Borrowings and Other Financing Instruments, Short-Term Borrowings (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Short-term Debt [Line Items]
Amount outstanding at period end	$ 14,000	$ 14,000	$ 382,000
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	4,000	1,000	4,000	0
Maximum amount outstanding	$ 34,000	$ 34,000	$ 97,000	$ 12,000
Weighted average interest rate, computed on a daily basis (percentage)	0.36%	0.41%	0.25%	0.36%
Commercial Paper
Short-term Debt [Line Items]
Borrowing limit	$ 700,000	$ 700,000	$ 700,000	$ 700,000
Amount outstanding at period end	14,000	14,000	382,000	0
Average amount outstanding	14,000	95,000	167,000	38,000
Maximum amount outstanding	$ 68,000	$ 449,000	$ 393,000	$ 332,000
Weighted average interest rate, computed on a daily basis (percentage)	0.50%	0.51%	0.31%	0.34%
Weighted average interest rate at period end (percentage)	0.60%	0.60%	0.65%