Borrowings and Other Financing Instruments, Commercial Paper (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	$ 78,000
Commercial Paper
Short-term Debt [Line Items]
Borrowing limit	150,000	150,000
Amount outstanding at period end	0	78,000
Average amount outstanding	0	46,000
Maximum amount outstanding	$ 0	$ 101,000
Weighted average interest rate, computed on a daily basis (percentage)		0.27%
Weighted average interest rate at period end (percentage)		0.55%