Borrowings and Other Financing Instruments, Short-Term Borrowings (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Sep. 30, 2015	Dec. 31, 2014
Short-term Debt [Line Items]
Amount outstanding at period end	$ 0	$ 382,000
Money Pool
Short-term Debt [Line Items]
Borrowing limit	250,000	250,000
Amount outstanding at period end	0	0
Average amount outstanding	0	4,000
Maximum amount outstanding	8,000	$ 97,000
Weighted average interest rate, computed on a daily basis (percentage)		0.25%
Commercial Paper
Short-term Debt [Line Items]
Borrowing limit	700,000	$ 700,000
Amount outstanding at period end	0	382,000
Average amount outstanding	8,000	167,000
Maximum amount outstanding	$ 67,000	$ 393,000
Weighted average interest rate, computed on a daily basis (percentage)	0.43%	0.31%
Weighted average interest rate at period end (percentage)		0.65%