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Borrowings and Other Financing Instruments, Credit Facility (Details) - Credit Facility - USD ($)
6 Months Ended
Jun. 30, 2018
Dec. 31, 2017
Line of Credit Facility [Line Items]    
Credit Facility [1] $ 150,000,000  
Drawn [2] 30,000,000  
Available $ 120,000,000  
Maturity Date Jun. 30, 2021  
Direct advances on the credit facility outstanding $ 0 $ 0
[1] This credit facility expires in June 2021.
[2] Includes outstanding commercial paper.