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Borrowings and Other Financing Instruments, Credit Facility (Details) - Credit Facility - USD ($)
Mar. 31, 2019
Dec. 31, 2018
Line of Credit Facility [Line Items]    
Credit Facility $ 400,000,000  
Drawn 139,000,000  
Available 261,000,000  
Direct advances on the credit facility outstanding $ 0 $ 0