XML 53 R41.htm IDEA: XBRL DOCUMENT v3.21.2
Borrowings and Other Financing Instruments Bilateral Credit Agreement (Details) - USD ($)
$ in Millions
9 Months Ended
Sep. 30, 2021
Dec. 31, 2020
Short-term Debt [Line Items]    
Amount outstanding at period end $ 1,747 $ 584
Letter of Credit    
Short-term Debt [Line Items]    
Debt Instrument, Term 1 year  
Amount outstanding at period end $ 19 $ 20
Bilateral Credit Agreement [Member] | NSP-Minnesota | Letter of Credit    
Short-term Debt [Line Items]    
Debt Instrument, Term 1 year  
Credit Facility $ 75  
Amount outstanding at period end 41  
Available $ 34