Southwestern Public Service 10-K 21 Feb 20 2019 FY Annual report Financial data

Cover Page

Cover Page - USD ($)	12 Months Ended
	Dec. 31, 2019	Feb. 21, 2020	Jun. 28, 2019
Cover page.
Document Type	10-K
Document Annual Report	true
Document Period End Date	Dec. 31, 2019
Document Transition Report	false
Entity File Number	001-03789
Entity Incorporation, State or Country Code	NM
Entity Tax Identification Number	75-0575400
Entity Address, Address Line One	790 South Buchanan Street,
Entity Address, City or Town	Amarillo,
Entity Address, State or Province	TX
Entity Address, Postal Zip Code	79101
City Area Code	(303)
Local Phone Number	571-7511
Entity Well-Known Season Issuer	Yes
Entity Voluntary Filers	No
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity Filer Category	Non-accelerated Filer
Entity Small Business	false
Entity Emerging Growth Company	false
Entity Shell Company	false
Entity Common Stock, Shares Outstanding		100
Entity Registrant Name	SOUTHWESTERN PUBLIC SERVICE CO
Entity Central Index Key	0000092521
Current Fiscal Year End Date	--12-31
Entity Public Float			$ 0
Document Fiscal Year Focus	2019
Document Fiscal Period Focus	FY
Amendment Flag	false

STATEMENTS OF CASH FLOWS

STATEMENTS OF CASH FLOWS - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Operating activities
Net income	$ 263.1	$ 213.3	$ 159.2
Adjustments to reconcile net income to cash provided by operating activities:
Depreciation and amortization	232.2	210	193.9
Demand side management program amortization	0	1.7	1.7
Deferred income taxes	29	22.1	126.5
Allowance for equity funds used during construction	(26.8)	(19.1)	(9.3)
Provision for bad debts	5.7	4.9	5.1
Net derivative losses	0	0.1	0.1
Changes in operating assets and liabilities:
Accounts receivable	(9)	(19.5)	(10.4)
Accrued unbilled revenues	(0.6)	15.3	(10.4)
Inventories	(20.5)	(16)	(1.9)
Prepayments and other	2.8	0.5	4.3
Accounts payable	(8.5)	(6.6)	11.8
Net regulatory assets and liabilities	13.8	38.2	38.1
Other current liabilities	5.8	11.6	3.4
Pension and other employee benefit obligations	(17.7)	(16)	(21.7)
Other, net	3.5	5.8	(19.9)
Net cash provided by operating activities	472.8	446.3	470.5
Investing activities
Utility capital/construction expenditures	(844.4)	(1,020.9)	(550.6)
Investments in utility money pool arrangement	(133)	(285)	(142)
Receipts from utility money pool arrangement	133	350	77
Other	0	0	0.5
Net cash used in investing activities	(844.4)	(955.9)	(616.1)
Financing activities
(Repayments of) proceeds from short-term borrowings, net	(42)	42	(50)
Proceeds from issuance of long-term debt	292.2	295	442.3
Repayment of long-term debt, including reacquisition premiums	0	0	(271.6)
Borrowings under utility money pool arrangement	296	595	335
Repayments under utility money pool arrangement	(296)	(595)	(335)
Capital contributions from parent	426.3	336.8	143.7
Dividends paid to parent	(332.7)	(131)	(108.8)
Net cash provided by financing activities	343.8	542.8	155.6
Net change in cash and cash equivalents	(27.8)	33.2	10
Cash and cash equivalents at beginning of period	44	10.8	0.8
Cash and cash equivalents at end of period	16.2	44	10.8
Supplemental disclosure of cash flow information:
Cash paid for interest (net of amounts capitalized)	(83.6)	(71.2)	(76)
Cash paid for income taxes, net	(11.9)	10.6	(41.5)
Supplemental disclosure of non-cash investing transactions:
Property, plant and equipment additions in accounts payable	94.5	71.5	85.1
Inventory transfer additions in property, plant and equipment	23.3	22.5	13.7
Operating lease right-of-use assets	548.3	0	0
Allowance for equity funds used during construction	$ 26.8	$ 19.1	$ 9.3

STATEMENTS OF COMMON STOCKHOLDE

STATEMENTS OF COMMON STOCKHOLDER'S EQUITY - USD ($) $ in Thousands	Total	Common stock	Additional Paid In Capital	Retained Earnings	AOCI Attributable to Parent
Beginning Balance (in shares) at Dec. 31, 2016		100
Beginning Balance at Dec. 31, 2016	$ 1,931,600	$ 0	$ 1,446,200	$ 486,700	$ (1,300)
Increase (Decrease) in Stockholder's Equity
Net income	159,200			159,200
Other comprehensive income	100				100
Common dividends declared to parent	(104,600)			(104,600)
Contribution of capital by parent	144,000		144,000
Adoption of ASU No. 2018-02	0			300	(300)
Ending Balance (in shares) at Dec. 31, 2017		100
Ending Balance at Dec. 31, 2017	2,130,300	$ 0	1,590,200	541,600	(1,500)
Increase (Decrease) in Stockholder's Equity
Net income	213,300			213,300
Other comprehensive income	100				100
Common dividends declared to parent	(149,200)			(149,200)
Contribution of capital by parent	$ 342,100		342,100
Ending Balance (in shares) at Dec. 31, 2018	100	100
Ending Balance at Dec. 31, 2018	$ 2,536,600	$ 0	1,932,300	605,700	(1,400)
Increase (Decrease) in Stockholder's Equity
Net income	263,100			263,100
Other comprehensive income	0				0
Common dividends declared to parent	(333,800)			(333,800)
Contribution of capital by parent	$ 418,600		418,600
Ending Balance (in shares) at Dec. 31, 2019	100	100
Ending Balance at Dec. 31, 2019	$ 2,884,500	$ 0	$ 2,350,900	$ 535,000	$ (1,400)

STATEMENTS OF INCOME

STATEMENTS OF INCOME - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Revenues [Abstract]
Operating revenue	$ 1,825.8	$ 1,933.2	$ 1,918
Operating expenses
Electric fuel and purchased power	875.4	1,043.5	1,055.3
Operating and maintenance expenses	285.3	282.7	285.4
Demand side management program expenses	16.6	17.7	15.5
Depreciation and amortization	229.9	209.6	193.9
Taxes (other than income taxes)	71.9	68	67
Total operating expenses	1,479.1	1,621.5	1,617.1
Operating income	346.7	311.7	300.9
Other income (expense), net	2.2	(3)	(1.8)
Allowance for funds used during construction — equity	26.8	19.1	9.3
Interest charges and financing costs
Interest charges — includes other financing costs of $3.4, $2.9 and $2.5, respectively	99.3	84.5	86.2
Allowance for funds used during construction — debt	(12.3)	(8.9)	(5.4)
Total interest charges and financing costs	87	75.6	80.8
Income before income taxes	288.7	252.2	227.6
Income taxes	25.6	38.9	68.4
Net income	$ 263.1	$ 213.3	$ 159.2

STATEMENTS OF INCOME (Parenthet

STATEMENTS OF INCOME (Parenthetical) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Interest charges and financing costs
Other financing costs	$ 3.4	$ 2.9	$ 2.5

BALANCE SHEETS

BALANCE SHEETS - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Current assets
Cash and cash equivalents	$ 16.2	$ 44
Accounts receivable, net	92.7	90.7
Accounts receivable from affiliates	4.2	10.5
Investments in money pool arrangements	0	0
Accrued unbilled revenues	115.1	114.5
Inventories	31	33.9
Regulatory assets	20	26
Derivative instruments	15	17.8
Prepaid taxes	0.8	14.2
Prepayments and other	21.4	10.7
Total current assets	316.4	362.3
Property, plant and equipment, net	6,631.6	5,946.4
Other assets
Regulatory assets	364	366.2
Derivative instruments	12.6	15.8
Operating lease right-of-use assets	522.4	0
Other	3.9	5.1
Total other assets	902.9	387.1
Total assets	7,850.9	6,695.8
Current liabilities
Short-term debt	0	42
Accounts payable	168.1	191.8
Accounts payable to affiliates	20.4	19.9
Regulatory liabilities	118.1	85.8
Taxes accrued	40.4	41.6
Accrued interest	26.2	25.8
Dividends payable	46.3	45.2
Derivative instruments	3.7	3.6
Operating lease liabilities	26.9	0
Other	30.7	28.3
Total current liabilities	480.8	484
Deferred credits and other liabilities
Deferred income taxes	671.8	619.1
Regulatory liabilities	732.3	780.9
Asset retirement obligations	77.3	32.4
Derivative instruments	12.8	16.4
Pension and employee benefit obligations	67	92.4
Operating lease liabilities	495.3	0
Other	9.4	7.9
Total deferred credits and other liabilities	2,065.9	1,549.1
Commitments and contingencies
Capitalization
Long-term debt	2,419.7	2,126.1
Common stock — 200 shares authorized of $1.00 par value; 100 shares outstanding at Dec. 31, 2019 and 2018, respectively	0	0
Additional paid in capital	2,350.9	1,932.3
Retained earnings	535	605.7
Accumulated other comprehensive loss	(1.4)	(1.4)
Total common stockholder’s equity	2,884.5	2,536.6
Total liabilities and equity	$ 7,850.9	$ 6,695.8

BALANCE SHEETS (Parenthetical)

BALANCE SHEETS (Parenthetical) - $ / shares	Dec. 31, 2019	Dec. 31, 2018
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

STATEMENTS OF COMPREHENSIVE INC

STATEMENTS OF COMPREHENSIVE INCOME - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Comprehensive income:
Net income	$ 263.1	$ 213.3	$ 159.2
Defined pension and other postretirement benefits
Net pension and retiree medical loss arising during the period, net of tax of $(0.1), $0 and $0, respectively	(0.2)	0	0
Reclassification of loss to net income, net of tax of $0	0.2	0	0.1
Derivative instruments:
Reclassification of loss to net income, net of tax of $0	0	0.1	0
Other comprehensive income	0	0.1	0.1
Comprehensive income	$ 263.1	$ 213.4	$ 159.3

STATEMENTS OF COMPREHENSIVE I_2

STATEMENTS OF COMPREHENSIVE INCOME (Parenthetical) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Defined pension and other postretirement benefits
Net pension and retiree medical loss arising during the period, tax	$ (0.1)	$ 0	$ 0
Reclassification of losses to net income, tax	0	0	0
Derivative instruments:
Reclassification of losses to net income, tax	$ 0	$ 0	$ 0

Summary of Significant Accounti

Summary of Significant Accounting Policies	12 Months Ended
	Dec. 31, 2019
Accounting Policies [Abstract]
Public Utilities, Inventory	General — SPS is engaged in the regulated generation, purchase, transmission, distribution and sale of electricity. SPS’ financial statements are presented in accordance with GAAP. All of SPS’ underlying accounting records also conform to the FERC uniform system of accounts or to systems required by various state regulatory commissions. Certain amounts in the 2018 and 2017 financial statements or notes have been reclassified to conform to the 2019 presentation for comparative purposes; however, such reclassifications did not affect net income, total assets, liabilities, equity or cash flows. SPS has evaluated events occurring after Dec. 31, 2019 up to the date of issuance of these financial statements. These statements contain all necessary adjustments and disclosures resulting from that evaluation. Use of Estimates — SPS uses estimates based on the best information available in recording transactions and balances resulting from business operations. Estimates are used on items such as plant depreciable lives or potential disallowances, AROs, certain regulatory assets and liabilities, tax provisions, uncollectible amounts, environmental costs, unbilled revenues, jurisdictional fuel and energy cost allocations and actuarially determined benefit costs. Recorded estimates are revised when better information becomes available or actual amounts can be determined. Revisions can affect operating results. Regulatory Accounting — SPS accounts for income and expense items in accordance with accounting guidance for regulated operations. Under this guidance: • Certain costs, which would otherwise be charged to expense or other comprehensive income, are deferred as regulatory assets based on the expected ability to recover the costs in future rates; and • Certain credits, which would otherwise be reflected as income or other comprehensive income, are deferred as regulatory liabilities based on the expectation the amounts will be returned to customers in future rates or because the amounts were collected in rates prior to the costs being incurred. Estimates of recovering deferred costs and returning deferred credits are based on specific ratemaking decisions or precedent for each item. Regulatory assets and liabilities are amortized consistent with the treatment in the rate setting process. If changes in the regulatory environment occur, SPS may no longer be eligible to apply this accounting treatment and may be required to eliminate regulatory assets and liabilities from its balance sheet. Such changes could have a material effect on SPS’ results of operations, financial condition and cash flows. See Note 4 for further information. Income Taxes — SPS accounts for income taxes using the asset and liability method, which requires recognition of deferred tax assets and liabilities for the expected future tax consequences of events that have been included in the financial statements. SPS defers income taxes for all temporary differences between pretax financial and taxable income and between the book and tax bases of assets and liabilities. SPS uses rates that are scheduled to be in effect when the temporary differences are expected to reverse. The effect of a change in tax rates on deferred tax assets and liabilities is recognized in the period that includes the enactment date. The effects of SPS’ tax rate changes are generally subject to a normalization method of accounting. Therefore, the revaluation of most its net deferred taxes upon a tax rate reduction results in the establishment of a net regulatory liability, which will be refundable to utility customers over the remaining life of the related assets. A tax rate increase would result in the establishment of a similar regulatory asset. Tax credits are recorded when earned unless there is a requirement to defer the benefit and amortize it over the book depreciable lives of the related property. The requirement to defer and amortize tax credits only applies to federal ITCs related to public utility property. Utility rate regulation also has resulted in the recognition of regulatory assets and liabilities related to income taxes. Deferred tax assets are reduced by a valuation allowance if it is more likely than not that some portion or all of the deferred tax asset will not be realized. SPS follows the applicable accounting guidance to measure and disclose uncertain tax positions that it has taken or expects to take in its income tax returns. SPS recognizes a tax position in its financial statements when it is more likely than not that the position will be sustained upon examination based on the technical merits of the position. Recognition of changes in uncertain tax positions are reflected as a component of income tax expense. SPS reports interest and penalties related to income taxes within the other income and interest charges in the statements of income. Xcel Energy Inc. and its subsidiaries, including SPS, files consolidated federal income tax returns as well as consolidated or separate state income tax returns. Federal income taxes paid by Xcel Energy Inc. are allocated to its subsidiaries based on separate company computations. A similar allocation is made for state income taxes paid by Xcel Energy Inc. in connection with consolidated state filings. Xcel Energy Inc. also allocates its own income tax benefits to its direct subsidiaries. See Note 7 for further information. Property, Plant and Equipment and Depreciation in Regulated Operations — Property, plant and equipment is stated at original cost. The cost of plant includes direct labor and materials, contracted work, overhead costs and AFUDC. The cost of plant retired is charged to accumulated depreciation and amortization. Amounts recovered in rates for future removal costs are recorded as regulatory liabilities. Significant additions or improvements extending asset lives are capitalized, while repairs and maintenance costs are charged to expense as incurred. Maintenance and replacement of items determined to be less than a unit of property are charged to operating expenses as incurred. Planned maintenance activities are charged to operating expense unless the cost represents the acquisition of an additional unit of property or the replacement of an existing unit of property. Property, plant and equipment is tested for impairment when it is determined that the carrying value of the assets may not be recoverable. A loss is recognized in the current period if it becomes probable that part of a cost of a plant under construction or recently completed plant will be disallowed for recovery from customers and a reasonable estimate of the disallowance can be made. For investments in property, plant and equipment that are abandoned and not expected to go into service, incurred costs and related deferred tax amounts are compared to the discounted estimated future rate recovery, and a loss is recognized, if necessary. SPS records depreciation expense using the straight-line method over the plant’s useful life. Actuarial life studies are performed and submitted to the state and federal commissions for review. Upon acceptance by the various commissions, the resulting lives and net salvage rates are used to calculate depreciation. Depreciation expense, expressed as a percentage of average depreciable property, was 2.9% in 2019 , 2.9% in 2018 and 2.8% in 2017 . See Note 3 for further information. AROs — SPS accounts for AROs under accounting guidance that requires a liability for the fair value of an ARO to be recognized in the period in which it is incurred if it can be reasonably estimated, with the offsetting associated asset retirement costs capitalized as a long-lived asset. The liability is generally increased over time by applying the effective interest method of accretion, and the capitalized costs are depreciated over the useful life of the long-lived asset. Changes resulting from revisions to the timing or amount of expected asset retirement cash flows are recognized as an increase or a decrease in the ARO. SPS also recovers through rates certain future plant removal costs in addition to AROs. The accumulated removal costs for these obligations are reflected in the balance sheets as a regulatory liability. See Note 10 for further information. Benefit Plans and Other Postretirement Benefits — SPS maintains pension and postretirement benefit plans for eligible employees. Recognizing the cost of providing benefits and measuring the projected benefit obligation of these plans requires management to make various assumptions and estimates. Certain unrecognized actuarial gains and losses and unrecognized prior service costs or credits are deferred as regulatory assets and liabilities, rather than recorded as other comprehensive income, based on regulatory recovery mechanisms. See Note 9 for further information. Environmental Costs — Environmental costs are recorded when it is probable SPS is liable for remediation costs and the liability can be reasonably estimated. Costs are deferred as a regulatory asset if it is probable that the costs will be recovered from customers in future rates. Otherwise, the costs are expensed. If an environmental expense is related to facilities currently in use, such as emission-control equipment, the cost is capitalized and depreciated over the life of the plant. Estimated remediation costs are regularly adjusted as estimates are revised and remediation proceeds. If other participating PRPs exist and acknowledge their potential involvement with a site, costs are estimated and recorded only for SPS’ expected share of the cost. Future costs of restoring sites are treated as a capitalized cost of plant retirement. The depreciation expense levels recoverable in rates include a provision for removal expenses. Removal costs recovered in rates before the related costs are incurred are classified as a regulatory liability. See Note 10 for further information. Revenue from Contracts with Customers — Performance obligations related to the sale of energy are satisfied as energy is delivered to customers. SPS recognizes revenue that corresponds 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. SPS does not recognize a separate financing component of its collections from customers as contract terms are short-term in nature. SPS presents its revenues net of any excise or sales 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 also recorded on a gross basis. Other revenues and charges related to participating and transacting in RTOs are recorded on a net basis in cost of sales. See Note 6 for further information. Cash and Cash Equivalents — SPS considers investments in instruments with a remaining maturity of three months or less at the time of purchase to be cash equivalents. Accounts Receivable and Allowance for Bad Debts — Accounts receivable are stated at the actual billed amount net of an allowance for bad debts. SPS establishes an allowance for uncollectible receivables based on a policy that reflects its expected exposure to the credit risk of customers. As of Dec. 31, 2019 and 2018 , the allowance for bad debts was $5.3 million and $5.6 million , respectively. Inventory — Inventory is recorded at average cost and consisted of the following: (Millions of Dollars) Dec. 31, 2019 Dec. 31, 2018 Inventories Materials and supplies $ 24.7 $ 25.7 Fuel 6.3 8.2 Total inventories $ 31.0 $ 33.9 Fair Value Measurements — SPS presents cash equivalents, interest rate derivatives and commodity derivatives at estimated fair values in its financial statements. Cash equivalents are recorded at cost plus accrued interest; money market funds are measured using quoted NAVs. For interest rate derivatives, quoted prices based primarily on observable market interest rate curves are used to establish fair value. For commodity derivatives, the most observable inputs available are generally used to determine the fair value of each contract. In the absence of a quoted price, SPS may use quoted prices for similar contracts or internally prepared valuation models to determine fair value. For the pension and postretirement plan assets published trading data and pricing models, generally using the most observable inputs available, are utilized to estimate fair value for each security. See Notes 8 and 9 for further information. Derivative Instruments — SPS uses derivative instruments in connection with its utility commodity price and interest rate activities, including forward contracts, futures, swaps and options. Any derivative instruments not qualifying for the normal purchases and normal sales exception are recorded on the balance sheets at fair value as derivative instruments. Classification of changes in fair value for those derivative instruments is dependent on the designation of a qualifying hedging relationship. Changes in fair value of derivative instruments not designated in a qualifying hedging relationship are reflected in current earnings or as a regulatory asset or liability. Classification as a regulatory asset or liability is based on expected recovery of derivative instrument settlements through fuel and purchased energy cost recovery mechanisms. Interest rate hedging transactions are recorded as a component of interest expense. Normal Purchases and Normal Sales — SPS enters into contracts for purchases and sales of commodities for use in its operations. At inception, contracts are evaluated to determine whether a derivative exists and/or whether an instrument may be exempted from derivative accounting if designated as a normal purchase or normal sale. See Note 8 for further information. Other Utility Items AFUDC — AFUDC represents the cost of capital used to finance utility construction activity. AFUDC is computed by applying a composite financing rate to qualified CWIP. The amount of AFUDC capitalized as a utility construction cost is credited to other nonoperating income (for equity capital) and interest charges (for debt capital). AFUDC amounts capitalized are included in SPS’ rate base for establishing utility rates. Alternative Revenue — Certain rate rider mechanisms (including DSM programs) qualify as alternative revenue programs. 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 expected collection within 24 months , revenue is recognized equal to the revenue requirement, which may include incentives and return on rate base items. Billing amounts are revised periodically for differences between total amount collected and revenue earned, which may increase or decrease the level of revenue collected from customers. Alternative revenues arising from these programs are presented on a gross basis and disclosed separately from revenue from contracts with customers. See Note 6 for further information. Conservation Programs — SPS has implemented programs in its jurisdictions to assist customers in conserving energy and reducing peak demand on the electric system. These programs include commercial motor, air conditioner and lighting upgrades, as well as residential rebates for participation in air conditioner interruption and home weatherization. The costs incurred for some DSM programs are deferred as permitted by the applicable regulatory jurisdiction. For those programs, costs are deferred if it is probable future revenue will be provided to permit recovery of the incurred cost. Revenues recognized for incentive programs designed for recovery of lost margins and/or conservation performance incentives are limited to amounts expected to be collected within 24 months from the annual period in which they are earned. SPS recovers approved conservation program costs in base rate revenue or through a rider. Emission Allowances — Emission allowances are recorded at cost, including broker commission fees. The inventory accounting model is utilized for all emission allowances and sales of these allowances are included in electric revenues. RECs — Cost of RECs that are utilized for compliance is recorded as electric fuel and purchased power expense. SPS reduces recoverable fuel costs for the cost of RECs and records that cost as a regulatory asset when the amount is recoverable in future rates. Sales of RECs are recorded in electric revenues on a gross basis. Cost of these RECs and amounts credited to customers under margin-sharing mechanisms are recorded in electric fuel and purchased power expense. Segment Information

Accounting Pronouncements

Accounting Pronouncements	12 Months Ended
	Dec. 31, 2019
New Accounting Pronouncements and Changes in Accounting Principles [Abstract]
Accounting Pronouncements	cently Issued Credit Losses — In 2016, the FASB issued Financial Instruments - Credit Losses, Topic 326 (ASC Topic 326), which changes how entities account for losses on receivables and certain other assets. The guidance requires use of a current expected credit loss model, which may result in earlier recognition of credit losses than under previous accounting standards. ASC Topic 326 is effective for interim and annual periods beginning on or after Dec. 15, 2019, and will be applied using a modified-retrospective approach, with a cumulative-effect adjustment to retained earnings as of Jan. 1, 2020. SPS expects the impact of adoption of the new standard to include first-time recognition of expected credit losses (i.e., bad debt expense) on unbilled revenues, with the initial allowance established at Jan. 1, 2020 charged to retained earnings. Recognition of this allowance and other impacts of adoption are expected to be immaterial to the financial statements. Recently Adopted Leases — In 2016, the FASB issued Leases , Topic 842 (ASC Topic 842) , which provides new accounting and disclosure guidance for leasing activities, most significantly requiring that operating leases be recognized on the balance sheet. SPS adopted the guidance on Jan. 1, 2019 utilizing the package of transition practical expedients provided by the new standard, including carrying forward prior conclusions on whether agreements existing before the adoption date contain leases and whether existing leases are operating or finance leases; ASC Topic 842 refers to capital leases as finance leases. Specifically for land easement contracts, SPS has elected the practical expedient provided by ASU No. 2018-01 Leases: Land Easement Practical Expedient for Transition to Topic 842 , and as a result, only those easement contracts entered on or after Jan. 1, 2019 will be evaluated to determine if lease treatment is appropriate. SPS also utilized the transition practical expedient offered by ASU No. 2018-11 Leases: Targeted Improvements to implement the standard on a prospective basis. As a result, reporting periods in the financial statements beginning Jan. 1, 2019 reflect the implementation of ASC Topic 842, while prior periods continue to be reported in accordance with Leases, Topic 840 (ASC Topic 840) . Other than first-time recognition of operating leases on its balance sheet, the implementation of ASC Topic 842 did not have a significant impact on SPS’ financial statements. Adoption resulted in recognition of approximately $0.5 billion of operating lease ROU assets and current/noncurrent operating lease liabilities. See Note 10 for leasing disclosures.

Property Plant and Equipment Pr

Property Plant and Equipment Property Plant and Equipment	12 Months Ended
	Dec. 31, 2019
Property, Plant and Equipment [Abstract]
Property, Plant and Equipment Disclosure	r classes of property, plant and equipment (Millions of Dollars) Dec. 31, 2019 Dec. 31, 2018 Property, plant and equipment Electric plant $ 8,453.0 $ 7,227.7 CWIP 485.4 847.3 Total property, plant and equipment 8,938.4 8,075.0 Less accumulated depreciation (2,306.8 ) (2,128.6 ) Property, plant and equipment, net $ 6,631.6 $ 5,946.4

Regulatory Assets and Liabiliti

Regulatory Assets and Liabilities	12 Months Ended
	Dec. 31, 2019
Regulatory Assets and Liabilities Disclosure [Abstract]
Regulatory Assets and Liabilities	gulatory assets and liabilities are created for amounts that regulators may allow to be collected or may require to be paid back to customers in future electric rates. SPS would be required to recognize the write-off of regulatory assets and liabilities in net income or other comprehensive income if changes in the utility industry no longer allow for the application of regulatory accounting guidance under GAAP. Components of regulatory assets: (Millions of Dollars) See Remaining Dec. 31, 2019 Dec. 31, 2018 Regulatory Assets Current Noncurrent Current Noncurrent Pension and retiree medical obligations 9 Various $ 11.1 $ 203.5 $ 12.6 $ 222.1 Excess deferred taxes — TCJA 7 Various 1.7 52.0 — 55.9 Recoverable deferred taxes on AFUDC recorded in plant Plant lives — 34.1 — 27.9 Net AROs (a) 1, 10 Plant lives — 26.9 — 25.7 Losses on reacquired debt Term of related debt 0.8 21.0 0.8 21.9 Conservation programs (b) 1 One to two years 0.6 1.1 0.7 0.6 Other Various 5.8 25.4 11.9 12.1 Total regulatory assets $ 20.0 $ 364.0 $ 26.0 $ 366.2 (a) Includes amounts recorded for future recovery of AROs. (b) Includes costs for conservation programs, as well as incentives allowed in certain jurisdictions. Components of regulatory liabilities: (Millions of Dollars) See Remaining Dec. 31, 2019 Dec. 31, 2018 Regulatory Liabilities Current Noncurrent Current Noncurrent Deferred income tax adjustments and TCJA refunds (a) 7 Various $ 6.9 $ 534.9 $ 2.2 $ 569.8 Plant removal costs 1, 10 Plant lives — 174.5 — 187.7 Revenue subject to refund One to two years 14.6 1.1 11.3 8.1 Gain from asset sales Various — 2.4 — 2.4 Deferred electric energy costs Less than one year 81.6 — 56.5 — Contract valuation adjustments (b) 1, 8 Less than one year 11.7 — 14.7 — Other Various 3.3 19.4 1.1 12.9 Total regulatory liabilities (c) $ 118.1 $ 732.3 $ 85.8 $ 780.9 (a) Includes the revaluation of recoverable/regulated plant ADIT and revaluation impact of non-plant ADIT due to the TCJA. (b) Includes the fair value of certain long-term PPAs used to meet energy capacity requirements. (c) Revenue subject to refund of $3.9 million for 2019 and none for 2018 is included in other current liabilities. At Dec. 31, 2019 and 2018 , SPS’ regulatory assets not earning a return primarily included the unfunded portion of pension and retiree medical obligations and net AROs. In addition, SPS’ regulatory assets included $56.5 million and $50.5 million at Dec. 31, 2019 and 2018, respectively, of past expenditures not earning a return. Amounts primarily related to formula rates, losses on reacquired debt and certain rate case expenditures.

Borrowings and Other Financing

Borrowings and Other Financing Instruments	12 Months Ended
	Dec. 31, 2019
Debt Disclosure [Abstract]
Borrowings and Other Financing Instruments	Short-Term Borrowings SPS meets its short-term liquidity requirements primarily through the issuance of commercial paper and borrowings under its credit facility and the money pool. 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: (Millions of Dollars, Except Interest Rates) Three Months Ended Dec. 31, 2019 Year Ended Dec. 31 2019 2018 2017 Borrowing limit $ 100 $ 100 $ 100 $ 100 Amount outstanding at period end — — — — Average amount outstanding 1 8 29 13 Maximum amount outstanding 12 100 100 100 Weighted average interest rate, computed on a daily basis 1.63 % 2.42 % 1.96 % 1.12 % Weighted average interest rate at end of period N/A N/A N/A N/A Commercial Paper — Commercial paper outstanding for SPS was as follows: (Millions of Dollars, Except Interest Rates) Three Months Ended Dec. 31, 2019 Year Ended Dec. 31 2019 2018 2017 Borrowing limit $ 500 $ 500 $ 400 $ 400 Amount outstanding at period end — — 42 — Average amount outstanding — 72 30 69 Maximum amount outstanding — 316 144 176 Weighted average interest rate, computed on a daily basis N/A 2.68 % 2.27 % 1.13 % Weighted average interest rate at end of period N/A N/A 2.80 NA Letters of Credit — SPS may use letters of credit, typically with terms of one year, to provide financial guarantees for certain operating obligations. At Dec. 31, 2019 and 2018 , there were $2 million 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. Amended Credit Agreement — In June 2019, SPS entered into an amended five-year credit agreement with a syndicate of banks. The amended credit agreements have substantially the same terms and conditions as the prior credit agreements with the exception of the following: • Maturity extended from June 2021 to June 2024; and • Borrowing limit increased from $400 million to $500 million. 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. Features of SPS’ credit facility: Debt-to-Total Capitalization Ratio (a) Amount Facility May Be Increased (millions) Additional Periods for Which a One-Year Extension May Be Requested (b) 2019 2018 46% 46% $50 2 (a) The SPS credit facility has a financial covenant requiring that the debt-to-total capitalization ratio be less than or equal to 65% . (b) All extension requests are subject to majority bank group approval. The credit facility has a cross-default provision that SPS will be in default on its borrowings under the facility if SPS or any of its future significant subsidiaries whose total assets exceed 15% of SPS’ total assets default on indebtedness in an aggregate principal amount exceeding $75 million . If SPS does not comply with the covenant, an event of default may be declared, and if not remedied, any outstanding amounts due under the facility can be declared due by the lender. As of Dec. 31, 2019 , SPS was in compliance with all financial covenants. SPS had the following committed credit facilities available as of Dec. 31, 2019 . Credit Facility (a) Drawn (b) Available $500 $2 $498 (a) This credit facility matures in June 2024 . (b) Includes letters of credit and outstanding commercial paper. 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 facility outstanding at Dec. 31, 2019 and 2018 . Long-Term Borrowings and Other Financing Instruments Generally, all property of SPS is subject to the lien of its first mortgage indenture. Debt premiums, discounts and expenses are amortized over the life of the related debt. The premiums, discounts and expenses for refinanced debt are deferred and amortized over the life of the new issuance. Long-term debt obligations for SPS as of Dec. 31 (millions of dollars): Financing Instrument Interest Rate Maturity Date 2019 2018 First mortgage bonds 3.30 % June 15, 2024 $ 150 $ 150 First mortgage bonds 3.30 June 15, 2024 200 200 Unsecured senior notes 6.00 Oct. 1, 2033 100 100 Unsecured senior notes 6.00 Oct. 1, 2036 250 250 First mortgage bonds 4.50 Aug. 15, 2041 200 200 First mortgage bonds 4.50 Aug. 15, 2041 100 100 First mortgage bonds 4.50 Aug. 15, 2041 100 100 First mortgage bonds 3.40 Aug. 15, 2046 300 300 First mortgage bonds 3.70 Aug. 15, 2047 450 450 First mortgage bonds (b) 4.40 Nov. 15, 2048 300 300 First mortgage bonds (a) 3.75 June 15, 2049 300 — Unamortized discount (7 ) (4 ) Unamortized debt issuance cost (23 ) (20 ) Total long-term debt $ 2,420 $ 2,126 (a) 2019 financing (b) 2018 financing Maturities of long-term debt: (Millions of Dollars) 2020 $ — 2021 — 2022 — 2023 — 2024 350 Deferred Financing Costs — Deferred financing costs of approximately $23 million and $20 million , net of amortization, are presented as a deduction from the carrying amount of long-term debt at Dec. 31, 2019 and 2018 , respectively. SPS is amortizing these financing costs over the remaining maturity periods of the related debt. Capital Stock — SPS has the following preferred stock: Preferred Stock Authorized (Shares) Par Value of Preferred Stock Preferred Stock Outstanding (Shares) 2019 and 2018 10,000,000 1.00 — Dividend Restrictions — SPS dividends are subject to the FERC’s jurisdiction, which prohibits the payment of dividends out of capital accounts. Dividends are solely to be paid from retained earnings. SPS is required to be current on particular interest payments before dividends can be paid. SPS’ state regulatory commissions additionally impose dividend limitations, which are more restrictive than those imposed by the FERC. Requirements and actuals as of Dec. 31, 2019 : Equity to Total Capitalization Ratio - Required Range Equity to Total Capitalization Ratio - Actual (a) Low High 2019 45.0 % 55.0 % 54.4 % (a) Excludes short-term debt. Unrestricted Retained Earnings Total Capitalization Limit on Total Capitalization (a) $ 535.0 million $ 5.3 billion N/A (a)

Revenues

Revenues	12 Months Ended
	Dec. 31, 2019
Revenue from Contract with Customer [Abstract]
Revenue from Contract with Customer [Text Block]	Revenue is classified by the type of goods/services rendered and market/customer type. SPS’ operating revenues consisted of the following: (Millions of Dollars) Year Ended Dec. 31, 2019 Major product lines Revenue from contracts with customers: Residential $ 351.9 C&I 800.3 Other 41.1 Total retail 1,193.3 Wholesale 361.0 Transmission 239.6 Other 3.3 Total revenue from contracts with customers 1,797.2 Alternative revenue and other 28.6 Total revenues $ 1,825.8 (Millions of Dollars) Year Ended Dec. 31, 2018 Major product lines Revenue from contracts with customers: Residential $ 363.7 C&I 828.3 Other 44.7 Total retail 1,236.7 Wholesale 426.0 Transmission 231.1 Other 12.8 Total revenue from contracts with customers 1,906.6 Alternative revenue and other 26.6 Total revenues $ 1,933.2

Income Taxes

Income Taxes	12 Months Ended
	Dec. 31, 2019
Income Tax Disclosure [Abstract]
Income Taxes	deral Tax Reform — In 2017, the TCJA was signed into law. The key provisions impacting Xcel Energy (which includes SPS), generally beginning in 2018, included: • Corporate federal tax rate reduction from 35% to 21% ; • Normalization of resulting plant-related excess deferred taxes; • Elimination of the corporate alternative minimum tax; • Continued interest expense deductibility and discontinued bonus depreciation for regulated public utilities; • Limitations on certain executive compensation deductions; • Limitations on certain deductions for NOLs arising after Dec. 31, 2017 (limited to 80% of taxable income); • Repeal of the section 199 manufacturing deduction; and • Reduced deductions for meals and entertainment as well as state and local lobbying. Xcel Energy estimated the effects of the TCJA, which have been reflected in the consolidated financial statements. Reductions in deferred tax assets and liabilities due to a decrease in corporate federal tax rates typically result in a net tax benefit. However, the impacts are primarily recognized as regulatory liabilities refundable to utility customers as a result of IRS requirements and past regulatory treatment. Estimated impacts of the new tax law for SPS in December 2017 included: • $426 million ( $559 million grossed-up for tax) of reclassifications of plant-related excess deferred taxes to regulatory liabilities upon valuation at the new 21% federal rate. The regulatory liabilities will be amortized consistent with IRS normalization requirements, resulting in customer refunds over the average remaining life of the related property; • $45 million and $28 million of reclassifications (grossed-up for tax) of excess deferred taxes for non-plant related deferred tax assets and liabilities, respectively, to regulatory assets and liabilities; and • $8 million of total estimated income tax benefit related to the federal tax reform implementation and a $2 million reduction to net income related to the allocation of Xcel Energy Services Inc.’s tax rate change on its deferred taxes. Xcel Energy accounted for the state tax impacts of federal tax reform based on enacted state tax laws. Any future state tax law changes related to the TCJA will be accounted for in the periods state laws are enacted. Federal Audit — SPS is a member of the Xcel Energy affiliated group that files a consolidated federal income tax return. Statute of limitations applicable to Xcel Energy’s consolidated federal income tax returns expire as follows: Tax Year(s) Expiration 2009 - 2013 June 2020 2014 - 2016 September 2020 In 2015, the IRS commenced an examination of tax years 2012 and 2013 . In 2017, the IRS concluded the audit of tax years 2012 and 2013 and proposed an adjustment that would impact Xcel Energy’s NOL and ETR. Xcel Energy filed a protest with the IRS. As of Dec. 31, 2019, the case has been forwarded to the Office of 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. In 2018, the IRS began an audit of tax years 2014 - 2016 . As of Dec. 31, 2019 no adjustments have been proposed. State Audits — SPS is a member of the Xcel Energy affiliated group that files consolidated state income tax returns. As of Dec. 31, 2019, 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 Tax Benefits — 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 to the taxing authority to an earlier period. Unrecognized tax benefits — permanent vs temporary: (Millions of Dollars) Dec. 31, 2019 Dec. 31, 2018 Unrecognized tax benefit — Permanent tax positions $ 3.7 $ 3.0 Unrecognized tax benefit — Temporary tax positions 1.5 1.5 Total unrecognized tax benefit $ 5.2 $ 4.5 Changes in unrecognized tax benefits: (Millions of Dollars) 2019 2018 2017 Balance at Jan. 1 $ 4.5 $ 4.3 $ 28.7 Additions based on tax positions related to the current year 0.7 0.6 0.9 Reductions based on tax positions related to the current year (0.1 ) (0.1 ) (0.6 ) Additions for tax positions of prior years 0.2 0.1 1.3 Reductions for tax positions of prior years (0.1 ) (0.3 ) (19.9 ) Settlements with taxing authorities — (0.1 ) (6.1 ) Balance at Dec. 31 $ 5.2 $ 4.5 $ 4.3 Unrecognized tax benefits were reduced by tax benefits associated with NOL and tax credit carryforwards: (Millions of Dollars) Dec. 31, 2019 Dec. 31, 2018 NOL and tax credit carryforwards $ (4.4 ) $ (3.8 ) Net deferred tax liability associated with the unrecognized tax benefit amounts and related NOLs and tax credits carryforwards were $1.4 million and $0.8 million at Dec. 31, 2019 and Dec. 31, 2018 , respectively. As the IRS Appeals and federal audit progresses and state audits resume, it is reasonably possible that the amount of unrecognized tax benefit could decrease up to approximately $3.7 million in the next 12 months. Payable for interest related to unrecognized tax benefits is partially offset by the interest benefit associated with NOL and tax credit carryforwards. Interest payable related to unrecognized tax benefits: (Millions of Dollars) 2019 2018 2017 Receivable (payable) for interest related to unrecognized tax benefits at Jan. 1 $ 0.7 $ 0.5 $ (0.9 ) Interest income related to unrecognized tax benefits — 0.2 1.4 Receivable for interest related to unrecognized tax benefits at Dec. 31 $ 0.7 $ 0.7 $ 0.5 No amounts were accrued for penalties related to unrecognized tax benefits as of Dec. 31, 2019 , 2018 , or 2017 . Other Income Tax Matters — NOL amounts represent the tax loss that is carried forward and tax credits represent the deferred tax asset. NOL and tax credit carryforwards as of Dec. 31 were as follows: (Millions of Dollars) 2019 2018 Federal tax credit carryforwards $ 29.5 $ 5.7 State NOL carryforwards 1.2 2.9 Federal carryforward periods expire between 2024 and 2039 and state carryforward periods expire between 2025 and 2036 . 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. Effective income tax rate for years ended Dec. 31: 2019 2018 (a) 2017 (a) Federal statutory rate 21.0 % 21.0 % 35.0 % State income tax on pretax income, net of federal tax effect 2.2 % 2.3 % 2.0 % Increases (decreases) in tax from: Wind PTCs (7.9 ) — — Plant regulatory differences (b) (5.0 ) (4.8 ) (0.9 ) Amortization of excess nonplant deferred taxes (0.9 ) (1.2 ) — Other tax credits, net of NOL & tax credit allowances (0.6 ) (0.7 ) (0.6 ) Adjustments attributable to tax returns (0.1 ) (1.5 ) (0.4 ) Change in unrecognized tax benefits 0.2 0.1 (1.0 ) Tax reform — — (3.5 ) Other, net — 0.2 (0.5 ) Effective income tax rate 8.9 % 15.4 % 30.1 % (a) Prior periods have been reclassified to conform to current year presentation. (b) Regulatory differences for income tax primarily relate to the credit of excess deferred taxes to customers through the average rate assumption method. Income tax benefits associated with the credit of excess deferred credits are offset by corresponding revenue reductions. Components of income tax expense for years ended Dec. 31: (Millions of Dollars) 2019 2018 2017 Current federal tax (benefit) expense $ (3.9 ) $ 12.3 $ (20.9 ) Current state tax expense (benefit) 0.6 2.3 (12.8 ) Current change in unrecognized tax expense (benefit) — 2.3 (24.3 ) Deferred federal tax expense 22.3 20.5 89.9 Deferred state tax expense 6.0 3.6 14.5 Deferred change in unrecognized tax expense (benefit) 0.7 (2.0 ) 22.1 Deferred ITCs (0.1 ) (0.1 ) (0.1 ) Total income tax expense $ 25.6 $ 38.9 $ 68.4 Components of deferred income tax expense as of Dec. 31: (Millions of Dollars) 2019 2018 2017 Deferred tax expense (benefit) excluding items below $ 52.7 $ 44.2 $ (414.2 ) Amortization and adjustments to deferred income taxes on income tax regulatory assets and liabilities (23.8 ) (22.0 ) 540.7 Tax benefit (expense) allocated to other comprehensive income, net of adoption of ASU No. 2018-02, and other 0.1 (0.1 ) — Deferred tax expense $ 29.0 $ 22.1 $ 126.5 Components of the net deferred tax liability as of Dec. 31: (Millions of Dollars) 2019 2018 (a) Deferred tax liabilities: Differences between book and tax bases of property $ 758.7 $ 680.6 Operating lease assets 115.8 — Regulatory assets 49.7 49.2 Pension expense 33.1 32.3 Total deferred tax liabilities $ 957.3 $ 762.1 Deferred tax assets: Regulatory liabilities $ 111.2 $ 116.8 Operating lease liabilities 115.8 — Tax credit carryforward 29.5 5.7 Deferred fuel costs 18.3 12.7 Other employee benefits 5.8 5.6 NOL carryforward 0.1 0.2 Other 4.8 2.0 Total deferred tax assets 285.5 143.0 Net deferred tax liability $ 671.8 $ 619.1 (a) Prior periods have been reclassified to conform to current year presentation.

Fair Value of Financial Assets

Fair Value of Financial Assets and Liabilities	12 Months Ended
	Dec. 31, 2019
Fair Value Disclosures [Abstract]
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 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. • 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; and • 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: Cash equivalents — The fair values of cash equivalents are generally based on cost plus accrued interest; money market funds are measured using quoted NAVs. 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 relate to inactive delivery locations or extend to periods beyond those readily observable on active exchanges or quoted by brokers, the significance of the use of less observable forecasts of 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 FTRs, purchased from SPP. FTRs purchased from an 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. 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 observability of important inputs to the value of FTRs between auction processes, including expected plant operating schedules and retail and wholesale demand, 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 relative to the electric utility operations of SPS, the numerous unobservable quantitative inputs pertinent to the value of FTRs are immaterial to the financial statements of SPS. Derivative 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 Dec. 31, 2019 , accumulated other comprehensive losses related to interest rate derivatives included $0.1 million 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 is allowed to conduct these activities within guidelines and limitations as approved by its risk management committee, comprised 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. Gross notional amounts of commodity FTRs at Dec. 31, 2019 and 2018 : (Amounts in Millions) (a) Dec. 31, 2019 Dec. 31, 2018 MWh of electricity 6.4 5.5 (a) Amounts are not reflective of net positions in the underlying commodities. Consideration of Credit Risk and Concentrations — SPS continuously monitors the creditworthiness of 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. Impact of credit risk was immaterial to the fair value of unsettled commodity derivatives presented in the balance sheets. 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. At Dec. 31, 2019 , three of the ten most significant counterparties for these activities, comprising $12.2 million or 35% of this credit exposure, had investment grade ratings from Standard & Poor’s, Moody’s or Fitch Ratings. Six of the ten most significant counterparties, comprising $22.1 million or 65% of this credit exposure, were not rated by external rating agencies, but based on SPS’ internal analysis, had credit quality consistent with investment grade. One of these significant counterparties, comprising $0.1 million or less than 1% of this credit exposure, had credit quality less than investment grade, based on internal analysis. Nine of these significant counterparties are municipal or cooperative electric entities, RTOs or other utilities. Qualifying Cash Flow Hedges — Financial impact of qualifying interest rate cash flow hedges on SPS’ accumulated other comprehensive loss, included in the statements of common stockholder’s equity and in the statements of comprehensive income: (Millions of Dollars) 2019 2018 2017 Accumulated other comprehensive loss related to cash flow hedges at Jan. 1 $ (0.7 ) $ (0.8 ) $ (0.7 ) After-tax net realized losses on derivative transactions reclassified into earnings — 0.1 — Adoption of ASU. 2018-02 (a) — — (0.1 ) Accumulated other comprehensive loss related to cash flow hedges at Dec. 31 $ (0.7 ) $ (0.7 ) $ (0.8 ) (a) In 2017, SPS implemented ASU No. 2018-02 related to TCJA, which resulted in reclassification of certain credit balances within net accumulated other comprehensive loss to retained earnings. Pre-tax losses related to interest rate derivatives reclassified from accumulated other comprehensive loss into earnings were immaterial, $0.1 million and $0.1 million for the years ended Dec. 31, 2019 , 2018 and 2017 , respectively. Changes in the fair value of FTRs resulting in pre-tax net gains of $6.5 million , $7.0 million and $0.5 million recognized for the years ended Dec. 31, 2019 , 2018 and 2017 , respectively, were reclassified 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 $6.0 million , $4.4 million and $0.8 million were recognized for the years ended Dec. 31, 2019 , 2018 and 2017 , respectively, and were 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 years ended Dec. 31, 2019 , 2018 and 2017 . 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 at Dec. 31, 2019 and 2018 : Dec. 31, 2019 Dec. 31, 2018 Fair Value Fair Value (Millions of Dollars) Level 1 Level 2 Level 3 Fair Value Total Netting (a) Total Level 1 Level 2 Level 3 Fair Value Total Netting (a) Total Current derivative assets Other derivative instruments: Electric commodity $ — $ — $ 11.8 $ 11.8 $ — $ 11.8 $ — $ — $ 14.9 $ 14.9 $ (0.2 ) $ 14.7 Total current derivative assets $ — $ — $ 11.8 $ 11.8 $ — 11.8 $ — $ — $ 14.9 $ 14.9 $ (0.2 ) 14.7 PPAs (b) 3.2 3.1 Current derivative instruments $ 15.0 $ 17.8 Noncurrent derivative assets PPAs (b) 12.6 15.8 Noncurrent derivative instruments $ 12.6 $ 15.8 Current derivative liabilities Other derivative instruments: Electric commodity $ — $ — $ 0.1 $ 0.1 $ — $ 0.1 $ — $ — $ 0.2 $ 0.2 $ (0.2 ) $ — Total current derivative liabilities $ — $ — $ 0.1 $ 0.1 $ — 0.1 $ — $ — $ 0.2 $ 0.2 $ (0.2 ) — PPAs (b) 3.6 3.6 Current derivative instruments $ 3.7 $ 3.6 Noncurrent derivative liabilities PPAs (b) 12.8 16.4 Noncurrent derivative instruments $ 12.8 $ 16.4 (a) 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, 2019 and 2018 . At both Dec. 31, 2019 and 2018 , derivative assets and liabilities include no obligations to return cash collateral or rights to reclaim cash collateral. The counterparty netting excludes settlement receivables and payables and non-derivative amounts that may be subject to the same master netting agreements. (b) 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 will be amortized over the remaining contract lives along with the offsetting regulatory assets and liabilities. Changes in Level 3 commodity derivatives for the years ended Dec. 31, 2019 , 2018 and 2017 : Year Ended Dec. 31 (Millions of Dollars) 2019 2018 2017 Balance at Jan. 1 $ 14.7 $ 12.7 $ 2.0 Purchases 26.7 32.3 41.2 Settlements (34.2 ) (41.6 ) (55.8 ) Net transactions recorded during the period: Net gains recognized as regulatory assets 4.5 11.3 25.3 Balance at Dec. 31 $ 11.7 $ 14.7 $ 12.7 SPS recognizes transfers between levels as of the beginning of each period. There were no transfers of amounts between levels for derivative instruments for 2017 – 2019 . Fair Value of Long-Term Debt As of Dec. 31, other financial instruments for which the carrying amount did not equal fair value: 2019 2018 (Millions of Dollars) Carrying Amount Fair Value Carrying Amount Fair Value Long-term debt, including current portion $ 2,419.7 $ 2,706.1 $ 2,126.1 $ 2,139.8 Fair value of SPS’ long-term debt is estimated based on recent trades and observable spreads from benchmark interest rates for similar securities. Fair value estimates are based on information available to management as of Dec. 31, 2019 and 2018 , and given the observability of the inputs, fair values presented for long-term debt were assigned as Level 2.

Benefit Plans and Other Postret

Benefit Plans and Other Postretirement Benefits	12 Months Ended
	Dec. 31, 2019
Retirement Benefits [Abstract]
Benefit Plans and Other Postretirement Benefits	nsion and Postretirement Health Care Benefits Xcel Energy, which includes SPS, has several noncontributory, defined benefit pension plans that cover almost all employees. Generally, benefits are based on a combination of years of service and average pay. Xcel Energy’s policy is to fully fund into an external trust the actuarially determined pension costs subject to the limitations of applicable employee benefit and tax laws. In addition to the qualified pension plans, Xcel Energy maintains a SERP and a nonqualified pension plan. The SERP is maintained for certain executives that were participants in the plan in 2008, when the SERP was closed to new participants. The nonqualified pension plan provides benefits for compensation that is in excess of the limits applicable to the qualified pension plans, with distributions funded by Xcel Energy’s consolidated operating cash flows. Obligations of the SERP and nonqualified plan as of Dec. 31, 2019 and 2018 were $39 million and $33 million , respectively, of which $2 million was attributable to SPS in both years. In 2019 and 2018 , Xcel Energy recognized net benefit cost for the SERP and nonqualified plans of $4 million in 2019 and 2018, of which immaterial amounts were attributable to SPS. Xcel Energy, which includes SPS, bases the investment-return assumption on expected long-term performance for each of the asset classes in its pension and postretirement health care portfolios. For pension assets, Xcel Energy considers the historical returns achieved by its asset portfolio over the past 20 years or longer period, as well as long-term projected return levels. Xcel Energy and SPS continually review pension assumptions. Pension cost determination assumes a forecasted mix of investment types over the long-term. • Investment returns in 2019 were above the assumed level of 6.78% ; • Investment returns in 2018 were below the assumed level of 6.78% ; • Investment returns in 2017 were above the assumed level of 6.78% ; and • In 2020 , Xcel Energy’s expected investment-return assumption is 6.78% . Pension plan and postretirement benefit assets are invested in a portfolio according to Xcel Energy’s return, liquidity and diversification objectives to provide a source of funding for plan obligations and minimize contributions to the plan, within appropriate levels of risk. The principal mechanism for achieving these objectives is the asset allocation given the long-term risk, return, correlation and liquidity characteristics of each particular asset class. There were no significant concentrations of risk in any industry, index or entity. Market volatility can impact even well-diversified portfolios and significantly affect the return levels achieved by the assets in any year. State agencies also have issued guidelines to the funding of postretirement benefit costs. SPS is required to fund postretirement benefit costs for Texas and New Mexico amounts collected in rates. These assets are invested in a manner consistent with the investment strategy for the pension plan. Xcel Energy’s ongoing investment strategy is based on plan-specific investment recommendations that seek to minimize potential investment and interest rate risk as a plan’s funded status increases over time. The investment recommendations result in a greater percentage of long-duration fixed income securities being allocated to specific plans having relatively higher funded status ratios and a greater percentage of growth assets being allocated to plans having relatively lower funded status ratios. Plan Assets For each of the fair value hierarchy levels, SPS’ pension plan assets measured at fair value: Dec. 31, 2019 (a) Dec. 31, 2018 (a) (Millions of Dollars) Level 1 Level 2 Level 3 Measured at NAV Total Level 1 Level 2 Level 3 Measured at NAV Total Cash equivalents $ 18.9 $ — $ — $ — $ 18.9 $ 21.6 $ — $ — $ — $ 21.6 Commingled funds 202.5 — — 144.8 347.3 128.6 — — 132.5 261.1 Debt securities — 98.2 0.6 — 98.8 — 98.1 — — 98.1 Equity securities 12.1 — — — 12.1 14.4 — — — 14.4 Other (16.8 ) 0.7 — (2.8 ) (18.9 ) 0.2 0.8 — (4.0 ) (3.0 ) Total $ 216.7 $ 98.9 $ 0.6 $ 142.0 $ 458.2 $ 164.8 $ 98.9 $ — $ 128.5 $ 392.2 (a) See Note 8 for further information on fair value measurement inputs and methods. For each of the fair value hierarchy levels, SPS’ proportionate allocation of the total postretirement benefit plan assets that were measured at fair value: Dec. 31, 2019 (a) Dec. 31, 2018 (a) (Millions of Dollars) Level 1 Level 2 Level 3 Measured at NAV Total Level 1 Level 2 Level 3 Measured at NAV Total Cash equivalents $ 2.2 $ — $ — $ — $ 2.2 $ 1.8 $ — $ — $ — $ 1.8 Insurance contracts — 4.9 — — 4.9 — 4.3 — — 4.3 Commingled funds: 6.7 — — 7.4 14.1 12.8 — — 3.8 16.6 Debt securities: — 22.1 0.1 — 22.2 — 17.2 — — 17.2 Equity securities: — — — — — — — — — — Other — 0.2 — — 0.2 — 0.1 — — 0.1 Total $ 8.9 $ 27.2 $ 0.1 $ 7.4 $ 43.6 $ 14.6 $ 21.6 $ — $ 3.8 $ 40.0 (a) See Note 8 for further information on fair value measurement inputs and methods. Immaterial assets were transferred in or out of Level 3 for 2019. No assets were transferred in or out of Level 3 for 2018. Funded Status — Comparisons of the actuarially computed benefit obligation, changes in plan assets and funded status of the pension and postretirement health care plans for Xcel Energy are presented in the following table: Pension Benefits Postretirement Benefits (Millions of Dollars) 2019 2018 2019 2018 Change in Benefit Obligation: Obligation at Jan. 1 $ 477.8 $ 515.9 $ 41.8 $ 47.0 Service cost 8.8 9.7 0.9 1.1 Interest cost 20.1 18.4 1.7 1.6 Plan amendments — — — — Actuarial loss (gain) 44.2 (34.8 ) 0.4 (5.1 ) Plan participants’ contributions — — 0.6 0.6 Benefit payments (a) (32.1 ) (31.4 ) (2.2 ) (3.4 ) Obligation at Dec. 31 $ 518.8 $ 477.8 $ 43.2 $ 41.8 Change in Fair Value of Plan Assets: Fair value of plan assets at Jan. 1 $ 392.2 $ 433.2 $ 40.0 $ 44.1 Actual return on plan assets 80.2 (17.6 ) 5.1 (1.3 ) Employer contributions 17.9 8.0 0.1 — Plan participants’ contributions — — 0.6 0.6 Benefit payments (32.1 ) (31.4 ) (2.2 ) (3.4 ) Fair value of plan assets at Dec. 31 $ 458.2 $ 392.2 $ 43.6 $ 40.0 Funded status of plans at Dec. 31 $ (60.6 ) $ (85.6 ) $ 0.4 $ (1.8 ) Amounts recognized in the Balance Sheet at Dec. 31: Noncurrent assets — — 0.4 — Noncurrent liabilities (60.6 ) (85.6 ) — (1.8 ) Net amounts recognized $ (60.6 ) $ (85.6 ) $ 0.4 $ (1.8 ) Significant Assumptions Used to Measure Benefit Obligations: Discount rate for year-end valuation 3.49 % 4.31 % 3.47 % 4.32 % Expected average long-term increase in compensation level 3.75 3.75 N/A N/A Mortality table Pri-2012 RP-2014 Pri-2012 RP-2014 Health care costs trend rate — initial: Pre-65 N/A N/A 6.00 % 6.50 % Health care costs trend rate — initial: Post-65 N/A N/A 5.10 % 5.30 % Ultimate trend assumption — initial: Pre-65 N/A N/A 4.50 % 4.50 % Ultimate trend assumption — initial: Post-65 N/A N/A 4.50 % 4.50 % Years until ultimate trend is reached N/A N/A 3 4 (a) Includes approximately $6.8 million in 2019 and $6.9 million in 2018 , of lump-sum benefit payments used in the determination of a settlement charge. Accumulated benefit obligation for the pension plan was $481.1 million and $445.8 million as of Dec. 31, 2019 and 2018 , respectively. Net Periodic Benefit Cost (Credit) — Net periodic benefit cost (credit) other than service cost component is included in other income in the statement of income. Components of net periodic benefit cost (credit) and the amounts recognized in other comprehensive income and regulatory assets and liabilities are as follows: Pension Benefits Postretirement Benefits (Millions of Dollars) 2019 2018 2017 2019 2018 2017 Service cost $ 8.8 $ 9.7 $ 9.8 $ 0.9 $ 1.1 $ 0.9 Interest cost 20.1 18.4 19.7 1.7 1.6 1.7 Expected return on plan assets (28.6 ) (28.3 ) (27.9 ) (2.0 ) (2.5 ) (2.4 ) Amortization of prior service credit (0.1 ) (0.1 ) — (0.5 ) (0.4 ) (0.4 ) Amortization of net loss 11.3 14.1 13.0 (0.4 ) (0.4 ) (0.6 ) Settlement charge (a) 2.4 3.2 — — — — Net periodic pension cost (credit) 13.9 17.0 14.6 (0.3 ) (0.6 ) (0.8 ) Costs not recognized due to effects of regulation 0.9 (2.2 ) 0.3 — — — Net benefit cost (credit) recognized for financial reporting $ 14.8 $ 14.8 $ 14.9 $ (0.3 ) $ (0.6 ) $ (0.8 ) Significant Assumptions Used to Measure Costs: Discount rate 4.31 % 3.63 % 4.13 % 4.32 % 3.62 % 4.13 % Expected average long-term increase in compensation level 3.75 3.75 3.75 — — — Expected average long-term rate of return on assets 6.78 6.78 6.78 5.30 5.80 5.80 (a) A settlement charge is required when the amount of all lump-sum distributions during the year is greater than the sum of the service and interest cost components of the annual net periodic pension cost. In 2019 and 2018 , as a result of lump-sum distributions during the 2019 and 2018 plan years, SPS recorded a total pension settlement charge of $2.4 million and $3.2 million in 2019 and 2018 , respectively. A total of $0.6 million and $0.7 million of that amount was recorded in the income statement in 2019 and 2018, respectively. Pension Benefits Postretirement Benefits (Millions of Dollars) 2019 2018 2019 2018 Amounts Not Yet Recognized as Components of Net Periodic Benefit Cost: Net loss $ 209.7 $ 230.9 $ (11.9 ) $ (9.6 ) Prior service credit (1.1 ) (1.2 ) (1.4 ) (1.8 ) Total $ 208.6 $ 229.7 $ (13.3 ) $ (11.4 ) Amounts Not Yet Recognized as Components of Net Periodic Benefit Cost Have Been Recorded as Follows Based Upon Expected Recovery in Rates: Current regulatory assets $ 11.0 $ 12.9 $ — $ — Noncurrent regulatory assets 197.6 216.8 — — Current regulatory liabilities — — (0.8 ) (0.9 ) Noncurrent regulatory liabilities — — (12.5 ) (10.5 ) Total $ 208.6 $ 229.7 $ (13.3 ) $ (11.4 ) Measurement date Dec. 31, 2019 Dec. 31, 2018 Dec. 31, 2019 Dec. 31, 2018 Cash Flows — Cash funding requirements can be impacted by changes to actuarial assumptions, actual asset levels and other calculations prescribed by the funding requirements of income tax and other pension-related regulations. Required contributions were made in 2017 - 2020 to meet minimum funding requirements. Total voluntary and required pension funding contributions across all four of Xcel Energy’s pension plans were as follows: • $150 million in January 2020 , of which $14 million was attributable to SPS; • $154 million in 2019 , of which $18 million was attributable to SPS; • $150 million in 2018 , of which $8 million was attributable to SPS; and • $162 million in 2017 , of which $24 million was attributable to SPS. For future years, Xcel Energy and SPS anticipate contributions will be made as necessary. The postretirement health care plans have no funding requirements under income tax and other retirement-related regulations other than fulfilling benefit payment obligations, when claims are presented and approved. Additional cash funding requirements are prescribed by certain state and federal rate regulatory authorities. Xcel Energy’s voluntary postretirement funding contributions were as follows: • Expects to contribute approximately $10 million during 2020 ; • $15 million during 2019 ; • $11 million during 2018 ; • $20 million during 2017 ; and • Amounts attributable to SPS were immaterial. Target asset allocations: Pension Benefits Postretirement Benefits 2019 2018 2019 2018 Domestic and international equity securities 37 % 35 % 15 % 18 % Long-duration fixed income securities 30 32 — — Short-to-intermediate fixed income securities 14 16 72 70 Alternative investments 17 15 9 8 Cash 2 2 4 4 Total 100 % 100 % 100 % 100 % Plan Amendments — Xcel Energy, which includes SPS, amended the Xcel Energy Inc. Nonbargaining Pension Plan (South) in 2017 to reduce supplemental benefits for non-bargaining participants as well as to allow the transfer of a portion of non-qualified pension obligations into the qualified plans. In 2019 and 2018 , there were no plan amendments made which affected the benefit obligation. Projected Benefit Payments SPS’ projected benefit payments: (Millions of Dollars) Projected Gross Projected Expected Net Projected 2020 $ 30.7 $ 2.9 $ — $ 2.9 2021 29.4 2.9 — 2.9 2022 30.3 2.9 — 2.9 2023 30.4 2.9 — 2.9 2024 30.4 2.8 — 2.8 2025-2029 153.5 13.2 0.1 13.1 Defined Contribution Plans Xcel Energy, which includes SPS, maintains 401(k) and other defined contribution plans that cover most employees. The expense to these plans for SPS was approximately $3 million in 2019 , 2018 and 2017 .

Commitments and Contingencies

Commitments and Contingencies	12 Months Ended
	Dec. 31, 2019
Commitments and Contingencies Disclosure [Abstract]
Commitments and Contingencies	10. Commitments and Contingencies Legal 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 complex judgments about future events. Management maintains accruals for losses that are probable of being incurred and subject to reasonable estimation. Management may be unable to estimate an amount or range of a reasonably possible loss in certain situations, including 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 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. Rate Matters Texas Fuel Reconciliation — In December 2018, SPS filed an application with the PUCT for reconciliation of fuel costs for the period Jan. 1, 2016, through June 30, 2018, to determine whether all fuel costs incurred were eligible for recovery. In December 2019, the PUCT issued an order disallowing recovery of costs for Texas customers related to two specific solar PPAs. These PPAs were previously approved by the NMPRC as reasonable, necessary and economic. SPS recorded a total disallowance of approximately $6 million in December 2019. SPP OATT Upgrade Costs — Under the SPP OATT, costs of transmission upgrades may be recovered from other SPP customers whose transmission service depends on capacity enabled by the upgrade. SPP had not been charging its customers for these upgrades, even though the SPP OATT had allowed SPP to do so since 2008. In 2016, the FERC granted SPP’s request to recover previously unbilled charges and SPP subsequently billed SPS approximately $13 million . In July 2018, SPS’ appeal to the D.C. Circuit over the FERC rulings granting SPP the right to recover previously unbilled charges was remanded to the FERC. In February 2019, the FERC reversed its 2016 decision and ordered SPP to refund charges retroactively collected from its transmission customers, including SPS, related to periods before September 2015. In April 2019, several parties, including SPP, filed requests for rehearing. Timing of a FERC response to rehearing requests is uncertain. Any refunds received by SPS are expected to be given back to SPS customers through future rates. In October 2017, SPS filed a separate complaint against SPP asserting SPP assessed upgrade charges to SPS in violation of the SPP OATT. The FERC granted a rehearing for further consideration in May 2018. Timing of FERC action on the SPS rehearing is uncertain. If SPS’ complaint results in additional charges or refunds, SPS will seek to recover or refund the amounts through future SPS customer rates. SPP Filing to Assign GridLiance Facilities to SPS Rate Zone — In August 2018, SPP filed a request with the FERC to amend its OATT to include costs of the GridLiance High Plains, LLC. facilities in the SPS rate zone. In a previous filing, the FERC determined that some of these facilities did not qualify as transmission facilities under the SPP OATT. In September 2018, SPS protested the proposed SPP tariff charges, and asked the FERC to reject the SPP filing. On Oct. 31, 2018, the FERC issued an order accepting the proposed charges, subject to refund, as of Nov. 1, 2018, and set the case for settlement hearing procedures. Hearings are scheduled for May 2020, with the ALJs’ initial decision expected in October 2020. SPS has incurred approximately $6 million in associated charges as of Dec. 31, 2019 . SPS Filing to Modify Wholesale Transmission Rates — In 2018, SPS filed revisions to its wholesale transmission formula rate. The proposal includes an update to depreciation rates for transmission plant. The new formula rate would also provide a credit to customers of “excess” ADIT resulting from the TCJA and recover certain wholesale regulatory commission expenses. Proposed changes would increase wholesale transmission revenues by approximately $9.4 million , with approximately $4.4 million of the total recovered in SPP regional transmission rates. SPS proposed formula rate changes be effective Feb. 1, 2019. Environmental New and changing federal and state environmental mandates can create financial liabilities for SPS, which are normally recovered through the regulated rate process. Site Remediation — Various federal and state environmental laws impose liability where hazardous substances or other regulated materials have been released to the environment. SPS may sometimes pay all or a portion of the cost to remediate sites where past activities of SPS’ predecessors or other parties have caused environmental contamination. Environmental contingencies could arise from various situations, including sites of former MGPs; and third-party sites, such as landfills, for which SPS is alleged to have sent wastes to that site. MGP, Landfill or Disposal Sites — SPS is currently remediating the site of a former facility. SPS has recognized its best estimate of costs/liabilities that will result from final resolution of these issues, however, the outcome and timing is unknown. In addition, there may be insurance recovery and/or recovery from other potentially responsible parties, offsetting a portion of costs incurred. Environmental Requirements — Water and Waste Federal CWA WOTUS Rule — In 2015, the EPA and Corps published a final rule that significantly broadened the scope of waters under the CWA that are subject to federal jurisdiction, referred to as “WOTUS”. In 2019, the EPA repealed the 2015 rule and published a draft replacement rule. Until a final rule is issued, SPS cannot estimate potential impacts, but anticipates costs will be recoverable through regulatory mechanisms. Federal CWA ELG — In 2015, the EPA issued a final ELG rule for power plants that discharge treated effluent to surface waters as well as utility-owned landfills that receive CCRs. In 2017, the EPA delayed the compliance date for flue gas desulfurization wastewater and bottom ash transport until November 2020. After 2020, SPS estimates that ELG compliance costs will be immaterial. The EPA, however, is conducting a rulemaking process to revise certain effluent limitations and pretreatment standards, which may impact compliance costs. SPS anticipates these costs will be fully recoverable through regulatory mechanisms. Environmental Requirements — Air Regional Haze Rules — The regional haze program requires SO 2 , nitrogen oxide and particulate matter emission controls at power plants to reduce visibility impairment in national parks and wilderness areas. The program includes BART and reasonable further progress. Texas’ first regional haze plan has undergone federal review as described below. BART Determination for Texas: The EPA has issued a revised final rule adopting a BART alternative Texas only SO 2 trading program that applies to all Harrington and Tolk units. Under the trading program, SPS expects the allowance allocations to be sufficient for SO 2 emissions. The anticipated costs of compliance are not expected to have a material impact; and SPS believes that compliance costs would be recoverable through regulatory mechanisms. Several parties have challenged whether the final rule issued by the EPA should be considered to have met the requirements imposed in a Consent Decree entered by the United States District Court for the District of Columbia that established deadlines for the EPA to take final action on state regional haze plan submissions. The court has required status reports from the parties while the EPA works on the reconsideration rulemaking. In December 2017, the National Parks Conservation Association, Sierra Club, and Environmental Defense Fund appealed the EPA’s 2017 final BART rule to the Fifth Circuit and filed a petition for administrative reconsideration. In January 2018, the court granted SPS’ motion to intervene in the Fifth Circuit litigation in support of the EPA’s final rule. The court has held the litigation in abeyance while the EPA decided whether to reconsider the rule. In August 2018, the EPA started a reconsideration rulemaking, which was supplemented by an additional agency notice in November 2019. It is not known when the EPA will make a final decision on this proposal. Reasonable Progress Rule: In 2016, the EPA adopted a final rule establishing a federal implementation plan for reasonable further progress under the regional haze program for the state of Texas. The rule imposes SO 2 emission limitations that would require the installation of dry scrubbers on Tolk Units 1 and 2, with compliance required by February 2021. Investment costs associated with dry scrubbers could be $600 million . SPS appealed the EPA’s decision and obtained a stay of the final rule. In March 2017, the Fifth Circuit remanded the rule to the EPA for reconsideration, leaving the stay in effect. In a future rulemaking, the EPA will address whether SO 2 emission reductions beyond those required in the BART alternative rule are needed at Tolk under the “reasonable progress” requirements. The EPA has not announced a schedule for acting on the remanded rule. Implementation of the NAAQS for SO 2 — The EPA has designated all areas near SPS’ generating plants as attaining the SO 2 NAAQS with an exception. The EPA issued final designations, which found the area near the Harrington plant as “unclassifiable.” The area near the Harrington plant is to be monitored for three years and a final designation is expected to be made by December 2020. If the area near the Harrington plant is designated nonattainment in 2020, the TCEQ will need to develop an implementation plan, designed to achieve the NAAQS by 2025. The TCEQ could require additional SO 2 controls at Harrington as part of such a plan. SPS cannot evaluate the impacts until the final designation is made and any required state plans are developed. SPS believes that should SO 2 AROs — AROs have been recorded for SPS’ assets. SPS’ AROs were as follows: 2019 (Millions of Dollars) Jan. 1, 2019 Amounts Incurred (a) Amounts (b) Accretion Cash Flow Revisions (c) Dec. 31, 2019 Electric Steam and other production $ 22.0 $ — $ (1.6 ) $ 1.4 $ 29.5 $ 51.3 Wind — 16.0 — 0.4 — 16.4 Distribution 9.1 — — 0.4 — 9.5 Miscellaneous 1.3 — — — (1.2 ) 0.1 Total liability $ 32.4 $ 16.0 $ (1.6 ) $ 2.2 $ 28.3 $ 77.3 (a) Amounts incurred related to the Hale wind farm placed in service in 2019. (b) Amounts settled related to asbestos abatement projects. (c) In 2019, AROs were revised for changes in timing and estimates of cash flows. Changes in steam production AROs primarily related to the cost estimates to remediate ponds at production facilities. 2018 (Millions of Dollars) Jan. 1, 2018 Accretion Cash Flow Revisions (a) Dec. 31, 2018 (b) Electric Steam and other production $ 20.3 $ 1.2 $ 0.5 $ 22.0 Distribution 7.0 0.3 1.8 9.1 Miscellaneous 1.2 0.1 — 1.3 Total liability $ 28.5 $ 1.6 $ 2.3 $ 32.4 (a) In 2018, AROs were revised for changes in timing and estimates of cash flows. Changes in electric distribution AROs were primarily related to increased labor costs. (b) There were no ARO amounts incurred or settled in 2018. Indeterminate AROs — Outside of the recorded asbestos AROs, other plants or buildings may contain asbestos due to the age of many of SPS’ facilities, but no confirmation or measurement of the cost of removal could be determined as of Dec. 31, 2019 . Therefore, an ARO has not been recorded for these facilities. Removal Costs — SPS records a regulatory liability for the plant removal costs that are recovered currently in rates. Removal costs have accumulated based on varying rates as authorized by the appropriate regulatory entities. SPS has estimated the amount of removal costs accumulated through historic depreciation expense based on current factors used in the existing depreciation rates. Removal costs as of Dec. 31, 2019 and 2018 were $174.5 million and $187.7 million , respectively. Leases SPS evaluates contracts that may contain leases, including PPAs and arrangements for the use of office space and other facilities, vehicles and equipment. Under ASC Topic 842, adopted by SPS on Jan. 1, 2019, a contract contains a lease if it conveys the exclusive right to control the use of a specific asset. A contract determined to contain a lease is evaluated further to determine if the arrangement is a finance lease. ROU assets represent SPS’ rights to use leased assets. Starting in 2019, the present value of future operating lease payments are recognized in current and noncurrent operating lease liabilities. These amounts, adjusted for any prepayments or incentives, are recognized as operating lease ROU assets. Most of SPS’ leases do not contain a readily determinable discount rate. Therefore, the present value of future lease payments is generally calculated using the estimated incremental borrowing rate (weighted-average of 4.4% ). SPS has elected the practical expedient under which non-lease components, such as asset maintenance costs included in payments, are not deducted from minimum lease payments for the purposes of lease accounting and disclosure. Leases with an initial term of 12 months or less are classified as short-term leases and are not recognized on the balance sheet. Operating lease ROU assets: (Millions of Dollars) Dec. 31, 2019 PPAs $ 500.3 Other 48.0 Gross operating lease ROU assets 548.3 Accumulated amortization (25.9 ) Net operating lease ROU assets $ 522.4 Components of lease expense: (Millions of Dollars) 2019 2018 2017 Operating leases PPA capacity payments $ 48.1 $ 51.1 $ 51.4 Other operating leases (a) 4.9 7.9 6.4 Total operating lease expense (b) $ 53.0 $ 59.0 $ 57.8 (a) Includes short-term lease expense of $1.5 million , $1.1 million and $1.2 million for 2019 , 2018 and 2017 , respectively. (b) PPA capacity payments are included in electric fuel and purchased power on the statements of income. Expense for other operating leases is included in O&M expense. Commitments under operating leases as of Dec. 31, 2019 : (Millions of Dollars) PPA (a) (b) Operating Leases Other Operating Leases Total Leases 2020 $ 46.2 $ 3.4 $ 49.6 2021 46.2 3.3 49.5 2022 46.2 3.4 49.6 2023 46.2 3.4 49.6 2024 46.2 3.5 49.7 Thereafter 404.5 51.3 455.8 Total minimum obligation 635.5 68.3 703.8 Interest component of obligation (160.0 ) (21.6 ) (181.6 ) Present value of minimum obligation 475.5 46.7 522.2 Less current portion (26.9 ) Noncurrent operating lease liabilities $ 495.3 Weighted-average remaining lease term in years 14.1 (a) Amounts do not include PPAs accounted for as executory contracts and/or contingent payments, such as energy payments on renewable PPAs. (b) PPA operating leases contractually expire at various dates through 2033. Commitments under operating leases as of Dec. 31, 2018 : (Millions of Dollars) PPA (a) (b) Operating Leases Other Operating Leases Total Leases 2019 $ 46.7 $ 5.2 $ 51.9 2020 46.2 5.2 51.4 2021 46.2 5.1 51.3 2022 46.2 5.1 51.3 2023 46.2 5.1 51.3 Thereafter 450.8 56.3 507.1 (a) Amounts do not include PPAs accounted for as executory contracts and/or contingent payments, such as energy payments on renewable PPAs. (b) PPA operating leases contractually expire at various dates through 2033. Non-Lease PPAs — SPS has entered into PPAs with other utilities and energy suppliers with various expiration dates through 2024 for purchased power to meet system load and energy requirements and operating reserve obligations. In general, these agreements provide for energy payments, based on actual energy delivered and capacity payments. Capacity payments are contingent on the IPP meeting contract obligations, including plant availability requirements. Certain contractual payments are adjusted based on market indices. The effects of price adjustments on financial results are mitigated through purchased energy cost recovery mechanisms. Included in electric fuel and purchased power expenses for PPAs accounted for as executory contracts, were payments for capacity of $19.9 million , $57.6 million and $58.4 million in 2019 , 2018 and 2017 , respectively. At Dec. 31, 2019 , the estimated future payments for capacity that SPS is obligated to purchase pursuant to these executory contracts, subject to availability, were as follows: (Millions of Dollars) Capacity 2020 $ 12.3 2021 12.5 2022 12.7 2023 13.0 2024 5.9 Thereafter — Total $ 56.4 Fuel Contracts — SPS has entered into various long-term commitments for the purchase and delivery of a significant portion of its coal and natural gas requirements. These contracts expire between 2020 and 2033 . SPS is required to pay additional amounts depending on actual quantities shipped under these agreements. Estimated minimum purchases under these contracts as of Dec. 31, 2019 : (Millions of Dollars) Coal Natural gas Natural gas 2020 $ 96.7 $ 12.3 $ 28.9 2021 67.7 — 23.3 2022 38.8 — 17.4 2023 — — 12.7 2024 — — 6.7 Thereafter — — 26.3 Total $ 203.2 $ 12.3 $ 115.3 VIEs Under certain PPAs, SPS purchases power from IPPs for which SPS is required to reimburse fuel costs, or to participate in tolling arrangements under which SPS procures the natural gas required to produce the energy that it purchases. SPS has determined that certain IPPs are VIEs. SPS is not subject to risk of loss from the operations of these entities, and no significant financial support is required other than contractual payments for energy and capacity. In addition, certain solar PPAs provide an option to purchase emission allowances or sharing provisions related to production credits generated by the solar facility under contract. These specific PPAs create a variable interest in the IPP. SPS evaluated each of these VIEs for possible consolidation, including review of qualitative factors such as the length and terms of the contract, control over O&M, control over dispatch of electricity, historical and estimated future fuel and electricity prices, and financing activities. SPS 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. SPS had approximately 1,197 MW of capacity under long-term PPAs at both Dec. 31, 2019 and 2018 with entities that have been determined to be VIEs. These agreements have expiration dates through 2041 . Fuel Contracts — SPS purchases all of its coal requirements for its Harrington and Tolk plant from TUCO Inc. under contracts that will expire in December 2022 . TUCO arranges for the purchase, receiving, transporting, unloading, handling, crushing, weighing, and delivery of coal to meet SPS’ requirements. TUCO is responsible for negotiating and administering contracts with coal suppliers, transporters and handlers. SPS has not provided any significant financial support to TUCO, other than contractual payments for delivered coal. However, the fuel contracts create a variable interest in TUCO due to SPS’ reimbursement of fuel procurement costs. SPS has determined that TUCO is a VIE. SPS has concluded that it is not the primary beneficiary of TUCO, because SPS does not have the power to direct the activities that most significantly impact TUCO’s economic performance.

Other Comprehensive Income

Other Comprehensive Income	12 Months Ended
	Dec. 31, 2019
Stockholders' Equity Note [Abstract]
Other Comprehensive Income	es in accumulated other comprehensive loss, net of tax, for the years ended Dec. 31: 2019 (Millions of Dollars) Gains and Losses on Cash Flow Hedges Defined Benefit Pension and Postretirement Items Total Accumulated other comprehensive loss at Jan. 1 $ (0.7 ) $ (0.7 ) $ (1.4 ) Other comprehensive loss before reclassifications (net of taxes of $0 and $(0.1), respectfully — (0.2 ) (0.2 ) Losses reclassified from net accumulated other comprehensive loss: Amortization of net actuarial loss (net of taxes of $0) — 0.2 (a) 0.2 Net current period other comprehensive income (loss) — — — Accumulated other comprehensive loss at Dec. 31 $ (0.7 ) $ (0.7 ) $ (1.4 ) (a) Included in the computation of net periodic pension and postretirement benefit costs. See Note 9 for further information. 2018 (Millions of Dollars) Gains and Losses on Cash Flow Hedges Defined Benefit Pension and Postretirement Items Total Accumulated other comprehensive loss at Jan. 1 $ (0.8 ) $ (0.7 ) $ (1.5 ) Losses reclassified from net accumulated other comprehensive loss: Interest rate derivatives (net of taxes of $0) 0.1 (a) — 0.1 Net current period other comprehensive income 0.1 — 0.1 Accumulated other comprehensive loss at Dec. 31 $ (0.7 ) $ (0.7 ) $ (1.4 ) (a) Included in interest charges.

Related Party Transactions

Related Party Transactions	12 Months Ended
	Dec. 31, 2019
Related Party Transactions [Abstract]
Related Party Transactions	l Energy Services Inc. provides management, administrative and other services for the subsidiaries of Xcel Energy Inc., including SPS. The services are provided and billed to each subsidiary in accordance with service agreements executed by each subsidiary. SPS uses the service provided by Xcel Energy Services Inc. whenever possible. Costs are charged directly to the subsidiary and are allocated if they cannot be directly assigned. Xcel Energy Inc., NSP-Minnesota, PSCo and SPS have established a utility money pool arrangement with the utility subsidiaries. See Note 5 for further information. Significant affiliate transactions among the companies and related parties for the years ended Dec. 31: (Millions of Dollars) 2019 2018 2017 Operating expenses: Purchased power $ — $ — $ 1.4 Other operating expenses — paid to Xcel Energy Services Inc. 192.0 195.1 196.6 Interest expense 0.2 0.6 — Accounts receivable and payable with affiliates at Dec. 31 were: 2019 2018 (Millions of Dollars) Accounts Accounts Accounts Accounts NSP-Minnesota $ 4.2 $ — $ 4.7 $ — PSCo — 0.4 — 0.7 Other subsidiaries of Xcel Energy Inc. — 20.0 5.8 19.2 $ 4.2 $ 20.4 $ 10.5 $ 19.9

Summarized Quarterly Financial

Summarized Quarterly Financial Data (Unaudited)	12 Months Ended
	Dec. 31, 2019
Quarterly Financial Information Disclosure [Abstract]
Summarized Quarterly Financial Data (Unaudited)	Quarter Ended (Millions of Dollars) March 31, 2019 June 30, 2019 Sept. 30, 2019 Dec. 31, 2019 Operating revenues $ 454.1 $ 410.5 $ 533.1 $ 428.1 Operating income 74.5 81.9 135.4 54.9 Net income 54.1 58.8 105.1 45.1 Quarter Ended (Millions of Dollars) March 31, 2018 June 30, 2018 Sept. 30, 2018 Dec. 31, 2018 Operating revenues $ 447.2 $ 481.3 $ 540.1 $ 464.6 Operating income (a) 57.1 87.6 111.0 56.0 Net income 33.1 58.5 81.5 40.2 (a) In 2018, SPS implemented ASU No. 2017-07 related to net periodic benefit cost, which resulted in retrospective reclassification of pension costs from O&M expense to other income.

Schedule II, Valuation and Qual

Schedule II, Valuation and Qualifying Accounts	12 Months Ended
	Dec. 31, 2019
SEC Schedule, 12-09, Valuation and Qualifying Accounts [Abstract]
Schedule II, Valuation and Qualifying Accounts	Southwestern Public Service Co. Valuation and Qualifying Accounts Years Ended Dec. 31 Allowance for bad debts (Millions of Dollars) 2019 2018 2017 Balance at Jan. 1 $ 5.6 $ 6.4 $ 6.4 Additions charged to costs and expenses 5.7 4.9 5.1 Additions charged to other accounts (a) 1.5 1.0 1.2 Deductions from reserves (b) (7.5 ) (6.7 ) (6.3 ) Balance at Dec. 31 $ 5.3 $ 5.6 $ 6.4 (a) Recovery of amounts previously written off. (b) Deductions related primarily to bad debt write-offs.

Summary of Significant Accoun_2

Summary of Significant Accounting Policies (Policies)	12 Months Ended
	Dec. 31, 2019
Accounting Policies [Abstract]
Business and System of Accounts	General — SPS is engaged in the regulated generation, purchase, transmission, distribution and sale of electricity. SPS’ financial statements are presented in accordance with GAAP. All of SPS’ underlying accounting records also conform to the FERC uniform system of accounts or to systems required by various state regulatory commissions. Certain amounts in the 2018 and 2017 financial statements or notes have been reclassified to conform to the 2019 presentation for comparative purposes; however, such reclassifications did not affect net income, total assets, liabilities, equity or cash flows.
Subsequent Events	SPS has evaluated events occurring after Dec. 31, 2019 up to the date of issuance of these financial statements. These statements contain all necessary adjustments and disclosures resulting from that evaluation.
Use of Estimates	Use of Estimates — SPS uses estimates based on the best information available in recording transactions and balances resulting from business operations. Estimates are used on items such as plant depreciable lives or potential disallowances, AROs, certain regulatory assets and liabilities, tax provisions, uncollectible amounts, environmental costs, unbilled revenues, jurisdictional fuel and energy cost allocations and actuarially determined benefit costs. Recorded estimates are revised when better information becomes available or actual amounts can be determined. Revisions can affect operating results.
Regulatory Accounting	Regulatory Accounting — SPS accounts for income and expense items in accordance with accounting guidance for regulated operations. Under this guidance: • Certain costs, which would otherwise be charged to expense or other comprehensive income, are deferred as regulatory assets based on the expected ability to recover the costs in future rates; and • Certain credits, which would otherwise be reflected as income or other comprehensive income, are deferred as regulatory liabilities based on the expectation the amounts will be returned to customers in future rates or because the amounts were collected in rates prior to the costs being incurred. Estimates of recovering deferred costs and returning deferred credits are based on specific ratemaking decisions or precedent for each item. Regulatory assets and liabilities are amortized consistent with the treatment in the rate setting process. If changes in the regulatory environment occur, SPS may no longer be eligible to apply this accounting treatment and may be required to eliminate regulatory assets and liabilities from its balance sheet. Such changes could have a material effect on SPS’ results of operations, financial condition and cash flows. See Note 4 for further information.
Income Taxes	Income Taxes — SPS accounts for income taxes using the asset and liability method, which requires recognition of deferred tax assets and liabilities for the expected future tax consequences of events that have been included in the financial statements. SPS defers income taxes for all temporary differences between pretax financial and taxable income and between the book and tax bases of assets and liabilities. SPS uses rates that are scheduled to be in effect when the temporary differences are expected to reverse. The effect of a change in tax rates on deferred tax assets and liabilities is recognized in the period that includes the enactment date. The effects of SPS’ tax rate changes are generally subject to a normalization method of accounting. Therefore, the revaluation of most its net deferred taxes upon a tax rate reduction results in the establishment of a net regulatory liability, which will be refundable to utility customers over the remaining life of the related assets. A tax rate increase would result in the establishment of a similar regulatory asset. Tax credits are recorded when earned unless there is a requirement to defer the benefit and amortize it over the book depreciable lives of the related property. The requirement to defer and amortize tax credits only applies to federal ITCs related to public utility property. Utility rate regulation also has resulted in the recognition of regulatory assets and liabilities related to income taxes. Deferred tax assets are reduced by a valuation allowance if it is more likely than not that some portion or all of the deferred tax asset will not be realized. SPS follows the applicable accounting guidance to measure and disclose uncertain tax positions that it has taken or expects to take in its income tax returns. SPS recognizes a tax position in its financial statements when it is more likely than not that the position will be sustained upon examination based on the technical merits of the position. Recognition of changes in uncertain tax positions are reflected as a component of income tax expense. SPS reports interest and penalties related to income taxes within the other income and interest charges in the statements of income. Xcel Energy Inc. and its subsidiaries, including SPS, files consolidated federal income tax returns as well as consolidated or separate state income tax returns. Federal income taxes paid by Xcel Energy Inc. are allocated to its subsidiaries based on separate company computations. A similar allocation is made for state income taxes paid by Xcel Energy Inc. in connection with consolidated state filings. Xcel Energy Inc. also allocates its own income tax benefits to its direct subsidiaries. See Note 7 for further information.
Property, Plant and Equipment and Depreciation	Property, Plant and Equipment and Depreciation in Regulated Operations — Property, plant and equipment is stated at original cost. The cost of plant includes direct labor and materials, contracted work, overhead costs and AFUDC. The cost of plant retired is charged to accumulated depreciation and amortization. Amounts recovered in rates for future removal costs are recorded as regulatory liabilities. Significant additions or improvements extending asset lives are capitalized, while repairs and maintenance costs are charged to expense as incurred. Maintenance and replacement of items determined to be less than a unit of property are charged to operating expenses as incurred. Planned maintenance activities are charged to operating expense unless the cost represents the acquisition of an additional unit of property or the replacement of an existing unit of property. Property, plant and equipment is tested for impairment when it is determined that the carrying value of the assets may not be recoverable. A loss is recognized in the current period if it becomes probable that part of a cost of a plant under construction or recently completed plant will be disallowed for recovery from customers and a reasonable estimate of the disallowance can be made. For investments in property, plant and equipment that are abandoned and not expected to go into service, incurred costs and related deferred tax amounts are compared to the discounted estimated future rate recovery, and a loss is recognized, if necessary. SPS records depreciation expense using the straight-line method over the plant’s useful life. Actuarial life studies are performed and submitted to the state and federal commissions for review. Upon acceptance by the various commissions, the resulting lives and net salvage rates are used to calculate depreciation. Depreciation expense, expressed as a percentage of average depreciable property, was 2.9% in 2019 , 2.9% in 2018 and 2.8% in 2017 . See Note 3 for further information.
Asset Retirement Obligations	AROs — SPS accounts for AROs under accounting guidance that requires a liability for the fair value of an ARO to be recognized in the period in which it is incurred if it can be reasonably estimated, with the offsetting associated asset retirement costs capitalized as a long-lived asset. The liability is generally increased over time by applying the effective interest method of accretion, and the capitalized costs are depreciated over the useful life of the long-lived asset. Changes resulting from revisions to the timing or amount of expected asset retirement cash flows are recognized as an increase or a decrease in the ARO. SPS also recovers through rates certain future plant removal costs in addition to AROs. The accumulated removal costs for these obligations are reflected in the balance sheets as a regulatory liability. See Note 10 for further information.
Benefit Plans and Other Postretirement Benefits	Benefit Plans and Other Postretirement Benefits — SPS maintains pension and postretirement benefit plans for eligible employees. Recognizing the cost of providing benefits and measuring the projected benefit obligation of these plans requires management to make various assumptions and estimates.
Environmental Costs	Environmental Costs — Environmental costs are recorded when it is probable SPS is liable for remediation costs and the liability can be reasonably estimated. Costs are deferred as a regulatory asset if it is probable that the costs will be recovered from customers in future rates. Otherwise, the costs are expensed. If an environmental expense is related to facilities currently in use, such as emission-control equipment, the cost is capitalized and depreciated over the life of the plant. Estimated remediation costs are regularly adjusted as estimates are revised and remediation proceeds. If other participating PRPs exist and acknowledge their potential involvement with a site, costs are estimated and recorded only for SPS’ expected share of the cost.
Revenue From Contracts With Customers	Revenue from Contracts with Customers — Performance obligations related to the sale of energy are satisfied as energy is delivered to customers. SPS recognizes revenue that corresponds 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. SPS does not recognize a separate financing component of its collections from customers as contract terms are short-term in nature. SPS presents its revenues net of any excise or sales 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 also recorded on a gross basis. Other revenues and charges related to participating and transacting in RTOs are recorded on a net basis in cost of sales. See Note 6 for further information.
Cash and Cash Equivalents	Cash and Cash Equivalents — SPS considers investments in instruments with a remaining maturity of three months or less at the time of purchase to be cash equivalents.
Accounts Receivable and Allowance for Bad Debts	Accounts Receivable and Allowance for Bad Debts — Accounts receivable are stated at the actual billed amount net of an allowance for bad debts. SPS establishes an allowance for uncollectible receivables based on a policy that reflects its expected exposure to the credit risk of customers. As of Dec. 31, 2019 and 2018 , the allowance for bad debts was $5.3 million and $5.6 million
Inventory	Inventory — Inventory is recorded at average cost and consisted of the following: (Millions of Dollars) Dec. 31, 2019 Dec. 31, 2018 Inventories Materials and supplies $ 24.7 $ 25.7 Fuel 6.3 8.2 Total inventories $ 31.0 $ 33.9
Fair Value Measurements	Fair Value Measurements
Derivative Instruments	Derivative Instruments — SPS uses derivative instruments in connection with its utility commodity price and interest rate activities, including forward contracts, futures, swaps and options. Any derivative instruments not qualifying for the normal purchases and normal sales exception are recorded on the balance sheets at fair value as derivative instruments. Classification of changes in fair value for those derivative instruments is dependent on the designation of a qualifying hedging relationship. Changes in fair value of derivative instruments not designated in a qualifying hedging relationship are reflected in current earnings or as a regulatory asset or liability. Classification as a regulatory asset or liability is based on expected recovery of derivative instrument settlements through fuel and purchased energy cost recovery mechanisms. Interest rate hedging transactions are recorded as a component of interest expense. Normal Purchases and Normal Sales — SPS enters into contracts for purchases and sales of commodities for use in its operations. At inception, contracts are evaluated to determine whether a derivative exists and/or whether an instrument may be exempted from derivative accounting if designated as a normal purchase or normal sale.
AFUDC	AFUDC — AFUDC represents the cost of capital used to finance utility construction activity. AFUDC is computed by applying a composite financing rate to qualified CWIP. The amount of AFUDC capitalized as a utility construction cost is credited to other nonoperating income (for equity capital) and interest charges (for debt capital). AFUDC amounts capitalized are included in SPS’ rate base for establishing utility rates.
Alternative Revenue Programs	Alternative Revenue — Certain rate rider mechanisms (including DSM programs) qualify as alternative revenue programs. 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 expected collection within 24 months , revenue is recognized equal to the revenue requirement, which may include incentives and return on rate base items. Billing amounts are revised periodically for differences between total amount collected and revenue earned, which may increase or decrease the level of revenue collected from customers. Alternative revenues arising from these programs are presented on a gross basis and disclosed separately from revenue from contracts with customers. See Note 6 for further information. Conservation Programs — SPS has implemented programs in its jurisdictions to assist customers in conserving energy and reducing peak demand on the electric system. These programs include commercial motor, air conditioner and lighting upgrades, as well as residential rebates for participation in air conditioner interruption and home weatherization. The costs incurred for some DSM programs are deferred as permitted by the applicable regulatory jurisdiction. For those programs, costs are deferred if it is probable future revenue will be provided to permit recovery of the incurred cost. Revenues recognized for incentive programs designed for recovery of lost margins and/or conservation performance incentives are limited to amounts expected to be collected within 24 months from the annual period in which they are earned. SPS recovers approved conservation program costs in base rate revenue or through a rider.
Emission Allowances	Emission Allowances — Emission allowances are recorded at cost, including broker commission fees. The inventory accounting model is utilized for all emission allowances and sales of these allowances are included in electric revenues.
Renewable Energy Credits	RECs — Cost of RECs that are utilized for compliance is recorded as electric fuel and purchased power expense. SPS reduces recoverable fuel costs for the cost of RECs and records that cost as a regulatory asset when the amount is recoverable in future rates.
Segment Reporting, Policy [Policy Text Block]	Segment Information — SPS has only one reportable segment. SPS is a wholly owned subsidiary of Xcel Energy Inc. and operates in the regulated electric utility industry providing wholesale and retail electric service in the states of Texas and New Mexico.

Summary of Significant Accoun_3

Summary of Significant Accounting Policies Inventory (Tables)	12 Months Ended
	Dec. 31, 2019
Balance Sheet Related Disclosure - Inventory [Abstract]
Schedule of Utility Inventory [Table Text Block]	Inventory is recorded at average cost and consisted of the following: (Millions of Dollars) Dec. 31, 2019 Dec. 31, 2018 Inventories Materials and supplies $ 24.7 $ 25.7 Fuel 6.3 8.2 Total inventories $ 31.0 $ 33.9

Property Plant and Equipment _2

Property Plant and Equipment Property Plant and Equipment (Tables)	12 Months Ended
	Dec. 31, 2019
Property, Plant and Equipment [Abstract]
Public Utility Property, Plant, and Equipment	Major classes of property, plant and equipment (Millions of Dollars) Dec. 31, 2019 Dec. 31, 2018 Property, plant and equipment Electric plant $ 8,453.0 $ 7,227.7 CWIP 485.4 847.3 Total property, plant and equipment 8,938.4 8,075.0 Less accumulated depreciation (2,306.8 ) (2,128.6 ) Property, plant and equipment, net $ 6,631.6 $ 5,946.4

Regulatory Assets and Liabili_2

Regulatory Assets and Liabilities (Tables)	12 Months Ended
	Dec. 31, 2019
Regulatory Assets and Liabilities Disclosure [Abstract]
Regulatory Assets	Components of regulatory assets: (Millions of Dollars) See Remaining Dec. 31, 2019 Dec. 31, 2018 Regulatory Assets Current Noncurrent Current Noncurrent Pension and retiree medical obligations 9 Various $ 11.1 $ 203.5 $ 12.6 $ 222.1 Excess deferred taxes — TCJA 7 Various 1.7 52.0 — 55.9 Recoverable deferred taxes on AFUDC recorded in plant Plant lives — 34.1 — 27.9 Net AROs (a) 1, 10 Plant lives — 26.9 — 25.7 Losses on reacquired debt Term of related debt 0.8 21.0 0.8 21.9 Conservation programs (b) 1 One to two years 0.6 1.1 0.7 0.6 Other Various 5.8 25.4 11.9 12.1 Total regulatory assets $ 20.0 $ 364.0 $ 26.0 $ 366.2 (a) Includes amounts recorded for future recovery of AROs. (b) Includes costs for conservation programs, as well as incentives allowed in certain jurisdictions.
Regulatory Liabilities	Components of regulatory liabilities: (Millions of Dollars) See Remaining Dec. 31, 2019 Dec. 31, 2018 Regulatory Liabilities Current Noncurrent Current Noncurrent Deferred income tax adjustments and TCJA refunds (a) 7 Various $ 6.9 $ 534.9 $ 2.2 $ 569.8 Plant removal costs 1, 10 Plant lives — 174.5 — 187.7 Revenue subject to refund One to two years 14.6 1.1 11.3 8.1 Gain from asset sales Various — 2.4 — 2.4 Deferred electric energy costs Less than one year 81.6 — 56.5 — Contract valuation adjustments (b) 1, 8 Less than one year 11.7 — 14.7 — Other Various 3.3 19.4 1.1 12.9 Total regulatory liabilities (c) $ 118.1 $ 732.3 $ 85.8 $ 780.9 (a) Includes the revaluation of recoverable/regulated plant ADIT and revaluation impact of non-plant ADIT due to the TCJA. (b) Includes the fair value of certain long-term PPAs used to meet energy capacity requirements. (c) Revenue subject to refund of $3.9 million for 2019 and none for 2018 is included in other current liabilities.

Borrowings and Other Financin_2

Borrowings and Other Financing Instruments (Tables)	12 Months Ended
	Dec. 31, 2019
Debt Disclosure [Abstract]
Money Pool [Table Text Block]	Money pool borrowings for SPS were as follows: (Millions of Dollars, Except Interest Rates) Three Months Ended Dec. 31, 2019 Year Ended Dec. 31 2019 2018 2017 Borrowing limit $ 100 $ 100 $ 100 $ 100 Amount outstanding at period end — — — — Average amount outstanding 1 8 29 13 Maximum amount outstanding 12 100 100 100 Weighted average interest rate, computed on a daily basis 1.63 % 2.42 % 1.96 % 1.12 % Weighted average interest rate at end of period N/A N/A N/A N/A
Short-Term Borrowings	Commercial paper outstanding for SPS was as follows: (Millions of Dollars, Except Interest Rates) Three Months Ended Dec. 31, 2019 Year Ended Dec. 31 2019 2018 2017 Borrowing limit $ 500 $ 500 $ 400 $ 400 Amount outstanding at period end — — 42 — Average amount outstanding — 72 30 69 Maximum amount outstanding — 316 144 176 Weighted average interest rate, computed on a daily basis N/A 2.68 % 2.27 % 1.13 % Weighted average interest rate at end of period N/A N/A 2.80 NA
Schedule Of Debt To Total Capitalization Ratio [Table Text Block]	Features of SPS’ credit facility: Debt-to-Total Capitalization Ratio (a) Amount Facility May Be Increased (millions) Additional Periods for Which a One-Year Extension May Be Requested (b) 2019 2018 46% 46% $50 2 (a) The SPS credit facility has a financial covenant requiring that the debt-to-total capitalization ratio be less than or equal to 65% . (b) All extension requests are subject to majority bank group approval.
Credit Facilities	SPS had the following committed credit facilities available as of Dec. 31, 2019 . Credit Facility (a) Drawn (b) Available $500 $2 $498 (a) This credit facility matures in June 2024 . (b) Includes letters of credit and outstanding commercial paper.
Schedule of Long-term Debt Instruments [Table Text Block]	Long-term debt obligations for SPS as of Dec. 31 (millions of dollars): Financing Instrument Interest Rate Maturity Date 2019 2018 First mortgage bonds 3.30 % June 15, 2024 $ 150 $ 150 First mortgage bonds 3.30 June 15, 2024 200 200 Unsecured senior notes 6.00 Oct. 1, 2033 100 100 Unsecured senior notes 6.00 Oct. 1, 2036 250 250 First mortgage bonds 4.50 Aug. 15, 2041 200 200 First mortgage bonds 4.50 Aug. 15, 2041 100 100 First mortgage bonds 4.50 Aug. 15, 2041 100 100 First mortgage bonds 3.40 Aug. 15, 2046 300 300 First mortgage bonds 3.70 Aug. 15, 2047 450 450 First mortgage bonds (b) 4.40 Nov. 15, 2048 300 300 First mortgage bonds (a) 3.75 June 15, 2049 300 — Unamortized discount (7 ) (4 ) Unamortized debt issuance cost (23 ) (20 ) Total long-term debt $ 2,420 $ 2,126 (a) 2019 financing (b) 2018 financing
Dividend Payment Restrictions [Text Block]	Requirements and actuals as of Dec. 31, 2019 : Equity to Total Capitalization Ratio - Required Range Equity to Total Capitalization Ratio - Actual (a) Low High 2019 45.0 % 55.0 % 54.4 % (a) Excludes short-term debt. Unrestricted Retained Earnings Total Capitalization Limit on Total Capitalization (a) $ 535.0 million $ 5.3 billion N/A (a)
Schedule of Maturities of Long-term Debt [Table Text Block]	Maturities of long-term debt: (Millions of Dollars) 2020 $ — 2021 — 2022 — 2023 — 2024 350
Schedule of Stock by Class [Table Text Block]	SPS has the following preferred stock: Preferred Stock Authorized (Shares) Par Value of Preferred Stock Preferred Stock Outstanding (Shares) 2019 and 2018 10,000,000 1.00 —

Revenues (Tables)

Revenues (Tables)	12 Months Ended
	Dec. 31, 2019
Revenue from Contract with Customer [Abstract]
Disaggregation of Revenue [Table Text Block]	SPS’ operating revenues consisted of the following: (Millions of Dollars) Year Ended Dec. 31, 2019 Major product lines Revenue from contracts with customers: Residential $ 351.9 C&I 800.3 Other 41.1 Total retail 1,193.3 Wholesale 361.0 Transmission 239.6 Other 3.3 Total revenue from contracts with customers 1,797.2 Alternative revenue and other 28.6 Total revenues $ 1,825.8 (Millions of Dollars) Year Ended Dec. 31, 2018 Major product lines Revenue from contracts with customers: Residential $ 363.7 C&I 828.3 Other 44.7 Total retail 1,236.7 Wholesale 426.0 Transmission 231.1 Other 12.8 Total revenue from contracts with customers 1,906.6 Alternative revenue and other 26.6 Total revenues $ 1,933.2

Income Taxes (Tables)

Income Taxes (Tables)	12 Months Ended
	Dec. 31, 2019
Income Tax Disclosure [Abstract]
Summary of Statute of Limitations Applicable to Open Tax Years	SPS is a member of the Xcel Energy affiliated group that files a consolidated federal income tax return. Statute of limitations applicable to Xcel Energy’s consolidated federal income tax returns expire as follows: Tax Year(s) Expiration 2009 - 2013 June 2020 2014 - 2016 September 2020
Reconciliation of Unrecognized Tax Benefits	Unrecognized tax benefits — permanent vs temporary: (Millions of Dollars) Dec. 31, 2019 Dec. 31, 2018 Unrecognized tax benefit — Permanent tax positions $ 3.7 $ 3.0 Unrecognized tax benefit — Temporary tax positions 1.5 1.5 Total unrecognized tax benefit $ 5.2 $ 4.5 Changes in unrecognized tax benefits: (Millions of Dollars) 2019 2018 2017 Balance at Jan. 1 $ 4.5 $ 4.3 $ 28.7 Additions based on tax positions related to the current year 0.7 0.6 0.9 Reductions based on tax positions related to the current year (0.1 ) (0.1 ) (0.6 ) Additions for tax positions of prior years 0.2 0.1 1.3 Reductions for tax positions of prior years (0.1 ) (0.3 ) (19.9 ) Settlements with taxing authorities — (0.1 ) (6.1 ) Balance at Dec. 31 $ 5.2 $ 4.5 $ 4.3
Tax Benefits Associated with NOL and Tax Credit Carryforwards	Unrecognized tax benefits were reduced by tax benefits associated with NOL and tax credit carryforwards: (Millions of Dollars) Dec. 31, 2019 Dec. 31, 2018 NOL and tax credit carryforwards $ (4.4 ) $ (3.8 )
Interest Payable related to Unrecognized Tax Benefits	Interest payable related to unrecognized tax benefits: (Millions of Dollars) 2019 2018 2017 Receivable (payable) for interest related to unrecognized tax benefits at Jan. 1 $ 0.7 $ 0.5 $ (0.9 ) Interest income related to unrecognized tax benefits — 0.2 1.4 Receivable for interest related to unrecognized tax benefits at Dec. 31 $ 0.7 $ 0.7 $ 0.5
NOL and Tax Credit Carryforwards	NOL amounts represent the tax loss that is carried forward and tax credits represent the deferred tax asset. NOL and tax credit carryforwards as of Dec. 31 were as follows: (Millions of Dollars) 2019 2018 Federal tax credit carryforwards $ 29.5 $ 5.7 State NOL carryforwards 1.2 2.9
Schedule of Effective Income Tax Rate Reconciliation	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. Effective income tax rate for years ended Dec. 31: 2019 2018 (a) 2017 (a) Federal statutory rate 21.0 % 21.0 % 35.0 % State income tax on pretax income, net of federal tax effect 2.2 % 2.3 % 2.0 % Increases (decreases) in tax from: Wind PTCs (7.9 ) — — Plant regulatory differences (b) (5.0 ) (4.8 ) (0.9 ) Amortization of excess nonplant deferred taxes (0.9 ) (1.2 ) — Other tax credits, net of NOL & tax credit allowances (0.6 ) (0.7 ) (0.6 ) Adjustments attributable to tax returns (0.1 ) (1.5 ) (0.4 ) Change in unrecognized tax benefits 0.2 0.1 (1.0 ) Tax reform — — (3.5 ) Other, net — 0.2 (0.5 ) Effective income tax rate 8.9 % 15.4 % 30.1 % (a) Prior periods have been reclassified to conform to current year presentation. (b) Regulatory differences for income tax primarily relate to the credit of excess deferred taxes to customers through the average rate assumption method. Income tax benefits associated with the credit of excess deferred credits are offset by corresponding revenue reductions.
Schedule of Components of Income Tax Expense (Benefit)	Components of income tax expense for years ended Dec. 31: (Millions of Dollars) 2019 2018 2017 Current federal tax (benefit) expense $ (3.9 ) $ 12.3 $ (20.9 ) Current state tax expense (benefit) 0.6 2.3 (12.8 ) Current change in unrecognized tax expense (benefit) — 2.3 (24.3 ) Deferred federal tax expense 22.3 20.5 89.9 Deferred state tax expense 6.0 3.6 14.5 Deferred change in unrecognized tax expense (benefit) 0.7 (2.0 ) 22.1 Deferred ITCs (0.1 ) (0.1 ) (0.1 ) Total income tax expense $ 25.6 $ 38.9 $ 68.4 Components of deferred income tax expense as of Dec. 31: (Millions of Dollars) 2019 2018 2017 Deferred tax expense (benefit) excluding items below $ 52.7 $ 44.2 $ (414.2 ) Amortization and adjustments to deferred income taxes on income tax regulatory assets and liabilities (23.8 ) (22.0 ) 540.7 Tax benefit (expense) allocated to other comprehensive income, net of adoption of ASU No. 2018-02, and other 0.1 (0.1 ) — Deferred tax expense $ 29.0 $ 22.1 $ 126.5
Schedule of Deferred Tax Assets and Liabilities	Components of the net deferred tax liability as of Dec. 31: (Millions of Dollars) 2019 2018 (a) Deferred tax liabilities: Differences between book and tax bases of property $ 758.7 $ 680.6 Operating lease assets 115.8 — Regulatory assets 49.7 49.2 Pension expense 33.1 32.3 Total deferred tax liabilities $ 957.3 $ 762.1 Deferred tax assets: Regulatory liabilities $ 111.2 $ 116.8 Operating lease liabilities 115.8 — Tax credit carryforward 29.5 5.7 Deferred fuel costs 18.3 12.7 Other employee benefits 5.8 5.6 NOL carryforward 0.1 0.2 Other 4.8 2.0 Total deferred tax assets 285.5 143.0 Net deferred tax liability $ 671.8 $ 619.1 (a) Prior periods have been reclassified to conform to current year presentation.

Fair Value of Financial Asset_2

Fair Value of Financial Assets and Liabilities (Tables)	12 Months Ended
	Dec. 31, 2019
Fair Value Disclosures [Abstract]
Gross Notional Amounts of Commodity FTRs	Gross notional amounts of commodity FTRs at Dec. 31, 2019 and 2018 : (Amounts in Millions) (a) Dec. 31, 2019 Dec. 31, 2018 MWh of electricity 6.4 5.5 (a) Amounts are not reflective of net positions in the underlying commodities.
Financial Impact of Qualifying Cash Flow Hedges on Accumulated Other Comprehensive Loss	(Millions of Dollars) 2019 2018 2017 Accumulated other comprehensive loss related to cash flow hedges at Jan. 1 $ (0.7 ) $ (0.8 ) $ (0.7 ) After-tax net realized losses on derivative transactions reclassified into earnings — 0.1 — Adoption of ASU. 2018-02 (a) — — (0.1 ) Accumulated other comprehensive loss related to cash flow hedges at Dec. 31 $ (0.7 ) $ (0.7 ) $ (0.8 ) (a) In 2017, SPS implemented ASU No. 2018-02 related to TCJA, which resulted in reclassification of certain credit balances within net accumulated other comprehensive loss to retained earnings.
Derivative Assets and Liabilities Measured at Fair Value on a Recurring Basis by Hierarchy Level	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 at Dec. 31, 2019 and 2018 : Dec. 31, 2019 Dec. 31, 2018 Fair Value Fair Value (Millions of Dollars) Level 1 Level 2 Level 3 Fair Value Total Netting (a) Total Level 1 Level 2 Level 3 Fair Value Total Netting (a) Total Current derivative assets Other derivative instruments: Electric commodity $ — $ — $ 11.8 $ 11.8 $ — $ 11.8 $ — $ — $ 14.9 $ 14.9 $ (0.2 ) $ 14.7 Total current derivative assets $ — $ — $ 11.8 $ 11.8 $ — 11.8 $ — $ — $ 14.9 $ 14.9 $ (0.2 ) 14.7 PPAs (b) 3.2 3.1 Current derivative instruments $ 15.0 $ 17.8 Noncurrent derivative assets PPAs (b) 12.6 15.8 Noncurrent derivative instruments $ 12.6 $ 15.8 Current derivative liabilities Other derivative instruments: Electric commodity $ — $ — $ 0.1 $ 0.1 $ — $ 0.1 $ — $ — $ 0.2 $ 0.2 $ (0.2 ) $ — Total current derivative liabilities $ — $ — $ 0.1 $ 0.1 $ — 0.1 $ — $ — $ 0.2 $ 0.2 $ (0.2 ) — PPAs (b) 3.6 3.6 Current derivative instruments $ 3.7 $ 3.6 Noncurrent derivative liabilities PPAs (b) 12.8 16.4 Noncurrent derivative instruments $ 12.8 $ 16.4 (a) 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, 2019 and 2018 . At both Dec. 31, 2019 and 2018 , derivative assets and liabilities include no obligations to return cash collateral or rights to reclaim cash collateral. The counterparty netting excludes settlement receivables and payables and non-derivative amounts that may be subject to the same master netting agreements. (b) 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 will be amortized over the remaining contract lives along with the offsetting regulatory assets and liabilities.
Changes in Level 3 Commodity Derivatives	Changes in Level 3 commodity derivatives for the years ended Dec. 31, 2019 , 2018 and 2017 : Year Ended Dec. 31 (Millions of Dollars) 2019 2018 2017 Balance at Jan. 1 $ 14.7 $ 12.7 $ 2.0 Purchases 26.7 32.3 41.2 Settlements (34.2 ) (41.6 ) (55.8 ) Net transactions recorded during the period: Net gains recognized as regulatory assets 4.5 11.3 25.3 Balance at Dec. 31 $ 11.7 $ 14.7 $ 12.7
Carrying Amount and Fair Value of Long-term Debt	2019 2018 (Millions of Dollars) Carrying Amount Fair Value Carrying Amount Fair Value Long-term debt, including current portion $ 2,419.7 $ 2,706.1 $ 2,126.1 $ 2,139.8

Benefit Plans and Other Postr_2

Benefit Plans and Other Postretirement Benefits (Tables)	12 Months Ended
	Dec. 31, 2019
Benefit Plans and Other Postretirement Benefits [Abstract]
Target Asset Allocations and Plan Assets Measured at Fair Value	or each of the fair value hierarchy levels, SPS’ pension plan assets measured at fair value: Dec. 31, 2019 (a) Dec. 31, 2018 (a) (Millions of Dollars) Level 1 Level 2 Level 3 Measured at NAV Total Level 1 Level 2 Level 3 Measured at NAV Total Cash equivalents $ 18.9 $ — $ — $ — $ 18.9 $ 21.6 $ — $ — $ — $ 21.6 Commingled funds 202.5 — — 144.8 347.3 128.6 — — 132.5 261.1 Debt securities — 98.2 0.6 — 98.8 — 98.1 — — 98.1 Equity securities 12.1 — — — 12.1 14.4 — — — 14.4 Other (16.8 ) 0.7 — (2.8 ) (18.9 ) 0.2 0.8 — (4.0 ) (3.0 ) Total $ 216.7 $ 98.9 $ 0.6 $ 142.0 $ 458.2 $ 164.8 $ 98.9 $ — $ 128.5 $ 392.2 (a) See Note 8 for further information on fair value measurement inputs and methods. For each of the fair value hierarchy levels, SPS’ proportionate allocation of the total postretirement benefit plan assets that were measured at fair value: Dec. 31, 2019 (a) Dec. 31, 2018 (a) (Millions of Dollars) Level 1 Level 2 Level 3 Measured at NAV Total Level 1 Level 2 Level 3 Measured at NAV Total Cash equivalents $ 2.2 $ — $ — $ — $ 2.2 $ 1.8 $ — $ — $ — $ 1.8 Insurance contracts — 4.9 — — 4.9 — 4.3 — — 4.3 Commingled funds: 6.7 — — 7.4 14.1 12.8 — — 3.8 16.6 Debt securities: — 22.1 0.1 — 22.2 — 17.2 — — 17.2 Equity securities: — — — — — — — — — — Other — 0.2 — — 0.2 — 0.1 — — 0.1 Total $ 8.9 $ 27.2 $ 0.1 $ 7.4 $ 43.6 $ 14.6 $ 21.6 $ — $ 3.8 $ 40.0 (a) See Note 8 for further information on fair value measurement inputs and methods. Target asset allocations: Pension Benefits Postretirement Benefits 2019 2018 2019 2018 Domestic and international equity securities 37 % 35 % 15 % 18 % Long-duration fixed income securities 30 32 — — Short-to-intermediate fixed income securities 14 16 72 70 Alternative investments 17 15 9 8 Cash 2 2 4 4 Total 100 % 100 % 100 % 100 %
Funded Status of Plans	Funded Status — Comparisons of the actuarially computed benefit obligation, changes in plan assets and funded status of the pension and postretirement health care plans for Xcel Energy are presented in the following table: Pension Benefits Postretirement Benefits (Millions of Dollars) 2019 2018 2019 2018 Change in Benefit Obligation: Obligation at Jan. 1 $ 477.8 $ 515.9 $ 41.8 $ 47.0 Service cost 8.8 9.7 0.9 1.1 Interest cost 20.1 18.4 1.7 1.6 Plan amendments — — — — Actuarial loss (gain) 44.2 (34.8 ) 0.4 (5.1 ) Plan participants’ contributions — — 0.6 0.6 Benefit payments (a) (32.1 ) (31.4 ) (2.2 ) (3.4 ) Obligation at Dec. 31 $ 518.8 $ 477.8 $ 43.2 $ 41.8 Change in Fair Value of Plan Assets: Fair value of plan assets at Jan. 1 $ 392.2 $ 433.2 $ 40.0 $ 44.1 Actual return on plan assets 80.2 (17.6 ) 5.1 (1.3 ) Employer contributions 17.9 8.0 0.1 — Plan participants’ contributions — — 0.6 0.6 Benefit payments (32.1 ) (31.4 ) (2.2 ) (3.4 ) Fair value of plan assets at Dec. 31 $ 458.2 $ 392.2 $ 43.6 $ 40.0 Funded status of plans at Dec. 31 $ (60.6 ) $ (85.6 ) $ 0.4 $ (1.8 ) Amounts recognized in the Balance Sheet at Dec. 31: Noncurrent assets — — 0.4 — Noncurrent liabilities (60.6 ) (85.6 ) — (1.8 ) Net amounts recognized $ (60.6 ) $ (85.6 ) $ 0.4 $ (1.8 ) Significant Assumptions Used to Measure Benefit Obligations: Discount rate for year-end valuation 3.49 % 4.31 % 3.47 % 4.32 % Expected average long-term increase in compensation level 3.75 3.75 N/A N/A Mortality table Pri-2012 RP-2014 Pri-2012 RP-2014 Health care costs trend rate — initial: Pre-65 N/A N/A 6.00 % 6.50 % Health care costs trend rate — initial: Post-65 N/A N/A 5.10 % 5.30 % Ultimate trend assumption — initial: Pre-65 N/A N/A 4.50 % 4.50 % Ultimate trend assumption — initial: Post-65 N/A N/A 4.50 % 4.50 % Years until ultimate trend is reached N/A N/A 3 4 (a) Includes approximately $6.8 million in 2019 and $6.9 million in 2018 , of lump-sum benefit payments used in the determination of a settlement charge. Accumulated benefit obligation for the pension plan was $481.1 million and $445.8 million as of Dec. 31, 2019 and 2018 , respectively.
Components of Net Periodic Benefit Costs	Components of net periodic benefit cost (credit) and the amounts recognized in other comprehensive income and regulatory assets and liabilities are as follows: Pension Benefits Postretirement Benefits (Millions of Dollars) 2019 2018 2017 2019 2018 2017 Service cost $ 8.8 $ 9.7 $ 9.8 $ 0.9 $ 1.1 $ 0.9 Interest cost 20.1 18.4 19.7 1.7 1.6 1.7 Expected return on plan assets (28.6 ) (28.3 ) (27.9 ) (2.0 ) (2.5 ) (2.4 ) Amortization of prior service credit (0.1 ) (0.1 ) — (0.5 ) (0.4 ) (0.4 ) Amortization of net loss 11.3 14.1 13.0 (0.4 ) (0.4 ) (0.6 ) Settlement charge (a) 2.4 3.2 — — — — Net periodic pension cost (credit) 13.9 17.0 14.6 (0.3 ) (0.6 ) (0.8 ) Costs not recognized due to effects of regulation 0.9 (2.2 ) 0.3 — — — Net benefit cost (credit) recognized for financial reporting $ 14.8 $ 14.8 $ 14.9 $ (0.3 ) $ (0.6 ) $ (0.8 ) Significant Assumptions Used to Measure Costs: Discount rate 4.31 % 3.63 % 4.13 % 4.32 % 3.62 % 4.13 % Expected average long-term increase in compensation level 3.75 3.75 3.75 — — — Expected average long-term rate of return on assets 6.78 6.78 6.78 5.30 5.80 5.80 (a) A settlement charge is required when the amount of all lump-sum distributions during the year is greater than the sum of the service and interest cost components of the annual net periodic pension cost. In 2019 and 2018 , as a result of lump-sum distributions during the 2019 and 2018 plan years, SPS recorded a total pension settlement charge of $2.4 million and $3.2 million in 2019 and 2018 , respectively. A total of $0.6 million and $0.7 million of that amount was recorded in the income statement in 2019 and 2018, respectively.
Projected Benefit Payments for the Pension and Postretirement Benefit Plans	Projected Benefit Payments SPS’ projected benefit payments: (Millions of Dollars) Projected Gross Projected Expected Net Projected 2020 $ 30.7 $ 2.9 $ — $ 2.9 2021 29.4 2.9 — 2.9 2022 30.3 2.9 — 2.9 2023 30.4 2.9 — 2.9 2024 30.4 2.8 — 2.8 2025-2029 153.5 13.2 0.1 13.1
Other Postretirement Benefits Plan [Member]
Benefit Plans and Other Postretirement Benefits [Abstract]
Amounts Not Yet Recognized as Components of Net Periodic Benefit Cost	Pension Benefits Postretirement Benefits (Millions of Dollars) 2019 2018 2019 2018 Amounts Not Yet Recognized as Components of Net Periodic Benefit Cost: Net loss $ 209.7 $ 230.9 $ (11.9 ) $ (9.6 ) Prior service credit (1.1 ) (1.2 ) (1.4 ) (1.8 ) Total $ 208.6 $ 229.7 $ (13.3 ) $ (11.4 ) Amounts Not Yet Recognized as Components of Net Periodic Benefit Cost Have Been Recorded as Follows Based Upon Expected Recovery in Rates: Current regulatory assets $ 11.0 $ 12.9 $ — $ — Noncurrent regulatory assets 197.6 216.8 — — Current regulatory liabilities — — (0.8 ) (0.9 ) Noncurrent regulatory liabilities — — (12.5 ) (10.5 ) Total $ 208.6 $ 229.7 $ (13.3 ) $ (11.4 ) Measurement date Dec. 31, 2019 Dec. 31, 2018 Dec. 31, 2019 Dec. 31, 2018

Commitments and Contingencies (

Commitments and Contingencies (Tables)	12 Months Ended
	Dec. 31, 2019
Commitments and Contingencies Disclosure [Abstract]
Assets and Liabilities, Lessee [Table Text Block]	Operating lease ROU assets: (Millions of Dollars) Dec. 31, 2019 PPAs $ 500.3 Other 48.0 Gross operating lease ROU assets 548.3 Accumulated amortization (25.9 ) Net operating lease ROU assets $ 522.4
Lease, Cost [Table Text Block]	Components of lease expense: (Millions of Dollars) 2019 2018 2017 Operating leases PPA capacity payments $ 48.1 $ 51.1 $ 51.4 Other operating leases (a) 4.9 7.9 6.4 Total operating lease expense (b) $ 53.0 $ 59.0 $ 57.8 (a) Includes short-term lease expense of $1.5 million , $1.1 million and $1.2 million for 2019 , 2018 and 2017 , respectively. (b) PPA capacity payments are included in electric fuel and purchased power on the statements of income. Expense for other operating leases is included in O&M expense.
Lessee, Operating Lease, Liability, Maturity [Table Text Block]	Commitments under operating leases as of Dec. 31, 2019 : (Millions of Dollars) PPA (a) (b) Operating Leases Other Operating Leases Total Leases 2020 $ 46.2 $ 3.4 $ 49.6 2021 46.2 3.3 49.5 2022 46.2 3.4 49.6 2023 46.2 3.4 49.6 2024 46.2 3.5 49.7 Thereafter 404.5 51.3 455.8 Total minimum obligation 635.5 68.3 703.8 Interest component of obligation (160.0 ) (21.6 ) (181.6 ) Present value of minimum obligation 475.5 46.7 522.2 Less current portion (26.9 ) Noncurrent operating lease liabilities $ 495.3 Weighted-average remaining lease term in years 14.1 (a) Amounts do not include PPAs accounted for as executory contracts and/or contingent payments, such as energy payments on renewable PPAs. (b) PPA operating leases contractually expire at various dates through 2033.
Estimated Minimum Purchases Under Fuel Contracts	Estimated minimum purchases under these contracts as of Dec. 31, 2019 : (Millions of Dollars) Coal Natural gas Natural gas 2020 $ 96.7 $ 12.3 $ 28.9 2021 67.7 — 23.3 2022 38.8 — 17.4 2023 — — 12.7 2024 — — 6.7 Thereafter — — 26.3 Total $ 203.2 $ 12.3 $ 115.3
Estimated Future Payments for Capacity and Energy Pursuant to Purchased Power Agreements	At Dec. 31, 2019 , the estimated future payments for capacity that SPS is obligated to purchase pursuant to these executory contracts, subject to availability, were as follows: (Millions of Dollars) Capacity 2020 $ 12.3 2021 12.5 2022 12.7 2023 13.0 2024 5.9 Thereafter — Total $ 56.4
Future Commitments Under Operating Leases	Commitments under operating leases as of Dec. 31, 2018 : (Millions of Dollars) PPA (a) (b) Operating Leases Other Operating Leases Total Leases 2019 $ 46.7 $ 5.2 $ 51.9 2020 46.2 5.2 51.4 2021 46.2 5.1 51.3 2022 46.2 5.1 51.3 2023 46.2 5.1 51.3 Thereafter 450.8 56.3 507.1 (a) Amounts do not include PPAs accounted for as executory contracts and/or contingent payments, such as energy payments on renewable PPAs. (b) PPA operating leases contractually expire at various dates through 2033.
Asset Retirement Obligations	SPS’ AROs were as follows: 2019 (Millions of Dollars) Jan. 1, 2019 Amounts Incurred (a) Amounts (b) Accretion Cash Flow Revisions (c) Dec. 31, 2019 Electric Steam and other production $ 22.0 $ — $ (1.6 ) $ 1.4 $ 29.5 $ 51.3 Wind — 16.0 — 0.4 — 16.4 Distribution 9.1 — — 0.4 — 9.5 Miscellaneous 1.3 — — — (1.2 ) 0.1 Total liability $ 32.4 $ 16.0 $ (1.6 ) $ 2.2 $ 28.3 $ 77.3 (a) Amounts incurred related to the Hale wind farm placed in service in 2019. (b) Amounts settled related to asbestos abatement projects. (c) In 2019, AROs were revised for changes in timing and estimates of cash flows. Changes in steam production AROs primarily related to the cost estimates to remediate ponds at production facilities. 2018 (Millions of Dollars) Jan. 1, 2018 Accretion Cash Flow Revisions (a) Dec. 31, 2018 (b) Electric Steam and other production $ 20.3 $ 1.2 $ 0.5 $ 22.0 Distribution 7.0 0.3 1.8 9.1 Miscellaneous 1.2 0.1 — 1.3 Total liability $ 28.5 $ 1.6 $ 2.3 $ 32.4 (a) In 2018, AROs were revised for changes in timing and estimates of cash flows. Changes in electric distribution AROs were primarily related to increased labor costs. (b) There were no ARO amounts incurred or settled in 2018.

Other Comprehensive Income (Tab

Other Comprehensive Income (Tables)	12 Months Ended
	Dec. 31, 2019
Stockholders' Equity Note [Abstract]
Changes in Accumulated Other Comprehensive Income (Loss), Net of Tax	Changes in accumulated other comprehensive loss, net of tax, for the years ended Dec. 31: 2019 (Millions of Dollars) Gains and Losses on Cash Flow Hedges Defined Benefit Pension and Postretirement Items Total Accumulated other comprehensive loss at Jan. 1 $ (0.7 ) $ (0.7 ) $ (1.4 ) Other comprehensive loss before reclassifications (net of taxes of $0 and $(0.1), respectfully — (0.2 ) (0.2 ) Losses reclassified from net accumulated other comprehensive loss: Amortization of net actuarial loss (net of taxes of $0) — 0.2 (a) 0.2 Net current period other comprehensive income (loss) — — — Accumulated other comprehensive loss at Dec. 31 $ (0.7 ) $ (0.7 ) $ (1.4 ) (a) Included in the computation of net periodic pension and postretirement benefit costs. See Note 9 for further information. 2018 (Millions of Dollars) Gains and Losses on Cash Flow Hedges Defined Benefit Pension and Postretirement Items Total Accumulated other comprehensive loss at Jan. 1 $ (0.8 ) $ (0.7 ) $ (1.5 ) Losses reclassified from net accumulated other comprehensive loss: Interest rate derivatives (net of taxes of $0) 0.1 (a) — 0.1 Net current period other comprehensive income 0.1 — 0.1 Accumulated other comprehensive loss at Dec. 31 $ (0.7 ) $ (0.7 ) $ (1.4 ) (a) Included in interest charges.

Related Party Transactions (Tab

Related Party Transactions (Tables)	12 Months Ended
	Dec. 31, 2019
Related Party Transactions [Abstract]
Related Party Transactions	Significant affiliate transactions among the companies and related parties for the years ended Dec. 31: (Millions of Dollars) 2019 2018 2017 Operating expenses: Purchased power $ — $ — $ 1.4 Other operating expenses — paid to Xcel Energy Services Inc. 192.0 195.1 196.6 Interest expense 0.2 0.6 — Accounts receivable and payable with affiliates at Dec. 31 were: 2019 2018 (Millions of Dollars) Accounts Accounts Accounts Accounts NSP-Minnesota $ 4.2 $ — $ 4.7 $ — PSCo — 0.4 — 0.7 Other subsidiaries of Xcel Energy Inc. — 20.0 5.8 19.2 $ 4.2 $ 20.4 $ 10.5 $ 19.9

Summarized Quarterly Financia_2

Summarized Quarterly Financial Data (Unaudited) (Tables)	12 Months Ended
	Dec. 31, 2019
Quarterly Financial Information Disclosure [Abstract]
Summarized Quarterly Financial Data (Unaudited)	Quarter Ended (Millions of Dollars) March 31, 2019 June 30, 2019 Sept. 30, 2019 Dec. 31, 2019 Operating revenues $ 454.1 $ 410.5 $ 533.1 $ 428.1 Operating income 74.5 81.9 135.4 54.9 Net income 54.1 58.8 105.1 45.1 Quarter Ended (Millions of Dollars) March 31, 2018 June 30, 2018 Sept. 30, 2018 Dec. 31, 2018 Operating revenues $ 447.2 $ 481.3 $ 540.1 $ 464.6 Operating income (a) 57.1 87.6 111.0 56.0 Net income 33.1 58.5 81.5 40.2 (a) In 2018, SPS implemented ASU No. 2017-07 related to net periodic benefit cost, which resulted in retrospective reclassification of pension costs from O&M expense to other income.

Summary of Significant Accoun_4

Summary of Significant Accounting Policies (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Property, Plant and Equipment [Abstract]
Depreciation expense expressed as a percentage of average depreciable property	2.90%	2.90%	2.80%
Cash and Cash Equivalents [Abstract]
Maximum number of months of remaining maturity at time of purchase to consider investments in certain instruments as cash equivalents	three months
Accounts and Financing Receivable, after Allowance for Credit Loss, Current and Noncurrent [Abstract]
Accounts Receivable, Allowance for Credit Loss, Current	$ 5.3	$ 5.6
Alternative Revenue Programs [Abstract]
Maximum number of months following end of annual period in which revenues are earned to be included in incentive programs	24 months
Public Utilities, Inventory [Line Items]
Inventory	$ 31	33.9
Supplies
Public Utilities, Inventory [Line Items]
Inventory	24.7	25.7
Fuel
Public Utilities, Inventory [Line Items]
Inventory	$ 6.3	$ 8.2

Accounting Pronouncements - Rec

Accounting Pronouncements - Recently Adopted (Details) - USD ($) $ in Millions	Dec. 31, 2019	Jan. 01, 2019	Dec. 31, 2018
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Operating Lease, Liability	$ 522.2
Operating Lease, Right-of-Use Asset	$ 522.4		$ 0
Accounting Standards Update 2016-02 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Operating Lease, Liability		$ 500
Operating Lease, Right-of-Use Asset		$ 500

Property Plant and Equipment _3

Property Plant and Equipment Property Plant and Equipment (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Property, Plant and Equipment [Line Items]
Property, plant and equipment	$ 8,938.4	$ 8,075
Accumulated Depreciation	2,306.8	2,128.6
Property, plant and equipment, net	6,631.6	5,946.4
Electric plant
Property, Plant and Equipment [Line Items]
Property, plant and equipment	8,453	7,227.7
CWIP
Property, Plant and Equipment [Line Items]
Property, plant and equipment	$ 485.4	$ 847.3

Regulatory Assets and Liabili_3

Regulatory Assets and Liabilities, Regulatory Assets (Details) - USD ($) $ in Millions		Dec. 31, 2019	Dec. 31, 2018
Regulatory Assets [Line Items]
Regulatory Asset, Current		$ 20	$ 26
Regulatory Asset, Noncurrent		364	366.2
Pension and Retiree Medical Obligations
Regulatory Assets [Line Items]
Regulatory Asset, Current		11.1	12.6
Regulatory Asset, Noncurrent		203.5	222.1
Excess deferred taxes - TCJA
Regulatory Assets [Line Items]
Regulatory Asset, Current		1.7	0
Regulatory Asset, Noncurrent		52	55.9
Recoverable Deferred Taxes on AFUDC Recorded in Plant
Regulatory Assets [Line Items]
Regulatory Asset, Current		0	0
Regulatory Asset, Noncurrent		34.1	27.9
Net AROs
Regulatory Assets [Line Items]
Regulatory Asset, Current	[1]	0	0
Regulatory Asset, Noncurrent	[1]	26.9	25.7
Losses on Reacquired Debt
Regulatory Assets [Line Items]
Regulatory Asset, Current		0.8	0.8
Regulatory Asset, Noncurrent		21	21.9
Conservation Programs
Regulatory Assets [Line Items]
Regulatory Asset, Current	[2]	0.6	0.7
Regulatory Asset, Noncurrent	[2]	1.1	0.6
Other
Regulatory Assets [Line Items]
Regulatory Asset, Current		5.8	11.9
Regulatory Asset, Noncurrent		$ 25.4	$ 12.1
[1]	Includes amounts recorded for future recovery of AROs.
[2]	Includes costs for conservation programs, as well as incentives allowed in certain jurisdictions.

Regulatory Assets and Liabili_4

Regulatory Assets and Liabilities, Regulatory Liabilities (Details) - USD ($) $ in Millions		Dec. 31, 2019	Dec. 31, 2018
Regulatory Liabilities [Line Items]
Regulatory Liability, Current		$ 118.1	$ 85.8
Regulatory Liability, Noncurrent		732.3	780.9
Regulatory assets not currently earning a return		56.5	50.5
Other Current Liabilities
Regulatory Liabilities [Line Items]
Entity's Recorded Provision for Revenue Subject To Refund		3.9	0
Deferred Income Tax Adjustments and TCJA Refunds
Regulatory Liabilities [Line Items]
Regulatory Liability, Current	[1]	6.9	2.2
Regulatory Liability, Noncurrent	[1]	534.9	569.8
Plant Removal Costs
Regulatory Liabilities [Line Items]
Regulatory Liability, Current		0	0
Regulatory Liability, Noncurrent		174.5	187.7
Revenue Subject to Refund
Regulatory Liabilities [Line Items]
Regulatory Liability, Current		14.6	11.3
Regulatory Liability, Noncurrent		1.1	8.1
Gain From Asset Sales
Regulatory Liabilities [Line Items]
Regulatory Liability, Current		0	0
Regulatory Liability, Noncurrent		2.4	2.4
Deferred Electric Energy Costs
Regulatory Liabilities [Line Items]
Regulatory Liability, Current		81.6	56.5
Regulatory Liability, Noncurrent		0	0
Contract Valuation Adjustments
Regulatory Liabilities [Line Items]
Regulatory Liability, Current	[2]	11.7	14.7
Regulatory Liability, Noncurrent	[2]	0	0
Other
Regulatory Liabilities [Line Items]
Regulatory Liability, Current		3.3	1.1
Regulatory Liability, Noncurrent		$ 19.4	$ 12.9
[1]	Includes the revaluation of recoverable/regulated plant ADIT and revaluation impact of non-plant ADIT due to the TCJA.
[2]	Includes costs for conservation programs, as well as incentives allowed in certain jurisdictions.

Regulatory Assets and Liabili_5

Regulatory Assets and Liabilities Regulatory Assets and Liabilities Phantom (Details)	12 Months Ended
	Dec. 31, 2019
Revenue Subject to Refund
Regulatory Assets [Line Items]
Regulatory liability, remaining amortization period, minimum	1 year
Regulatory liability remaining amortization period, maximum	2 years
Conservation Programs
Regulatory Assets [Line Items]
Regulatory Asset Amortization Period, minimum	1 year
Regulatory Asset Amortization Period, maximum	2 years

Short-Term Borrowings (Details)

Short-Term Borrowings (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Short-term Debt [Line Items]
Amount outstanding at period end	$ 0	$ 0	$ 42
Money Pool
Short-term Debt [Line Items]
Borrowing limit	100	100	100	$ 100
Amount outstanding at period end	0	0	0	0
Average amount outstanding	1	8	29	13
Maximum amount outstanding	$ 12	$ 100	$ 100	$ 100
Weighted average interest rate, computed on a daily basis (percentage)	1.63%	2.42%	1.96%	1.12%
Commercial Paper
Short-term Debt [Line Items]
Borrowing limit	$ 500	$ 500	$ 400	$ 400
Amount outstanding at period end	0	0	42	0
Average amount outstanding	0	72	30	69
Maximum amount outstanding	$ 0	$ 316	$ 144	$ 176
Weighted average interest rate, computed on a daily basis (percentage)		2.68%	2.27%	1.13%
Weighted average interest rate at period end (percentage)			2.80%

Letters of Credit (Details)

Letters of Credit (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Line of Credit Facility [Line Items]
Amount outstanding at period end	$ 0	$ 42
Letter of Credit
Line of Credit Facility [Line Items]
Amount outstanding at period end	$ 2
Letter of Credit | Letter of Credit
Line of Credit Facility [Line Items]
Line of Credit Facility, Expiration Period	1 year

Credit Facility (Details)

Credit Facility (Details) - Credit Facility		12 Months Ended
		Dec. 31, 2019USD ($)	Dec. 31, 2018
Line of Credit Facility [Line Items]
Line Of Credit Facility Debt To Total Capitalization Ratio	[1]	46.00%	46.00%
Line Of Credit Facility Maximum Amount Credit Facility May Be Increased		$ 50,000,000
Number Of Additional Periods Revolving Termination Date Can Be Extended Subject To Majority Bank Group Approval	[2]	2
Term Of Each Additional Period Revolving Termination Date Can Be Extended Subject To Majority Bank Group Approval		1
Line Of Credit Facility Maximum Debt To Total Capitalization Ratio Allowed		65.00%
Line Of Credit Facility Minimum Threshhold Percentage Of Subsidiary Assets To Consolidated Assets Required To Initiate Cross Default Provisions		15.00%
Line of Credit Facility, Minimum Amount of Indebtedness in Default to Initiate Cross Default Provisions		$ 75,000,000
Credit facility	[3]	500,000,000
Drawn	[4]	2,000,000
Available		498,000,000
Direct advances on the credit outstanding		$ 0
[1]	The SPS credit facility has a financial covenant requiring that the debt-to-total capitalization ratio be less than or equal to 65% .
[2]	All extension requests are subject to majority bank group approval.
[3]	This credit facility matures in June 2024 .
[4]	Includes letters of credit and outstanding commercial paper.

Long-Term Borrowings (Details)

Long-Term Borrowings (Details) - USD ($) $ in Millions		Dec. 31, 2019	Dec. 31, 2018
Debt Instrument [Line Items]
Long-term Debt, Maturities, Repayments of Principal in Next Twelve Months		$ 0
Debt Instrument, Unamortized Discount (Premium), Net		(7)	$ (4)
Unamortized Debt Issuance Expense		(23)	(20)
Deferred Finance Costs, Noncurrent, Net		23	20
Long-term Debt, Maturities, Repayments of Principal in Year Two		0
Long-term Debt, Maturities, Repayments of Principal in Year Three		0
Long-term Debt, Maturities, Repayments of Principal in Year Four		0
Long-term Debt, Maturities, Repayments of Principal in Year Five		350
Long-term Debt, Excluding Current Maturities		$ 2,419.7	2,126.1
First Mortgage Bonds, Series due: | Series Due June 15, 2024 [Member]
Debt Instrument [Line Items]
Interest Rate, Stated Percentage (in hundredths)		3.30%
Face Amount		$ 150	150
Long-term Debt, Excluding Current Maturities		$ 2,420	2,126
First Mortgage Bonds, Series due: | Series Due June 15, 2024 2 [Member]
Debt Instrument [Line Items]
Interest Rate, Stated Percentage (in hundredths)		3.30%
Face Amount		$ 200	200
First Mortgage Bonds, Series due: | Series Due Aug. 15, 2041 [Member]
Debt Instrument [Line Items]
Interest Rate, Stated Percentage (in hundredths)		4.50%
Face Amount		$ 200	200
First Mortgage Bonds, Series due: | Series Due Aug. 15, 2041 2 [Member]
Debt Instrument [Line Items]
Interest Rate, Stated Percentage (in hundredths)		4.50%
Face Amount		$ 100	100
First Mortgage Bonds, Series due: | Series Due Aug. 15, 2041 3 [Member]
Debt Instrument [Line Items]
Interest Rate, Stated Percentage (in hundredths)		4.50%
Face Amount		$ 100	100
First Mortgage Bonds, Series due: | Series Due August 15, 2046 [Member]
Debt Instrument [Line Items]
Interest Rate, Stated Percentage (in hundredths)		3.40%
Face Amount		$ 300	300
First Mortgage Bonds, Series due: | Series Due August 15, 2047 [Member]
Debt Instrument [Line Items]
Interest Rate, Stated Percentage (in hundredths)		3.70%
Face Amount		$ 450	450
First Mortgage Bonds, Series due: | Series Due Nov. 15, 2048 [Member]
Debt Instrument [Line Items]
Interest Rate, Stated Percentage (in hundredths)	[1]	4.40%
Face Amount	[1]	$ 300	300
First Mortgage Bonds, Series due: | Series Due June 15, 2049 [Domain]
Debt Instrument [Line Items]
Interest Rate, Stated Percentage (in hundredths)	[2]	3.75%
Face Amount	[2]	$ 300	0
Unsecured Debt [Member] | Senior C and D Due Oct. 1, 2033 [Member]
Debt Instrument [Line Items]
Interest Rate, Stated Percentage (in hundredths)		6.00%
Face Amount		$ 100	100
Unsecured Debt [Member] | Senior F Due Oct. 1, 2036 [Member]
Debt Instrument [Line Items]
Interest Rate, Stated Percentage (in hundredths)		6.00%
Face Amount		$ 250	$ 250
[1]	2018 financing
[2]	2019 financing

Borrowings and Other Financin_3

Borrowings and Other Financing Instruments Preferred Stock (Details)	Dec. 31, 2019$ / sharesshares
Preferred Stock [Abstract]
Preferred Stock, Shares Authorized	10,000,000
Preferred Stock, Par or Stated Value Per Share | $ / shares	$ 1
Preferred Stock, Shares Outstanding	0

Borrowings and Other Financin_4

Borrowings and Other Financing Instruments Dividend and Other Restrictions (Details) $ in Millions	Dec. 31, 2019USD ($)
Dividend and Other Restrictions [Abstract]
Equity to total capitalization ratio (excluding short-term debt), low end of range	45.00%
Equity to total capitalization ratio (excluding short-term debt), high end of range	55.00%
Equity to total capitalization ratio (excluding short-term debt)	54.40%	[1]
Unrestricted Retained Earnings Per State Regulatory Commissions Dividend Restrictions	$ 535
Capitalization, Short term debt, long term debt and equity	$ 5,300
[1]	Excludes short-term debt.

Revenues (Details)

Revenues (Details) - Regulated Electric [Member] - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Retail
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	$ 1,193.3	$ 1,236.7
Retail | Residential
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	351.9	363.7
Retail | Commercial and industrial (C&I)
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	800.3	828.3
Retail | Other
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	41.1	44.7
Wholesale
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	361	426
Transmission
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	239.6	231.1
Other
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	3.3	12.8
Product [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	1,797.2	1,906.6
Alternative and Other [Member]
Disaggregation of Revenue [Line Items]
Revenue Not from Contract with Customer	28.6	26.6
Operating Segments [Member]
Disaggregation of Revenue [Line Items]
Revenues	$ 1,825.8	$ 1,933.2

Income Taxes (Details)

Income Taxes (Details) - USD ($)		1 Months Ended	3 Months Ended	12 Months Ended
		Dec. 31, 2017	Dec. 31, 2018	Dec. 31, 2019	Dec. 31, 2018		Dec. 31, 2017		Dec. 31, 2016	Dec. 31, 2019	Dec. 31, 2018		Dec. 31, 2017	Dec. 31, 2016
Federal Tax Reform [Abstract]
Tax Cuts and Jobs Act of 2017, Corporate Federal Tax Rate		21.00%
Tax Cuts and Jobs Act of 2017, Net Operating Loss Deduction Limitation, Percent of Taxable income		80.00%
Tax Cuts and Jobs Act of 2017, Incomplete Accounting, Change in Tax Rate, Regulatory Liability, Provisional Income Tax (Expense) Benefit, Gross		$ 559,000,000
Tax Cuts and Jobs Act, Incomplete Accounting, Provisional Income Tax Expense (Benefit)		8,000,000
Tax Cuts and Jobs Act of 2017, Incomplete Accounting, Change in Tax Rate, Net Income Reduction		2,000,000
Tax Audits [Abstract]
Unrecognized Tax Benefits, Interest on Income Taxes Accrued										$ 700,000	$ 700,000		$ 500,000	$ 900,000
Interest Income (Expense) related to unrecognized tax benefits				$ 0	$ (200,000)		$ (1,400,000)
Unrecognized Tax Benefits [Abstract]
Unrecognized tax benefit — Permanent tax positions										3,700,000	3,000,000
Unrecognized tax benefit — Temporary tax positions										1,500,000	1,500,000
Total unrecognized tax benefit		4,300,000	$ 4,500,000	5,200,000	4,300,000		4,300,000		$ 28,700,000	5,200,000	4,500,000		4,300,000	$ 28,700,000
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Balance at Jan. 1				4,500,000	4,300,000		28,700,000
Unrecognized Tax Benefits, Increase Resulting from Current Period Tax Positions				700,000	600,000		900,000
Unrecognized Tax Benefits, Decrease Resulting from Current Period Tax Positions				(100,000)	(100,000)		(600,000)
Unrecognized Tax Benefits Increases Resulting From Prior Period Tax Positions				200,000	100,000		1,300,000
Unrecognized Tax Benefits Decreases Resulting From Prior Period Tax Positions				(100,000)	(300,000)		(19,900,000)
Unrecognized Tax Benefits, Decrease Resulting from Settlements with Taxing Authorities				0	(100,000)		(6,100,000)
Balance at Dec. 31		4,300,000	4,500,000	$ 5,200,000	$ 4,500,000		$ 4,300,000		$ 28,700,000
Tax Benefits Associated With NOL And Tax Credit Carryforwards [Abstract]
NOL and tax credit carryforwards										(4,400,000)	(3,800,000)
Net Deferred Tax Liability associated with the Unrecognized Tax Benefit Amounts and Related NOLs and Tax Credit Carryforwards										(1,400,000)	(800,000)
Decrease in Unrecognized Tax Benefits is Reasonably Possible										3,700,000
Unrecognized Tax Benefits, Income Tax Penalties Accrued										0	0		$ 0
Effective Income Tax Rate Reconciliation, Percent [Abstract]
Effective Income Tax Rate Reconciliation, at Federal Statutory Income Tax Rate, Percent				21.00%	21.00%	[1]	35.00%	[1]	35.00%
Effective Income Tax Rate Reconciliation, State and Local Income Taxes, Percent				2.20%	2.30%	[1]	2.00%	[1]
Effective Income Tax Rate Reconciliation, Tax Credit, Percent				(7.90%)	0.00%	[1]	0.00%	[1]
Effective Income Tax Reconciliation, Adjustments Attributable to Tax Returns, Percent				0.10%	1.50%	[1]	0.40%	[1]
Effective Income Tax Rate Reconciliation, Other Regulatory Items, Percent				(0.60%)	(0.70%)	[1]	(0.60%)	[1]
Amortization of excess nonplant deferred taxes				(0.90%)	(1.20%)	[1]	0.00%	[1]
Effective Income Tax Rate Reconciliation Regulatory Differences Utility Plant Items	[2]			(5.00%)	(4.80%)	[1]	(0.90%)	[1]
Effective Income Tax Rate Reconciliation, Tax Cuts and Jobs Act of 2017, Change in Tax Rate, Percent				0.00%	0.00%	[1]	(3.50%)	[1]
Effective Income Tax Rate Reconciliation,Other Reconciling Items, Percent				0.00%	0.20%	[1]	(0.50%)	[1]
Effective Income Tax Rate Reconciliation Change In Unrecognized Tax Benefits				0.20%	0.10%	[1]	(1.00%)	[1]
Effective Income Tax Rate Reconciliation, Percent				8.90%	15.40%	[1]	30.10%	[1]
Components of Income Tax Expense (Benefit), Continuing Operations [Abstract]
Current Federal Tax Expense (Benefit)				$ (3,900,000)	$ 12,300,000		$ (20,900,000)
Current State and Local Tax Expense (Benefit)				600,000	2,300,000		(12,800,000)
Current Change In Unrecognized Tax Expense (Benefit)				0	2,300,000		(24,300,000)
Deferred Federal Income Tax Expense (Benefit)				22,300,000	20,500,000		89,900,000
Deferred State and Local Income Tax Expense (Benefit)				6,000,000	3,600,000		14,500,000
Deferred Change In Unrecognized Tax Expense (Benefit)				700,000	(2,000,000)		22,100,000
Deferred investment tax credits				(100,000)	(100,000)		(100,000)
Income Tax Expense (Benefit)				25,600,000	38,900,000		68,400,000
Deferred Income Tax Expense (Benefit), Continuing Operations [Abstract]
Deferred tax expense (benefit) excluding selected items				52,700,000	44,200,000		(414,200,000)
Amortization and adjustments to deferred income taxes on income tax regulatory assets and liabilities				(23,800,000)	(22,000,000)		540,700,000
Other Comprehensive Income (Loss), Tax				100,000	(100,000)		0
Deferred Income Tax Expense (Benefit)				$ 29,000,000	$ 22,100,000		$ 126,500,000
Deferred Tax Liabilities, Gross [Abstract]
Deferred Tax Liabilities, Property, Plant and Equipment										758,700,000	680,600,000	[3]
Deferred Tax Liabilities, Operating Lease Asset										115,800,000	0	[3]
Deferred Tax Liabilities, Regulatory Assets										49,700,000	49,200,000	[3]
Deferred Tax Liabilities, Compensation and Benefits, Employee Benefits										33,100,000	32,300,000	[3]
Deferred Tax Liabilities, Gross										957,300,000	762,100,000	[3]
Deferred Tax Assets, Gross [Abstract]
Deferred Tax Assets Regulatory Liabilities										111,200,000	116,800,000	[3]
Deferred Tax Assets, Operating Lease Liabilities										115,800,000	0	[3]
Deferred Tax Assets Unbilled Revenue Fuel Costs										18,300,000	12,700,000	[3]
Deferred Tax Assets, Tax Deferred Expense, Compensation and Benefits, Employee Benefits										5,800,000	5,600,000	[3]
Deferred Tax Assets, Operating Loss Carryforwards										100,000	200,000	[3]
Deferred Tax Assets, Tax Credit Carryforwards										29,500,000	5,700,000	[3]
Deferred Tax Assets, Other										4,800,000	2,000,000	[3]
Deferred Tax Assets, Net of Valuation Allowance										285,500,000	143,000,000	[3]
Deferred Tax Liabilities, Net										671,800,000	619,100,000	[3]
Internal Revenue Service (IRS)
Tax Audits [Abstract]
Tax Credit Carryforward, Amount										29,500,000	5,700,000
Potential Tax Adjustments			$ 0
State and Local Jurisdiction
Tax Audits [Abstract]
Operating Loss Carryforwards										$ 1,200,000	$ 2,900,000
Plant Related Regulatory Liability [Member]
Federal Tax Reform [Abstract]
Tax Cuts and Jobs Act of 2017, Incomplete Accounting, Change in Tax Rate, Regulatory Liability, Provisional Income Tax (Expense) Benefit		426,000,000
Non-Plant Related Regulated Liability [Member]
Federal Tax Reform [Abstract]
Tax Cuts and Jobs Act of 2017, Incomplete Accounting, Change in Tax Rate, Regulatory Liability, Provisional Income Tax (Expense) Benefit		28,000,000
Non-Plant Related Regulatory Asset [Member]
Federal Tax Reform [Abstract]
Tax Cuts and Jobs Act of 2017, Incomplete Accounting, Change in Tax Rate, Regulatory Asset, Provisional Income Tax Expense (Benefit)		$ 45,000,000
[1]	Prior periods have been reclassified to conform to current year presentation.
[2]	Regulatory differences for income tax primarily relate to the credit of excess deferred taxes to customers through the average rate assumption method. Income tax benefits associated with the credit of excess deferred credits are offset by corresponding revenue reductions.
[3]	Prior periods have been reclassified to conform to current year presentation.

Interest Rate Derivatives (Deta

Interest Rate Derivatives (Details) $ in Millions	Dec. 31, 2019USD ($)
Interest Rate Swap
Interest Rate Derivatives [Abstract]
Amount of accumulated other comprehensive gains (losses) related to interest rate derivatives expected to be reclassified into earnings within the next twelve months	$ (0.1)

Fair Value of Financial Asset_3

Fair Value of Financial Assets and Liabilities Commodity Derivatives (Details) - MWh MWh in Millions		Dec. 31, 2019	Dec. 31, 2018
Electric Commodity [Member]
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount	[1]	6.4	5.5
[1]	Amounts are not reflective of net positions in the underlying commodities.

Fair Value of Financial Asset_4

Fair Value of Financial Assets and Liabilities Consideration of Credit Risk and Concentrations (Details) - Credit Concentration Risk $ in Millions	Dec. 31, 2019USD ($)Counterparty
Derivative [Line Items]
Number of most significant counterparties for wholesale, trading and non-trading commodity activities with credit exposure	10
Municipal or Cooperative Entities or Other Utilities
Derivative [Line Items]
Number of most significant counterparties for wholesale, trading and non-trading commodity activities with credit exposure	9
External Credit Rating, Investment Grade
Derivative [Line Items]
Number of most significant counterparties for wholesale, trading and non-trading commodity activities with credit exposure	3
Wholesale, trading and non trading commodity credit exposure for the most significant counterparties | $	$ 12.2
Percentage of wholesale, trading and non trading commodity credit exposure for the most significant counterparties	35.00%
No Investment Grade Ratings from External Credit Rating Agencies
Derivative [Line Items]
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 | $	$ 22.1
Percentage of wholesale, trading and non trading commodity credit exposure for the most significant counterparties	65.00%
External Credit Rating, Non Investment Grade
Derivative [Line Items]
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 | $	$ 0.1
Percentage of wholesale, trading and non trading commodity credit exposure for the most significant counterparties	1.00%

Qualifying Cash Flow Hedges (De

Qualifying Cash Flow Hedges (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Financial Impact of Qualifying Cash Flow Hedges on Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Accumulated other comprehensive loss related to cash flow hedges at Jan. 1		$ (0.7)	$ (0.8)	$ (0.7)
After-tax net realized losses on derivative transactions reclassified into earnings		0	(0.1)	0
Adoption of ASU. 2018-02 (a)	[1]	0	0	(0.1)
Accumulated other comprehensive loss related to cash flow hedges at Dec. 31		(0.7)	(0.7)	(0.8)
Fair Value Hedges, Net		0	0	0
Interest Rate Contract [Member] | Designated as Hedging Instrument [Member] | Cash Flow Hedging [Member]
Financial Impact of Qualifying Cash Flow Hedges on Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Derivative Instruments, Gain (Loss) Reclassified from Accumulated OCI into Income, Effective Portion, Net			0.1	0.1
Electric Commodity Contract | Not Designated as Hedging Instrument
Financial Impact of Qualifying Cash Flow Hedges on Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Derivative Instruments Gain (Loss) Reclassified To Regulatory Assets And Liabilities Net		6.5	7	0.5
Pre-tax gains (losses) reclassified into income during the period from regulatory assets and (liabilities)		$ 6	$ 4.4	$ 0.8
[1]	In 2017, SPS implemented ASU No. 2018-02 related to TCJA, which resulted in reclassification of certain credit balances within net accumulated other comprehensive loss to retained earnings.

Recurring Fair Value Measuremen

Recurring Fair Value Measurements (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Derivatives, Fair Value [Line Items]
Return Cash Collateral		$ 0	$ 0
Commodity Contract [Member]
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Balance at beginning of period		14.7	12.7	$ 2
Purchases		26.7	32.3	41.2
Settlements		(34.2)	(41.6)	(55.8)
Gains (losses) recognized as regulatory assets		4.5	11.3	25.3
Balance at end of period		11.7	14.7	12.7
Transfers Between Levels, Net		0	0	$ 0
Other Current Assets
Derivatives, Fair Value [Line Items]
Derivative Asset, Net		15	17.8
Other Noncurrent Assets
Derivatives, Fair Value [Line Items]
Derivative Asset, Net		12.6	15.8
Other Current Liabilities
Derivatives, Fair Value [Line Items]
Derivative Liability, Net		3.7	3.6
Other Noncurrent Liabilities
Derivatives, Fair Value [Line Items]
Derivative Liability, Net		12.8	16.4
Fair Value Measured on a Recurring Basis | Other Current Assets
Derivatives, Fair Value [Line Items]
Derivative Asset, Gross		11.8	14.9
Netting	[1]	0	(0.2)
Derivative Asset, Net		11.8	14.7
Fair Value Measured on a Recurring Basis | Other Current Assets | Level 1
Derivatives, Fair Value [Line Items]
Derivative Asset, Gross		0	0
Fair Value Measured on a Recurring Basis | Other Current Assets | Level 2
Derivatives, Fair Value [Line Items]
Derivative Asset, Gross		0	0
Fair Value Measured on a Recurring Basis | Other Current Assets | Level 3
Derivatives, Fair Value [Line Items]
Derivative Asset, Gross		11.8	14.9
Fair Value Measured on a Recurring Basis | Other Current Assets | Other Derivative Instruments | Electric Commodity Contract
Derivatives, Fair Value [Line Items]
Derivative Asset, Gross		11.8	14.9
Netting	[1]	0	(0.2)
Derivative Asset, Net		11.8	14.7
Fair Value Measured on a Recurring Basis | Other Current Assets | Other Derivative Instruments | Electric Commodity Contract | Level 1
Derivatives, Fair Value [Line Items]
Derivative Asset, Gross		0	0
Fair Value Measured on a Recurring Basis | Other Current Assets | Other Derivative Instruments | Electric Commodity Contract | Level 2
Derivatives, Fair Value [Line Items]
Derivative Asset, Gross		0	0
Fair Value Measured on a Recurring Basis | Other Current Assets | Other Derivative Instruments | Electric Commodity Contract | Level 3
Derivatives, Fair Value [Line Items]
Derivative Asset, Gross		11.8	14.9
Fair Value Measured on a Recurring Basis | Other Current Liabilities
Derivatives, Fair Value [Line Items]
Derivative Liability, Gross		0.1	0.2
Netting	[1]	0	(0.2)
Derivative Liability, Net		0.1	0
Fair Value Measured on a Recurring Basis | Other Current Liabilities | Level 1
Derivatives, Fair Value [Line Items]
Derivative Liability, Gross		0	0
Fair Value Measured on a Recurring Basis | Other Current Liabilities | Level 2
Derivatives, Fair Value [Line Items]
Derivative Liability, Gross		0	0
Fair Value Measured on a Recurring Basis | Other Current Liabilities | Level 3
Derivatives, Fair Value [Line Items]
Derivative Liability, Gross		0.1	0.2
Fair Value Measured on a Recurring Basis | Other Current Liabilities | Other Derivative Instruments | Electric Commodity Contract
Derivatives, Fair Value [Line Items]
Derivative Liability, Gross		0.1	0.2
Netting	[1]	0	(0.2)
Derivative Liability, Net		0.1	0
Fair Value Measured on a Recurring Basis | Other Current Liabilities | Other Derivative Instruments | Electric Commodity Contract | Level 1
Derivatives, Fair Value [Line Items]
Derivative Liability, Gross		0	0
Fair Value Measured on a Recurring Basis | Other Current Liabilities | Other Derivative Instruments | Electric Commodity Contract | Level 2
Derivatives, Fair Value [Line Items]
Derivative Liability, Gross		0	0
Fair Value Measured on a Recurring Basis | Other Current Liabilities | Other Derivative Instruments | Electric Commodity Contract | Level 3
Derivatives, Fair Value [Line Items]
Derivative Liability, Gross		0.1	0.2
Fair Value, Measurements, Nonrecurring | Other Current Assets | Purchased Power Agreements
Derivatives, Fair Value [Line Items]
Derivative Asset, Net	[2]	3.2	3.1
Fair Value, Measurements, Nonrecurring | Other Noncurrent Assets | Purchased Power Agreements
Derivatives, Fair Value [Line Items]
Derivative Asset, Net		12.6	15.8
Fair Value, Measurements, Nonrecurring | Other Current Liabilities | Purchased Power Agreements
Derivatives, Fair Value [Line Items]
Derivative Liability, Net	[2]	3.6	3.6
Fair Value, Measurements, Nonrecurring | Other Noncurrent Liabilities | Purchased Power Agreements
Derivatives, Fair Value [Line Items]
Derivative Liability, Net	[2]	$ 12.8	$ 16.4
[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 Dec. 31, 2019 and 2018 . At both Dec. 31, 2019 and 2018 , derivative assets and liabilities include no obligations to return cash collateral or rights to reclaim cash collateral. The counterparty netting excludes settlement receivables and payables and non-derivative amounts that may be subject to the same master netting agreements.
[2]	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 will be amortized over the remaining contract lives along with the offsetting regulatory assets and liabilities.

Fair Value of Financial Asset_5

Fair Value of Financial Assets and Liabilities, Fair Value of Long-Term Debt (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Financial Liabilities, Balance Sheet Groupings [Abstract]
Long-term Debt, Gross	$ 2,419.7	$ 2,126.1
Long-term debt, Fair Value	$ 2,706.1	$ 2,139.8

Fair Value of Financial Asset_6

Fair Value of Financial Assets and Liabilities Fair Value Phantom (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value Hedges, Net	$ 0	$ 0	$ 0
Return Cash Collateral	0	0
Reclaim Cash Collateral	0	0
Commodity Contract [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Transfers Between Levels, Net	$ 0	$ 0	$ 0

Benefit Plans and Other Postr_3

Benefit Plans and Other Postretirement Benefits, Pension Benefits (Details) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019USD ($)Plan	Dec. 31, 2018USD ($)Plan	Dec. 31, 2017USD ($)Plan
Target Pension Asset Allocations [Abstract]
Defined Contribution Plan, Cost		$ 3	$ 3	$ 3
Pension Plan [Member]
Pension Benefits [Abstract]
Total benefit obligation		518.8	477.8	515.9
Net benefit cost recognized for financial reporting		$ 14.8	$ 14.8	$ 14.9
Minimum number of years historical achieved weighted average annual returns used to determine investment return assumptions (in years)		20 years
Expected average long-term rate of return on assets (as a percent)		6.78%	6.78%	6.78%
Target Pension Asset Allocations [Abstract]
Target pension asset allocations (as a percent)		100.00%	100.00%
Pension Plan [Member] | Domestic and international equity securities
Target Pension Asset Allocations [Abstract]
Target pension asset allocations (as a percent)		37.00%	35.00%
Pension Plan [Member] | Long-Duration Fixed Income and Long-duration fixed income and interest rate swap securities
Target Pension Asset Allocations [Abstract]
Target pension asset allocations (as a percent)		30.00%	32.00%
Pension Plan [Member] | Short-to-intermediate fixed income securities
Target Pension Asset Allocations [Abstract]
Target pension asset allocations (as a percent)		14.00%	16.00%
Pension Plan [Member] | Alternative investments
Target Pension Asset Allocations [Abstract]
Target pension asset allocations (as a percent)		17.00%	15.00%
Pension Plan [Member] | Cash
Target Pension Asset Allocations [Abstract]
Target pension asset allocations (as a percent)		2.00%	2.00%
Supplemental Executive Retirement Plan (SERP) and Nonqualified Pension Plan
Pension Benefits [Abstract]
Total benefit obligation		$ 2	$ 2
Net benefit cost recognized for financial reporting		4	4
Parent Company [Member]
Target Pension Asset Allocations [Abstract]
Total contributions to Xcel Energy's postretirement health care plans during the year		$ 15	$ 11	$ 20
Parent Company [Member] | Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan number of pension plans to which contributions were made | Plan		4	4	4
Parent Company [Member] | Supplemental Executive Retirement Plan (SERP) and Nonqualified Pension Plan
Pension Benefits [Abstract]
Total benefit obligation		$ 39	$ 33
Forecast [Member] | Pension Plan [Member]
Pension Benefits [Abstract]
Expected average long-term rate of return on assets for next fiscal year (as a percent)	6.78%

Benefit Plans and Other Postr_4

Benefit Plans and Other Postretirement Benefits, Fair Value of Pension Plan Assets (Details) - Pension Plan [Member] - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Plan Assets Measured at Fair Value for Each of the Fair Value Hierarchy Levels [Abstract]
Fair value of plan assets	$ 458.2	$ 392.2	$ 433.2
Plan asset investments measured at net asset value	142	128.5
Level 1
Plan Assets Measured at Fair Value for Each of the Fair Value Hierarchy Levels [Abstract]
Fair value of plan assets	216.7	164.8
Level 2
Plan Assets Measured at Fair Value for Each of the Fair Value Hierarchy Levels [Abstract]
Fair value of plan assets	98.9	98.9
Level 3
Plan Assets Measured at Fair Value for Each of the Fair Value Hierarchy Levels [Abstract]
Fair value of plan assets	0.6	0
Debt Securities [Member]
Plan Assets Measured at Fair Value for Each of the Fair Value Hierarchy Levels [Abstract]
Fair value of plan assets	98.8	98.1
Plan asset investments measured at net asset value	0	0
Debt Securities [Member] | Level 1
Plan Assets Measured at Fair Value for Each of the Fair Value Hierarchy Levels [Abstract]
Fair value of plan assets	0	0
Debt Securities [Member] | Level 2
Plan Assets Measured at Fair Value for Each of the Fair Value Hierarchy Levels [Abstract]
Fair value of plan assets	98.2	98.1
Debt Securities [Member] | Level 3
Plan Assets Measured at Fair Value for Each of the Fair Value Hierarchy Levels [Abstract]
Fair value of plan assets	0.6	0
Domestic and international equity securities
Plan Assets Measured at Fair Value for Each of the Fair Value Hierarchy Levels [Abstract]
Fair value of plan assets	12.1	14.4
Plan asset investments measured at net asset value	0	0
Domestic and international equity securities | Level 1
Plan Assets Measured at Fair Value for Each of the Fair Value Hierarchy Levels [Abstract]
Fair value of plan assets	12.1	14.4
Domestic and international equity securities | Level 2
Plan Assets Measured at Fair Value for Each of the Fair Value Hierarchy Levels [Abstract]
Fair value of plan assets	0	0
Domestic and international equity securities | Level 3
Plan Assets Measured at Fair Value for Each of the Fair Value Hierarchy Levels [Abstract]
Fair value of plan assets	0	0
Cash
Plan Assets Measured at Fair Value for Each of the Fair Value Hierarchy Levels [Abstract]
Fair value of plan assets	18.9	21.6
Plan asset investments measured at net asset value	0	0
Cash | Level 1
Plan Assets Measured at Fair Value for Each of the Fair Value Hierarchy Levels [Abstract]
Fair value of plan assets	18.9	21.6
Cash | Level 2
Plan Assets Measured at Fair Value for Each of the Fair Value Hierarchy Levels [Abstract]
Fair value of plan assets	0	0
Cash | Level 3
Plan Assets Measured at Fair Value for Each of the Fair Value Hierarchy Levels [Abstract]
Fair value of plan assets	0	0
Commingled funds
Plan Assets Measured at Fair Value for Each of the Fair Value Hierarchy Levels [Abstract]
Fair value of plan assets	347.3	261.1
Plan asset investments measured at net asset value	144.8	132.5
Commingled funds | Level 1
Plan Assets Measured at Fair Value for Each of the Fair Value Hierarchy Levels [Abstract]
Fair value of plan assets	202.5	128.6
Commingled funds | Level 2
Plan Assets Measured at Fair Value for Each of the Fair Value Hierarchy Levels [Abstract]
Fair value of plan assets	0	0
Commingled funds | Level 3
Plan Assets Measured at Fair Value for Each of the Fair Value Hierarchy Levels [Abstract]
Fair value of plan assets	0	0
Other
Plan Assets Measured at Fair Value for Each of the Fair Value Hierarchy Levels [Abstract]
Fair value of plan assets	(18.9)	(3)
Plan asset investments measured at net asset value	(2.8)	(4)
Other | Level 1
Plan Assets Measured at Fair Value for Each of the Fair Value Hierarchy Levels [Abstract]
Fair value of plan assets	(16.8)	0.2
Other | Level 2
Plan Assets Measured at Fair Value for Each of the Fair Value Hierarchy Levels [Abstract]
Fair value of plan assets	0.7	0.8
Other | Level 3
Plan Assets Measured at Fair Value for Each of the Fair Value Hierarchy Levels [Abstract]
Fair value of plan assets	$ 0	$ 0

Benefit Plans and Other Postr_5

Benefit Plans and Other Postretirement Benefits, Pension Plan Benefit Obligations, Cash Flows and Benefit Costs (Details) $ in Millions	1 Months Ended	12 Months Ended
	Jan. 31, 2020USD ($)	Dec. 31, 2019USD ($)Plan	Dec. 31, 2018USD ($)Plan	Dec. 31, 2017USD ($)Plan
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Plan Assets, Payment for Settlement		$ 6.8	$ 6.9
Operating and maintenance expenses		285.3	282.7	$ 285.4
Significant Assumptions Used to Measure Costs [Abstract]
Liability, Defined Benefit Plan, Noncurrent		(67)	(92.4)
Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Plan Assets, Payment for Settlement		2.4	3.2
Accumulated Benefit Obligation at Dec. 31		481.1	445.8
Operating and maintenance expenses		0.6	0.7
Change in Projected Benefit Obligation [Roll Forward]
Obligation at Jan. 1	$ 518.8	477.8	515.9
Service cost		8.8	9.7	9.8
Interest cost		20.1	18.4	19.7
Plan amendments		0	0
Actuarial loss		44.2	(34.8)
Benefit payments		(32.1)	(31.4)
Obligation at Dec. 31		518.8	477.8	515.9
Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets at Jan. 1	458.2	392.2	433.2
Actual return on plan assets		80.2	(17.6)
Employer contributions		17.9	8
Benefit payments		(32.1)	(31.4)
Fair value of plan assets at Dec. 31		458.2	392.2	433.2
Funded Status of Plans at Dec. 31 [Abstract]
Funded status		(60.6)	(85.6)
Amounts Not Yet Recognized as Components of Net Periodic Benefit Cost Have Been Recorded as Follows Based Upon Expected Recovery Rates [Abstract]
Net loss		209.7	230.9
Prior service credit		(1.1)	(1.2)
Total		208.6	229.7
Amounts Not Yet Recognized as Components of Net Periodic Benefit Cost Have Been Recorded as Follows Based Upon Expected Recovery in Rates [Abstract]
Current regulatory assets		11	12.9
Noncurrent regulatory assets		197.6	216.8
Amounts Not Yet Recognized As Components Of Net Periodic Benefit Cost Recorded As Current Regulatory Liabilities		0	0
Amounts Not Yet Recognized As Components Of Net Periodic Benefit Cost Recorded As Noncurrent Regulatory Liabilities		0	0
Total		$ 208.6	$ 229.7
Significant Assumptions Used to Measure Benefit Obligations [Abstract]
Discount rate for year-end valuation (as a percent)		3.49%	4.31%
Expected average long-term increase in compensation level (as a percent)		3.75%	3.75%
Mortality table		Pri-2012	RP-2014
Cash Flows [Abstract]
Payment for Pension Benefits		$ 18	$ 8	24
Components of Net Periodic Benefit Cost (Credit) [Abstract]
Service cost		8.8	9.7	9.8
Interest cost		20.1	18.4	19.7
Expected return on plan assets		(28.6)	(28.3)	(27.9)
Amortization of prior service cost		(0.1)	(0.1)	0
Amortization of net loss		11.3	14.1	13
Net periodic pension cost		13.9	17	14.6
Net benefit cost recognized for financial reporting		$ 14.8	$ 14.8	$ 14.9
Significant Assumptions Used to Measure Costs [Abstract]
Discount rate (as a percent)		4.31%	3.63%	4.13%
Expected average long-term increase in compensation level (as a percent)		3.75%	3.75%	3.75%
Expected average long-term rate of return on assets (as a percent)		6.78%	6.78%	6.78%
Plan participant's contributions		$ 0	$ 0
Plan participant's contributions		0	0
Liability, Defined Benefit Plan, Noncurrent		(60.6)	(85.6)
Defined Benefit Plan, Amounts for Asset (Liability) Recognized in Statement of Financial Position		(60.6)	(85.6)
Defined Benefit Plan, Net Periodic Benefit Cost (Credit), Gain (Loss) Due to Settlement		2.4	3.2	$ 0
Defined Benefit Plan, Costs Not Recognized Due To Regulation		$ 0.9	$ (2.2)	$ 0.3
Subsequent Event | Pension Plan [Member]
Cash Flows [Abstract]
Payment for Pension Benefits	14
Parent Company [Member] | Pension Plan [Member]
Cash Flows [Abstract]
Number of pension plans to which contributions were made | Plan		4	4	4
Payment for Pension Benefits		$ 154	$ 150	$ 162
Parent Company [Member] | Subsequent Event | Pension Plan [Member]
Cash Flows [Abstract]
Payment for Pension Benefits	$ 150

Benefit Plans and Other Postr_6

Benefit Plans and Other Postretirement Benefits, Defined Contribution Plans (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Defined Contribution Plans [Abstract]
Contributions to 401(k) and other defined contribution plans	$ 3	$ 3	$ 3

Benefit Plans and Other Postr_7

Benefit Plans and Other Postretirement Benefits, Postretirement Health Care Benefits (Details) - Other Postretirement Benefits Plan [Member]	Dec. 31, 2019	Dec. 31, 2018
Postretirement Health Care Benefits [Abstract]
Target pension asset allocations (as a percent)	100.00%	100.00%
Domestic and international equity securities
Postretirement Health Care Benefits [Abstract]
Target pension asset allocations (as a percent)	15.00%	18.00%
Long-Duration Fixed Income and Long-duration fixed income and interest rate swap securities
Postretirement Health Care Benefits [Abstract]
Target pension asset allocations (as a percent)	0.00%	0.00%
Short-to-intermediate fixed income securities
Postretirement Health Care Benefits [Abstract]
Target pension asset allocations (as a percent)	72.00%	70.00%
Alternative investments
Postretirement Health Care Benefits [Abstract]
Target pension asset allocations (as a percent)	9.00%	8.00%
Cash
Postretirement Health Care Benefits [Abstract]
Target pension asset allocations (as a percent)	4.00%	4.00%

Benefit Plans and Other Postr_8

Benefit Plans and Other Postretirement Benefits, Fair Value of Postretirement Benefit Plan Assets (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Other Postretirement Benefits Plan [Member]
Plan Assets Measured at Fair Value for Each of the Fair Value Hierarchy Levels [Abstract]
Fair value of plan assets	$ 43.6	$ 40	$ 44.1
Plan assets at net asset value	7.4	3.8
Other Postretirement Benefits Plan [Member] | Level 1
Plan Assets Measured at Fair Value for Each of the Fair Value Hierarchy Levels [Abstract]
Fair value of plan assets	8.9	14.6
Other Postretirement Benefits Plan [Member] | Level 2
Plan Assets Measured at Fair Value for Each of the Fair Value Hierarchy Levels [Abstract]
Fair value of plan assets	27.2	21.6
Other Postretirement Benefits Plan [Member] | Level 3
Plan Assets Measured at Fair Value for Each of the Fair Value Hierarchy Levels [Abstract]
Fair value of plan assets	0.1	0
Other Postretirement Benefits Plan [Member] | Debt Securities [Member]
Plan Assets Measured at Fair Value for Each of the Fair Value Hierarchy Levels [Abstract]
Fair value of plan assets	22.2	17.2
Plan assets at net asset value	0	0
Other Postretirement Benefits Plan [Member] | Debt Securities [Member] | Level 1
Plan Assets Measured at Fair Value for Each of the Fair Value Hierarchy Levels [Abstract]
Fair value of plan assets	0	0
Other Postretirement Benefits Plan [Member] | Debt Securities [Member] | Level 2
Plan Assets Measured at Fair Value for Each of the Fair Value Hierarchy Levels [Abstract]
Fair value of plan assets	22.1	17.2
Other Postretirement Benefits Plan [Member] | Debt Securities [Member] | Level 3
Plan Assets Measured at Fair Value for Each of the Fair Value Hierarchy Levels [Abstract]
Fair value of plan assets	0.1	0
Other Postretirement Benefits Plan [Member] | Cash
Plan Assets Measured at Fair Value for Each of the Fair Value Hierarchy Levels [Abstract]
Fair value of plan assets	2.2	1.8
Plan assets at net asset value	0	0
Other Postretirement Benefits Plan [Member] | Cash | Level 1
Plan Assets Measured at Fair Value for Each of the Fair Value Hierarchy Levels [Abstract]
Fair value of plan assets	2.2	1.8
Other Postretirement Benefits Plan [Member] | Cash | Level 2
Plan Assets Measured at Fair Value for Each of the Fair Value Hierarchy Levels [Abstract]
Fair value of plan assets	0	0
Other Postretirement Benefits Plan [Member] | Cash | Level 3
Plan Assets Measured at Fair Value for Each of the Fair Value Hierarchy Levels [Abstract]
Fair value of plan assets	0	0
Other Postretirement Benefits Plan [Member] | Insurance contracts
Plan Assets Measured at Fair Value for Each of the Fair Value Hierarchy Levels [Abstract]
Fair value of plan assets	4.9	4.3
Plan assets at net asset value	0	0
Other Postretirement Benefits Plan [Member] | Insurance contracts | Level 1
Plan Assets Measured at Fair Value for Each of the Fair Value Hierarchy Levels [Abstract]
Fair value of plan assets	0	0
Other Postretirement Benefits Plan [Member] | Insurance contracts | Level 2
Plan Assets Measured at Fair Value for Each of the Fair Value Hierarchy Levels [Abstract]
Fair value of plan assets	4.9	4.3
Other Postretirement Benefits Plan [Member] | Insurance contracts | Level 3
Plan Assets Measured at Fair Value for Each of the Fair Value Hierarchy Levels [Abstract]
Fair value of plan assets	0	0
Other Postretirement Benefits Plan [Member] | Other
Plan Assets Measured at Fair Value for Each of the Fair Value Hierarchy Levels [Abstract]
Fair value of plan assets	0.2	0.1
Plan assets at net asset value	0	0
Other Postretirement Benefits Plan [Member] | Other | Level 1
Plan Assets Measured at Fair Value for Each of the Fair Value Hierarchy Levels [Abstract]
Fair value of plan assets	0	0
Other Postretirement Benefits Plan [Member] | Other | Level 2
Plan Assets Measured at Fair Value for Each of the Fair Value Hierarchy Levels [Abstract]
Fair value of plan assets	0.2	0.1
Other Postretirement Benefits Plan [Member] | Other | Level 3
Plan Assets Measured at Fair Value for Each of the Fair Value Hierarchy Levels [Abstract]
Fair value of plan assets	0	0
Other Postretirement Benefits Plan [Member] | Commingled funds
Plan Assets Measured at Fair Value for Each of the Fair Value Hierarchy Levels [Abstract]
Fair value of plan assets	14.1	16.6
Plan assets at net asset value	7.4	3.8
Other Postretirement Benefits Plan [Member] | Commingled funds | Level 1
Plan Assets Measured at Fair Value for Each of the Fair Value Hierarchy Levels [Abstract]
Fair value of plan assets	6.7	12.8
Other Postretirement Benefits Plan [Member] | Commingled funds | Level 2
Plan Assets Measured at Fair Value for Each of the Fair Value Hierarchy Levels [Abstract]
Fair value of plan assets	0	0
Other Postretirement Benefits Plan [Member] | Commingled funds | Level 3
Plan Assets Measured at Fair Value for Each of the Fair Value Hierarchy Levels [Abstract]
Fair value of plan assets	0	0
Other Postretirement Benefits Plan [Member] | Domestic and international equity securities
Plan Assets Measured at Fair Value for Each of the Fair Value Hierarchy Levels [Abstract]
Fair value of plan assets	0	0
Plan assets at net asset value	0	0
Other Postretirement Benefits Plan [Member] | Domestic and international equity securities | Level 1
Plan Assets Measured at Fair Value for Each of the Fair Value Hierarchy Levels [Abstract]
Fair value of plan assets	0	0
Other Postretirement Benefits Plan [Member] | Domestic and international equity securities | Level 2
Plan Assets Measured at Fair Value for Each of the Fair Value Hierarchy Levels [Abstract]
Fair value of plan assets	0	0
Other Postretirement Benefits Plan [Member] | Domestic and international equity securities | Level 3
Plan Assets Measured at Fair Value for Each of the Fair Value Hierarchy Levels [Abstract]
Fair value of plan assets	0	0
Parent Company [Member]
Defined Benefit Plan Disclosure [Line Items]
Payment for Other Postretirement Benefits	$ 15	$ 11	$ 20

Benefit Plans and Other Postr_9

Benefit Plans and Other Postretirement Benefits, Postretirement Benefit Plan Benefit Obligations, Cash Flows and Benefit Costs (Details) - USD ($) $ in Millions	1 Months Ended	12 Months Ended
	Jan. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2020
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Plan Assets, Payment for Settlement		$ 6.8	$ 6.9
Significant Assumptions Used to Measure Costs [Abstract]
Liability, Defined Benefit Plan, Noncurrent		(67)	(92.4)
Operating and maintenance expenses		285.3	282.7	$ 285.4
Other Postretirement Benefits Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan amendments		0	0
Change in Projected Benefit Obligation [Roll Forward]
Obligation at Jan. 1	$ 43.2	41.8	47
Service cost		0.9	1.1	0.9
Interest cost		1.7	1.6	1.7
Plan participant's contributions		0.6	0.6
Actuarial loss		0.4	(5.1)
Benefit payments		(2.2)	(3.4)
Obligation at Dec. 31		43.2	41.8	47
Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets at Jan. 1	43.6	40	44.1
Actual return on plan assets		5.1	(1.3)
Plan participant's contributions		0.6	0.6
Employer contributions		0.1	0
Benefit payments		(2.2)	(3.4)
Fair value of plan assets at Dec. 31		43.6	40	44.1
Funded Status of Plans at Dec. 31 [Abstract]
Funded status		0.4	(1.8)
Amounts Not Yet Recognized as Components of Net Periodic Benefit Cost Have Been Recorded as Follows Based Upon Expected Recovery Rates [Abstract]
Net loss		(11.9)	(9.6)
Prior service credit		(1.4)	(1.8)
Total		(13.3)	(11.4)
Amounts Not Yet Recognized as Components of Net Periodic Benefit Cost (Credit) Have Been Recorded as Follows Based Upon Expected Recovery in Rates [Abstract]
Amounts Not Yet Recognized As Components Of Net Periodic Benefit Cost Recorded As Current Regulatory Assets		0	0
Amounts Not Yet Recognized As Components Of Net Periodic Benefit Cost Recorded As Noncurrent Regulatory Assets		0	0
Current regulatory liabilities		(0.8)	(0.9)
Noncurrent regulatory liabilities		(12.5)	(10.5)
Total		$ (13.3)	$ (11.4)
Significant Assumptions Used to Measure Benefit Obligations [Abstract]
Discount rate for year-end valuation (as a percent)		3.47%	4.32%
Mortality table		Pri-2012	RP-2014
Defined Benefit Plan, Health Care Cost Trend Rate Assumed, Pre-65		6.00%	6.50%
Defined Benefit Plan, Health Care Cost Trend Rate Assumed, Post-65		5.10%	5.30%
Ultimate health care trend assumption rate (as a percent)		4.50%	4.50%
Period until ultimate trend rate is reached (in years)		3 years	4 years
Components of Net Periodic Benefit Cost (Credit) [Abstract]
Service cost		$ 0.9	$ 1.1	0.9
Interest cost		1.7	1.6	1.7
Expected return on plan assets		(2)	(2.5)	(2.4)
Amortization of prior service cost		(0.5)	(0.4)	(0.4)
Amortization of net loss		(0.4)	(0.4)	(0.6)
Defined Benefit Plan, Net Periodic Benefit Cost (Credit), Gain (Loss) Due to Settlement		0	0	0
Net periodic pension cost		$ (0.3)	$ (0.6)	$ (0.8)
Significant Assumptions Used to Measure Costs [Abstract]
Discount rate (as a percent)		4.32%	3.62%	4.13%
Expected average long-term increase in compensation level (as a percent)		0.00%	0.00%	0.00%
Expected average long-term rate of return on assets (as a percent)		5.30%	5.80%	5.80%
Liability, Defined Benefit Plan, Noncurrent		$ 0	$ (1.8)
Defined Benefit Plan, Amounts for Asset (Liability) Recognized in Statement of Financial Position		0.4	(1.8)
Defined Benefit Plan, Costs Not Recognized Due To Regulation		0	0	$ 0
Net benefit cost recognized for financial reporting		(0.3)	(0.6)	(0.8)
Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Plan Assets, Payment for Settlement		2.4	3.2
Payment for Pension Benefits		18	8	24
Plan amendments		0	0
Change in Projected Benefit Obligation [Roll Forward]
Obligation at Jan. 1	518.8	477.8	515.9
Service cost		8.8	9.7	9.8
Interest cost		20.1	18.4	19.7
Plan participant's contributions		0	0
Actuarial loss		44.2	(34.8)
Benefit payments		(32.1)	(31.4)
Obligation at Dec. 31		518.8	477.8	515.9
Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets at Jan. 1	458.2	392.2	433.2
Actual return on plan assets		80.2	(17.6)
Plan participant's contributions		0	0
Employer contributions		17.9	8
Benefit payments		(32.1)	(31.4)
Fair value of plan assets at Dec. 31		458.2	392.2	433.2
Funded Status of Plans at Dec. 31 [Abstract]
Funded status		(60.6)	(85.6)
Amounts Not Yet Recognized as Components of Net Periodic Benefit Cost Have Been Recorded as Follows Based Upon Expected Recovery Rates [Abstract]
Net loss		209.7	230.9
Prior service credit		(1.1)	(1.2)
Total		208.6	229.7
Amounts Not Yet Recognized as Components of Net Periodic Benefit Cost (Credit) Have Been Recorded as Follows Based Upon Expected Recovery in Rates [Abstract]
Amounts Not Yet Recognized As Components Of Net Periodic Benefit Cost Recorded As Current Regulatory Assets		11	12.9
Amounts Not Yet Recognized As Components Of Net Periodic Benefit Cost Recorded As Noncurrent Regulatory Assets		197.6	216.8
Current regulatory liabilities		0	0
Noncurrent regulatory liabilities		0	0
Total		$ 208.6	$ 229.7
Significant Assumptions Used to Measure Benefit Obligations [Abstract]
Discount rate for year-end valuation (as a percent)		3.49%	4.31%
Defined Benefit Plan, Assumptions Used Calculating Benefit Obligation, Rate of Compensation Increase		3.75%	3.75%
Mortality table		Pri-2012	RP-2014
Components of Net Periodic Benefit Cost (Credit) [Abstract]
Service cost		$ 8.8	$ 9.7	9.8
Interest cost		20.1	18.4	19.7
Expected return on plan assets		(28.6)	(28.3)	(27.9)
Amortization of prior service cost		(0.1)	(0.1)	0
Amortization of net loss		11.3	14.1	13
Defined Benefit Plan, Net Periodic Benefit Cost (Credit), Gain (Loss) Due to Settlement		(2.4)	(3.2)	0
Net periodic pension cost		$ 13.9	$ 17	$ 14.6
Significant Assumptions Used to Measure Costs [Abstract]
Discount rate (as a percent)		4.31%	3.63%	4.13%
Expected average long-term increase in compensation level (as a percent)		3.75%	3.75%	3.75%
Expected average long-term rate of return on assets (as a percent)		6.78%	6.78%	6.78%
Liability, Defined Benefit Plan, Noncurrent		$ (60.6)	$ (85.6)
Defined Benefit Plan, Amounts for Asset (Liability) Recognized in Statement of Financial Position		(60.6)	(85.6)
Defined Benefit Plan, Costs Not Recognized Due To Regulation		(0.9)	2.2	$ (0.3)
Net benefit cost recognized for financial reporting		14.8	14.8	14.9
Operating and maintenance expenses		0.6	0.7
Parent Company [Member]
Cash Flows [Abstract]
Total contributions to Xcel Energy's postretirement health care plans during the year		15	11	20
Parent Company [Member] | Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Payment for Pension Benefits		$ 154	$ 150	$ 162
Subsequent Event | Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Payment for Pension Benefits	14
Subsequent Event | Parent Company [Member]
Cash Flows [Abstract]
Expected contribution to postretirement health care plans during 2019					$ 10
Subsequent Event | Parent Company [Member] | Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Payment for Pension Benefits	$ 150

Benefit Plans and Other Post_10

Benefit Plans and Other Postretirement Benefits, Projected Benefit Payments (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Accumulated Benefit Obligation	$ 481.1	$ 445.8
Defined Benefit Plan, Gross Projected Benefit Payments [Abstract]
2020	30.7
2021	29.4
2022	30.3
2023	30.4
2024	30.4
2025-2029	153.5
Other Postretirement Benefits Plan [Member]
Defined Benefit Plan, Gross Projected Benefit Payments [Abstract]
2020	2.9
2021	2.9
2022	2.9
2023	2.9
2024	2.8
2025-2029	13.2
Expected Medicare Part D Subsidies [Abstract]
2020	0
2021	0
2022	0
2023	0
2024	0
2025-2029	0.1
Defined Benefit Plan, Net Projected Benefit Payments [Abstract]
2020	2.9
2021	2.9
2022	2.9
2023	2.9
2024	2.8
2025-2029	$ 13.1

Commitments and Contingencies T

Commitments and Contingencies Texas Fuel Reconciliation (Details) $ in Millions	12 Months Ended
	Dec. 31, 2019USD ($)
NMPRC [Member] | SPS [Member] | Alternative Energy [Member]
Public Utilities, General Disclosures [Line Items]
Fuel Costs Disallowed	$ 6

Commitments and Contingencies S

Commitments and Contingencies SPP OATT Upgrade Costs (Details) $ in Millions	12 Months Ended
	Dec. 31, 2017USD ($)
Southwest Power Pool (SPP) [Member] | SPP Open Access Transmission Tariff Upgrade Costs [Member] | SPS [Member]
Public Utilities, General Disclosures [Line Items]
Public Utilities, Billed Charges For Transmission Service Upgrades	$ 13

Commitments and Contingencies_2

Commitments and Contingencies SPP Filing to Assign GridLiance Facilities to SPS Rate Zone (Details) $ in Millions	1 Months Ended
	Aug. 31, 2018USD ($)
Southwest Power Pool (SPP) [Member] | SPP Open Access Transmission Tariff Upgrade Costs [Member] | SPS [Member]
Public Utilities, General Disclosures [Line Items]
Public Utilities, Annual Transmission Revenue Requirement Increase (SPS)	$ 6

Commitments and Contingencies_3

Commitments and Contingencies SPS Filing to Modify Wholesale Transmission Rates (Details) $ in Millions	12 Months Ended
	Dec. 31, 2019USD ($)
Commitments and Contingencies Disclosure [Abstract]
Public Utilities, Amount proposed changes would increase wholesale transmission revenues	$ 9.4
Public Utilities, Amount of proposed changes would be recoverable	$ 4.4

Commitments and Contingencies,

Commitments and Contingencies, Legal Contingencies (Details) - USD ($) $ in Millions	1 Months Ended	12 Months Ended
	Aug. 31, 2018	Dec. 31, 2019	Dec. 31, 2017
Loss Contingencies [Line Items]
Public Utilities, Amount proposed changes would increase wholesale transmission revenues		$ 9.4
Legal Contingencies [Abstract]
Public Utilities, Amount of proposed changes would be recoverable		$ 4.4
SPS [Member] | Southwest Power Pool (SPP) [Member] | SPP Open Access Transmission Tariff Upgrade Costs [Member]
Loss Contingencies [Line Items]
Public Utilities, Billed Charges For Transmission Service Upgrades			$ 13
Legal Contingencies [Abstract]
Public Utilities, Annual Transmission Revenue Requirement Increase (SPS)	$ 6

Commitments and Contingencies E

Commitments and Contingencies Environmental Requirements - Air (Details) $ in Millions	12 Months Ended
	Dec. 31, 2019USD ($)
Implementation of the National Ambient Air Quality Standard for sulfur dioxide [Member] | Harrington Units 1 and 2 [Member]
Loss Contingencies [Line Items]
Number of years unclassifiable areas will be monitored	3 years
Capital Addition Purchase Commitments [Member] | Regional Haze Rules [Member] | Tolk Units 1 and 2 [Member]
Loss Contingencies [Line Items]
Liability for estimated cost to comply with regulation	$ 600

Commitments and Contingencies M

Commitments and Contingencies MGP Sites (Details)	Dec. 31, 2019
Other MGP, Landfill, or Disposal Sites [Domain]
Loss Contingencies [Line Items]
Number of identified MGP sites under current investigation and/or remediation	1

Commitments and Contingencies_4

Commitments and Contingencies, Asset Retirement Obligations (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2019		Dec. 31, 2018
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Beginning balance		$ 32.4	[1]	$ 28.5
Amounts Incurred		16	[2]	0	[1]
Amounts Settled		(1.6)	[3]	0	[1]
Accretion		2.2		1.6
Cash flow revisions		28.3	[4]	2.3	[5]
Ending balance		77.3		32.4	[1]
Electric Plant Steam Production Asbestos
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Beginning balance		22	[1]	20.3
Amounts Incurred		0	[2]	0
Amounts Settled		(1.6)	[3]	0
Accretion		1.4		1.2
Cash flow revisions		29.5	[4]	0.5	[5]
Ending balance		51.3		22	[1]
Electric Plant Wind Production [Member]
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Beginning balance		0
Amounts Incurred	[2]	16
Amounts Settled	[3]	0
Accretion		0.4
Cash flow revisions	[4]	0
Ending balance		16.4		0
Electric Plant Distribution
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Beginning balance		9.1	[1]	7
Amounts Incurred		0	[2]	0
Amounts Settled		0	[3]	0
Accretion		0.4		0.3
Cash flow revisions		0	[4]	1.8	[5]
Ending balance		9.5		9.1	[1]
Electric Plant Other Sources
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Beginning balance		1.3	[1]	1.2
Amounts Incurred		0	[2]	0
Amounts Settled		0	[3]	0
Accretion		0		0.1
Cash flow revisions		(1.2)	[4]	0	[5]
Ending balance		$ 0.1		$ 1.3	[1]
[1]	There were no ARO amounts incurred or settled in 2018.
[2]	Amounts incurred related to the Hale wind farm placed in service in 2019.
[3]	Amounts settled related to asbestos abatement projects.
[4]	In 2019, AROs were revised for changes in timing and estimates of cash flows. Changes in steam production AROs primarily related to the cost estimates to remediate ponds at production facilities.
[5]	In 2018, AROs were revised for changes in timing and estimates of cash flows. Changes in electric distribution AROs were primarily related to increased labor costs.

Commitments and Contingencies R

Commitments and Contingencies Removal Costs (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Removal Costs [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities	$ 174.5	$ 187.7

Commitments and Contingencies A

Commitments and Contingencies ARO Phantom (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019		Dec. 31, 2018
Asset Retirement Obligations [Line Items]
Amounts Settled	$ 1.6	[1]	$ 0	[2]
Amounts Incurred	16	[3]	0	[2]
Electric Plant Steam Production Asbestos
Asset Retirement Obligations [Line Items]
Amounts Settled	1.6	[1]	0
Amounts Incurred	0	[3]	0
Electric Plant Distribution
Asset Retirement Obligations [Line Items]
Amounts Settled	0	[1]	0
Amounts Incurred	0	[3]	0
Electric Plant Other Sources
Asset Retirement Obligations [Line Items]
Amounts Settled	0	[1]	0
Amounts Incurred	$ 0	[3]	$ 0
[1]	Amounts settled related to asbestos abatement projects.
[2]	There were no ARO amounts incurred or settled in 2018.
[3]	Amounts incurred related to the Hale wind farm placed in service in 2019.

Commitments and Contingencies_5

Commitments and Contingencies, Leases (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Lessee, Lease, Description [Line Items]
Operating Lease, Weighted Average Discount Rate, Percent		4.40%
Short-term Lease, Cost		$ 1.5	$ 1.1	$ 1.2
Operating Lease, Assets and Liabilities, Lessee [Abstract]
Operating Lease, Right-of-Use Asset, Gross		548.3
Operating Lease, Right-of-Use Asset, Accumulated Depreciation		(25.9)
Operating lease right-of-use assets		522.4	0
Lease, Cost [Abstract]
Operating Lease, Cost	[1]	53	59	57.8
Lessee, Operating Lease, Liability, Payment, Due [Abstract]
2020		49.6
2021		49.5
2022		49.6
2023		49.6
2024		49.7
Thereafter		455.8
Total minimum obligation		703.8
Interest component of obligation		(181.6)
Present value of minimum obligation		522.2
Less current portion		(26.9)	0
Operating Lease, Liability, Noncurrent		$ 495.3	0
Weighted-average remaining lease term in years		14 years 1 month 6 days
Operating Leases, Future Minimum Payments Due, Fiscal Year Maturity [Abstract]
2019			51.9
2020			51.4
2021			51.3
2022			51.3
2023			51.3
Thereafter			507.1
Property, Plant and Equipment, Other Types [Member]
Operating Lease, Assets and Liabilities, Lessee [Abstract]
Operating Lease, Right-of-Use Asset, Gross		$ 48
Lease, Cost [Abstract]
Operating Lease, Cost	[2]	4.9	7.9	6.4
Lessee, Operating Lease, Liability, Payment, Due [Abstract]
2020		3.4
2021		3.3
2022		3.4
2023		3.4
2024		3.5
Thereafter		51.3
Total minimum obligation		68.3
Interest component of obligation		(21.6)
Present value of minimum obligation		46.7
Operating Leases, Future Minimum Payments Due, Fiscal Year Maturity [Abstract]
2019			5.2
2020			5.2
2021			5.1
2022			5.1
2023			5.1
Thereafter			56.3
Purchased Power Agreements
Operating Lease, Assets and Liabilities, Lessee [Abstract]
Operating Lease, Right-of-Use Asset, Gross		500.3
Lease, Cost [Abstract]
Operating Lease, Cost		48.1	51.1	$ 51.4
Lessee, Operating Lease, Liability, Payment, Due [Abstract]
2020	[3],[4]	46.2
2021	[3],[4]	46.2
2022	[3],[4]	46.2
2023	[3],[4]	46.2
2024	[3],[4]	46.2
Thereafter	[3],[4]	404.5
Total minimum obligation	[3],[4]	635.5
Interest component of obligation	[3],[4]	(160)
Present value of minimum obligation	[3],[4]	$ 475.5
Operating Leases, Future Minimum Payments Due, Fiscal Year Maturity [Abstract]
2019	[5],[6]		46.7
2020	[5],[6]		46.2
2021	[5],[6]		46.2
2022	[5],[6]		46.2
2023	[5],[6]		46.2
Thereafter	[5],[6]		$ 450.8
[1]	PPA capacity payments are included in electric fuel and purchased power on the statements of income. Expense for other operating leases is included in O&M expense.
[2]	Includes short-term lease expense of $1.5 million , $1.1 million and $1.2 million for 2019 , 2018 and 2017 , respectively.
[3]	Amounts do not include PPAs accounted for as executory contracts and/or contingent payments, such as energy payments on renewable PPAs.
[4]	PPA operating leases contractually expire at various dates through 2033.
[5]	Amounts do not include PPAs accounted for as executory contracts and/or contingent payments, such as energy payments on renewable PPAs.
[6]	PPA operating leases contractually expire at various dates through 2033.

Commitments and Contingencies_6

Commitments and Contingencies, Purchased Power Agreements (Details) - Capacity - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Purchased Power Agreements (PPAs) [Abstract]
Payments for capacity	$ 19.9	$ 57.6	$ 58.4
Estimated Future Payments Under PPAs [Abstract]
2020	12.3
2021	12.5
2022	12.7
2023	13
2024	5.9
Thereafter	0
Total	$ 56.4

Commitments and Contingencies_7

Commitments and Contingencies, Fuel Contracts (Details) $ in Thousands	Dec. 31, 2019USD ($)
Coal
Fuel Contracts [Abstract]
2020	$ 96,700
2021	67,700
2022	38,800
2023	0
2024	0
Thereafter	0
Total	203,200
Natural Gas Supply [Member]
Fuel Contracts [Abstract]
2020	12,300
2021	0
2022	0
2023	0
2024	0
Thereafter	0
Total	12,300
Natural Gas Storage and Transportation [Member]
Fuel Contracts [Abstract]
2020	28,900
2021	23,300
2022	17,400
2023	12,700
2024	6,700
Thereafter	26,300
Total	$ 115,300

Commitments and Contingencies_8

Commitments and Contingencies, Variable Interest Entities (Details) - MW	Dec. 31, 2019	Dec. 31, 2018
Independent Power Producing Entities
Purchased Power Agreements [Abstract]
Generating capacity (in MW)	1,197	1,197

Other Comprehensive Income (Det

Other Comprehensive Income (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2019	Dec. 31, 2018
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Accumulated other comprehensive loss at beginning of period		$ 2,536.6
Accumulated other comprehensive loss at end of period		2,884.5	$ 2,536.6
Gains and Losses on Cash Flow Hedges
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Accumulated other comprehensive loss at beginning of period		(0.7)	(0.8)
Other Comprehensive Income (Loss) before reclassifications		0
Amortization of net actuarial loss		0
Net current period other comprehensive income (loss)		0	0.1
Accumulated other comprehensive loss at end of period		(0.7)	(0.7)
Gains and Losses on Cash Flow Hedges | Interest Rate Swap
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Amortization of net actuarial loss	[1]		0.1
Defined Benefit Pension and Postretirement Items
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Accumulated other comprehensive loss at beginning of period		(0.7)	(0.7)
Other Comprehensive Income (Loss) before reclassifications		(0.2)
Amortization of net actuarial loss	[2]	0.2
Net current period other comprehensive income (loss)		0	0
Accumulated other comprehensive loss at end of period		(0.7)	(0.7)
Defined Benefit Pension and Postretirement Items | Interest Rate Swap
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Amortization of net actuarial loss			0
Total
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Accumulated other comprehensive loss at beginning of period		(1.4)	(1.5)
Other Comprehensive Income (Loss) before reclassifications		(0.2)
Amortization of net actuarial loss		0.2
Net current period other comprehensive income (loss)		0	0.1
Accumulated other comprehensive loss at end of period		$ (1.4)	(1.4)
Total | Interest Rate Swap
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Amortization of net actuarial loss			$ 0.1
[1]	Included in interest charges.
[2]	Included in the computation of net periodic pension and postretirement benefit costs. See Note 9 for further information.

Other Comprehensive Income Othe

Other Comprehensive Income Other Comprehensive Income Phantom (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Accumulated Other Comprehensive Income (Loss) [Line Items]
Income Tax Expense (Benefit)	$ 25.6	$ 38.9	$ 68.4
Reclassification out of Accumulated Other Comprehensive Income | Gains and Losses on Cash Flow Hedges
Accumulated Other Comprehensive Income (Loss) [Line Items]
Other Comprehensive Income (Loss) before reclassifications, Tax	0	0
Income Tax Expense (Benefit)	0	0
Reclassification from AOCI, Current Period, Tax	0	0
Reclassification out of Accumulated Other Comprehensive Income | Defined Benefit Pension and Postretirement Items
Accumulated Other Comprehensive Income (Loss) [Line Items]
Other Comprehensive Income (Loss) before reclassifications, Tax	0.1	0
Income Tax Expense (Benefit)	0	0
Reclassification from AOCI, Current Period, Tax	$ 0	$ 0

Related Party Transactions (Det

Related Party Transactions (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Operating expenses [Abstract]
Interest expense	$ 0.2	$ 0.6	$ 0
Accounts Receivable and Payable with Affiliates [Abstract]
Accounts receivable	4.2	10.5
Accounts payable	20.4	19.9
NSP-Minnesota
Accounts Receivable and Payable with Affiliates [Abstract]
Accounts receivable	4.2	4.7
Accounts payable	0	0
PSCo
Accounts Receivable and Payable with Affiliates [Abstract]
Accounts receivable	0	0
Accounts payable	0.4	0.7
Other Subsidiaries of Xcel Energy Inc.
Accounts Receivable and Payable with Affiliates [Abstract]
Accounts receivable	0	5.8
Accounts payable	20	19.2
Purchased Power [Member]
Operating expenses [Abstract]
Costs and Expenses, Related Party	0	0	1.4
Other Expense [Member]
Operating expenses [Abstract]
Costs and Expenses, Related Party	$ 192	$ 195.1	$ 196.6

Summarized Quarterly Financia_3

Summarized Quarterly Financial Data (Unaudited) (Details) - USD ($) $ in Millions	3 Months Ended												12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018		Sep. 30, 2018		Jun. 30, 2018		Mar. 31, 2018		Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Quarterly Financial Information Disclosure [Abstract]
Regulated and Unregulated Operating Revenue	$ 428.1	$ 533.1	$ 410.5	$ 454.1	$ 464.6		$ 540.1		$ 481.3		$ 447.2
Operating income	54.9	135.4	81.9	74.5	56	[1]	111	[1]	87.6	[1]	57.1	[1]	$ 346.7	$ 311.7	$ 300.9
Net income	$ 45.1	$ 105.1	$ 58.8	$ 54.1	$ 40.2		$ 81.5		$ 58.5		$ 33.1		$ 263.1	$ 213.3	$ 159.2
[1]	In 2018, SPS implemented ASU No. 2017-07 related to net periodic benefit cost, which resulted in retrospective reclassification of pension costs from O&M expense to other income.

Schedule II, Valuation and Qu_2

Schedule II, Valuation and Qualifying Accounts (Details) - Allowance for Bad Debts - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
SEC Schedule, 12-09, Movement in Valuation Allowances and Reserves [Roll Forward]
Balance at Jan. 1		$ 5.6	$ 6.4	$ 6.4
Charged to costs and expenses		5.7	4.9	5.1
Charged to other accounts	[1]	1.5	1	1.2
Deductions from reserves	[2]	(7.5)	(6.7)	(6.3)
Balance at Dec. 31		$ 5.3	$ 5.6	$ 6.4
[1]	Recovery of amounts previously written off.
[2]	Deductions related primarily to bad debt write-offs