Borrowings and Other Financing Instruments, Commercial Paper (Details) - USD ($)	3 Months Ended	12 Months Ended
	Mar. 31, 2016	Dec. 31, 2015
Short-term Debt [Line Items]
Amount outstanding at period end	$ 183,000,000	$ 846,000,000
Commercial Paper
Short-term Debt [Line Items]
Borrowing limit	2,750,000,000	2,750,000,000
Amount outstanding at period end	183,000,000	846,000,000
Average amount outstanding	774,000,000	601,000,000
Maximum amount outstanding	$ 1,183,000,000	$ 1,360,000,000
Weighted average interest rate, computed on a daily basis (percentage)	0.73%	0.48%
Weighted average interest rate at period end (percentage)	0.63%	0.82%