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Borrowings and Other Financing Instruments (Tables)
 12 Months Ended
Dec. 31, 2024
Debt Disclosure [Abstract]  
Money Pool [Table Text Block]
Money pool borrowings:
(Millions of Dollars, Except Interest Rates)Three Months Ended Dec. 31, 2024Year Ended Dec. 31
202420232022
Borrowing limit$100 $100 $100 $100 
Amount outstanding at period end7 7 27 — 
Average amount outstanding6 24 46 35 
Maximum amount outstanding60 100 100 100 
Weighted average interest rate, computed on a daily basis4.58 %5.29 %5.04 %0.44 %
Weighted average interest rate at end of period4.58 4.58 5.34 N/A
Short-Term Borrowings Commercial paper outstanding:
(Millions of Dollars, Except Interest Rates)Three Months Ended Dec. 31, 2024Year Ended Dec. 31
202420232022
Borrowing limit$500 $500 $500 $500 
Amount outstanding at period end145 145 75 34 
Average amount outstanding13 35 50 100 
Maximum amount outstanding145 160 119 324 
Weighted average interest rate, computed on a daily basis4.70 %5.47 %5.12 %0.79 %
Weighted average interest rate at end of period4.66 4.66 5.56 4.55 
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 of dollars)
Additional Periods for Which a One-Year Extension May Be Requested (b)
20242023
46.6%46.1%$502
(a)The 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. 		[1],[2]
Credit Facilities
SPS had the following committed credit facility available as of Dec. 31, 2024 (in millions) of dollars:
Credit Facility (a)
Drawn (b)
Available
$500 $145 $355 
(a)This credit facility matures in September 2027.
(b)Includes letters of credit and outstanding commercial paper.
 [3],[4]
Schedule of Long-term Debt Instruments [Table Text Block]
Long-term debt obligations for SPS as of Dec. 31 (in millions of dollars):
Financing InstrumentInterest RateMaturity Date20242023
First mortgage bonds3.30 %June 15, 2024$— $150 
First mortgage bonds3.30 June 15, 2024— 200 
Unsecured senior notes6.00 Oct. 1, 2033100 100 
Unsecured senior notes6.00 Oct. 1, 2036250 250 
First mortgage bonds4.50 Aug. 15, 2041200 200 
First mortgage bonds4.50 Aug. 15, 2041100 100 
First mortgage bonds4.50 Aug. 15, 2041100 100 
First mortgage bonds3.40 Aug. 15, 2046300 300 
First mortgage bonds3.70 Aug. 15, 2047450 450 
First mortgage bonds4.40 Nov. 15, 2048300 300 
First mortgage bonds 3.75 June 15, 2049300 300 
First mortgage bonds3.15 May 1, 2050350 350 
First mortgage bonds3.15 May 1, 2050250 250 
First mortgage bonds5.15 June 1, 2052200 200 
First mortgage bonds (a)
6.00 Sept. 15, 2053100 100 
First mortgage bonds (b)
6.00 June 1, 2054600 — 
Unamortized discount(14)(10)
Unamortized debt issuance cost(35)(29)
Current maturities— (350)
Total long-term debt$3,551 $2,961 
(a)2023 financing.
(b)2024 financing.
 [5]
Schedule of Stock by Class [Table Text Block] SPS has the following preferred stock:
Preferred Stock Authorized (Shares)Par Value of Preferred StockPreferred Stock Outstanding (Shares) 2024 and 2023
10,000,000 1.00 — 
Dividend Payment Restrictions [Text Block]
Requirements and actuals as of Dec. 31, 2024:
Equity to Total Capitalization Ratio
Required Range
Equity to Total Capitalization Ratio Actual (a)
LowHigh2024
45.0 %55.0 %54.4 %
(a)Excludes short-term debt.
Unrestricted Retained EarningsTotal Capitalization
Limit on Total Capitalization (a)
$592  million$7,789 millionN/A
(a)SPS may not pay a dividend that would cause it to lose its investment grade bond rating.
[1] All extension requests are subject to majority bank group approval.
[2] The credit facility has a financial covenant requiring that the debt-to-total capitalization ratio be less than or equal to 65%.
[3] Includes letters of credit and outstanding commercial paper.
[4] This credit facility matures in September 2027.
[5] 2024 financing