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Borrowings and Other Financing Instruments - Credit Facility (Details) - Credit Facility - USD ($)
Mar. 31, 2021
Dec. 31, 2020
Line of Credit Facility [Line Items]    
Credit Facility [1] $ 500,000,000  
Outstanding [2] 2,000,000  
Available 498,000,000  
Direct advances on the credit facility outstanding $ 0 $ 0
[1] Expires in June 2024
[2] Includes outstanding letters of credit.