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Borrowings and Other Financing Instruments - Letters of Credit (Details) - USD ($)
9 Months Ended
Sep. 30, 2020
Dec. 31, 2019
Letter of Credit    
Line of Credit Facility [Line Items]    
Amount outstanding at period end $ 2,000,000 $ 2,000,000
Debt Instrument, Term 1 year  
Revolving Credit Facility [Member]    
Line of Credit Facility [Line Items]    
Direct advances on the credit facility outstanding $ 0 $ 0