Borrowings and Other Financing Instruments, Commercial Paper (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Short-term Debt [Line Items]
Amount outstanding at period end	$ 846,000	$ 846,000	$ 1,019,500
Commercial Paper
Short-term Debt [Line Items]
Borrowing Limit	2,750,000	2,750,000	2,750,000	$ 2,450,000
Amount outstanding at period end	846,000	846,000	1,020,000	759,000
Average amount outstanding	290,000	601,000	841,000	481,000
Maximum amount outstanding	$ 846,000	$ 1,360,000	$ 1,200,000	$ 1,160,000
Weighted average interest rate, computed on a daily basis (percentage)	0.56%	0.48%	0.33%	0.31%
Weighted average interest rate at period end (percentage)	0.82%	0.82%	0.56%	0.25%