Borrowings and Other Financing Instruments, Short-Term Borrowings (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	$ 50,000	$ 50,000	$ 15,000
Money Pool
Short-term Debt [Line Items]
Borrowing limit	100,000	100,000	100,000	$ 100,000
Amount outstanding at period end	0	0	0	16,000
Average amount outstanding	20,000	28,000	21,000	9,000
Maximum amount outstanding	$ 64,000	$ 100,000	$ 100,000	$ 100,000
Weighted average interest rate, computed on a daily basis (percentage)	0.83%	0.67%	0.40%	0.22%
Weighted average interest rate at period end (percentage)				0.45%
Commercial Paper
Short-term Debt [Line Items]
Borrowing limit	$ 400,000	$ 400,000	$ 400,000	$ 400,000
Amount outstanding at period end	50,000	50,000	15,000	37,000
Average amount outstanding	19,000	43,000	100,000	83,000
Maximum amount outstanding	$ 75,000	$ 140,000	$ 246,000	$ 241,000
Weighted average interest rate, computed on a daily basis (percentage)	0.74%	0.67%	0.46%	0.26%
Weighted average interest rate at period end (percentage)	0.95%	0.95%	0.60%	0.47%