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Borrowings and Other Financing Instruments Bilateral Credit Agreement (Details) - USD ($)
$ in Millions
6 Months Ended
Jun. 30, 2020
Jun. 30, 2019
Dec. 31, 2019
Short-term Debt [Line Items]      
Amount outstanding at period end $ 1,410   $ 595
Letter of Credit      
Short-term Debt [Line Items]      
Debt Instrument, Term 1 year    
Amount outstanding at period end $ 20   $ 20
Bilateral Credit Agreement [Member] | NSP Minnesota [Member] | Letter of Credit      
Short-term Debt [Line Items]      
Debt Instrument, Term 1 year 1 year  
Credit Facility $ 75    
Amount outstanding at period end 31    
Available $ 44