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Borrowings and Other Financing Instruments, Credit Facility (Details) - USD ($)
3 Months Ended
Mar. 31, 2016
Dec. 31, 2015
Line of Credit Facility [Line Items]    
Short-term debt $ 73,000,000 $ 223,000,000
Credit Facility    
Line of Credit Facility [Line Items]    
Debt Instrument, Maturity Date Oct. 31, 2019  
Credit Facility [1] $ 500,000,000  
Drawn [2] 91,000,000  
Available 409,000,000  
Direct advances on the credit facility outstanding $ 0 $ 0
[1] This credit facility expires in October 2019.
[2] Includes outstanding commercial paper and letters of credit.