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	$ 60,000	$ 60,000	$ 10,000
Commercial Paper
Short-term Debt [Line Items]
Borrowing limit	150,000	150,000	150,000	$ 150,000
Amount outstanding at period end	60,000	60,000	10,000	78,000
Average amount outstanding	32,000	15,000	39,000	46,000
Maximum amount outstanding	$ 64,000	$ 64,000	$ 122,000	$ 101,000
Weighted average interest rate, computed on a daily basis (percentage)	0.73%	0.69%	0.44%	0.27%
Weighted average interest rate at period end (percentage)	0.95%	0.95%	0.70%	0.55%