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Borrowings and Other Financing Instruments, Commercial Paper (Details) - USD ($)
3 Months Ended 12 Months Ended
Mar. 31, 2017
Dec. 31, 2016
Short-term Debt [Line Items]    
Amount outstanding at period end $ 605,000,000 $ 392,000,000
Commercial Paper    
Short-term Debt [Line Items]    
Borrowing limit 2,750,000,000 2,750,000,000
Amount outstanding at period end 605,000,000 392,000,000
Average amount outstanding 557,000,000 485,000,000
Maximum amount outstanding $ 719,000,000 $ 1,183,000,000
Weighted average interest rate, computed on a daily basis (percentage) 0.97% 0.74%
Weighted average interest rate at period end (percentage) 1.18% 0.95%