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Borrowings and Other Financing Instruments Bilateral Credit Agreement (Details) - USD ($)
$ in Millions
9 Months Ended
Sep. 30, 2025
Dec. 31, 2024
Short-term Debt [Line Items]    
Amount outstanding at period end $ 1,330 $ 695
Letter of Credit    
Short-term Debt [Line Items]    
Debt Instrument, Term 1 year  
Amount outstanding at period end $ 74 $ 42
Bilateral Credit Agreement [Member] | NSP-Minnesota | Letter of Credit    
Short-term Debt [Line Items]    
Debt Instrument, Term 1 year  
Amount outstanding at period end $ 69  
Credit Facility $ 75