858	$ 2,065	$ 10,389	$ 4,010