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Borrowings and Other Financing Instruments Term Loan (Details) - USD ($)
9 Months Ended
Sep. 30, 2021
Sep. 30, 2020
Feb. 17, 2021
Dec. 31, 2020
Short-term Debt [Line Items]        
Amount outstanding at period end $ 1,747,000,000     $ 584,000,000
Proceeds from issuance of common stock 13,000,000 $ 5,000,000    
Expiration Period, Line of Credit     $ 364  
Letter of Credit        
Short-term Debt [Line Items]        
Amount outstanding at period end 19,000,000     $ 20,000,000
364-Day Term Loan [Member] | Short-term Debt [Member] | Xcel Energy Inc.        
Short-term Debt [Line Items]        
Credit Facility 1,200,000,000   $ 1,200,000,000  
Required debt to total capitalization ratio     6500.00%  
Amount outstanding at period end 1,200,000,000      
Available $ 0