Borrowings and Other Financing Instruments, Commercial Paper (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Short-term Debt [Line Items]
Amount outstanding at period end	$ 392,000	$ 392,000	$ 846,000
Commercial Paper
Short-term Debt [Line Items]
Borrowing Limit	2,750,000	2,750,000	2,750,000	$ 2,750,000
Amount outstanding at period end	392,000	392,000	846,000	1,020,000
Average amount outstanding	290,000	485,000	601,000	841,000
Maximum amount outstanding	$ 582,000	$ 1,183,000	$ 1,360,000	$ 1,200,000
Weighted average interest rate, computed on a daily basis (percentage)	0.75%	0.74%	0.48%	0.33%
Weighted average interest rate at period end (percentage)	0.95%	0.95%	0.82%	0.56%