Borrowings and Other Financing Instruments, Commercial Paper (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Short-term Debt [Line Items]
Amount outstanding at period end	$ 11,000	$ 11,000	$ 60,000
Commercial Paper
Short-term Debt [Line Items]
Borrowing limit	150,000	150,000	150,000	$ 150,000
Amount outstanding at period end	11,000	11,000	60,000	10,000
Average amount outstanding	70,000	52,000	15,000	39,000
Maximum amount outstanding	$ 129,000	$ 129,000	$ 64,000	$ 122,000
Weighted average interest rate, computed on a daily basis (percentage)	1.38%	1.23%	0.69%	0.44%
Weighted average interest rate at period end (percentage)	1.73%	1.73%	0.95%	0.70%