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	$ 129,000	$ 129,000	$ 14,000
Money Pool
Short-term Debt [Line Items]
Borrowing limit	250,000	250,000	250,000	$ 250,000
Amount outstanding at period end	0	0	0	0
Average amount outstanding	56,000	21,000	1,000	4,000
Maximum amount outstanding	$ 141,000	$ 141,000	$ 34,000	$ 97,000
Weighted average interest rate, computed on a daily basis (percentage)	0.78%	0.73%	0.41%	0.25%
Commercial Paper
Short-term Debt [Line Items]
Borrowing limit	$ 700,000	$ 700,000	$ 700,000	$ 700,000
Amount outstanding at period end	129,000	129,000	14,000	382,000
Average amount outstanding	33,000	24,000	95,000	167,000
Maximum amount outstanding	$ 151,000	$ 154,000	$ 449,000	$ 393,000
Weighted average interest rate, computed on a daily basis (percentage)	0.78%	0.70%	0.51%	0.31%
Weighted average interest rate at period end (percentage)	0.95%	0.95%	0.60%	0.65%