Borrowings and Other Financing Instruments, Short-Term Borrowings (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Short-term Debt [Line Items]
Amount outstanding at period end	$ 0	$ 50,000
Money Pool
Short-term Debt [Line Items]
Borrowing limit	100,000	100,000	$ 100,000
Amount outstanding at period end	0	0	0
Average amount outstanding	13,000	28,000	21,000
Maximum amount outstanding	$ 100,000	$ 100,000	$ 100,000
Weighted average interest rate, computed on a daily basis (percentage)	1.12%	0.67%	0.40%
Commercial Paper
Short-term Debt [Line Items]
Borrowing limit	$ 400,000	$ 400,000	$ 400,000
Amount outstanding at period end	0	50,000	15,000
Average amount outstanding	69,000	43,000	100,000
Maximum amount outstanding	$ 176,000	$ 140,000	$ 246,000
Weighted average interest rate, computed on a daily basis (percentage)	1.13%	0.67%	0.46%
Weighted average interest rate at period end (percentage)		0.95%	0.60%