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	$ 37,000
Money Pool
Short-term Debt [Line Items]
Borrowing limit	100,000	100,000
Amount outstanding at period end	18,000	16,000
Average amount outstanding	47,000	9,000
Maximum amount outstanding	$ 100,000	$ 100,000
Weighted average interest rate, computed on a daily basis (percentage)	0.41%	0.22%
Weighted average interest rate at period end (percentage)	0.38%	0.45%
Commercial Paper
Short-term Debt [Line Items]
Borrowing limit	$ 400,000	$ 400,000
Amount outstanding at period end	0	37,000
Average amount outstanding	135,000	83,000
Maximum amount outstanding	$ 246,000	$ 241,000
Weighted average interest rate, computed on a daily basis (percentage)	0.45%	0.26%
Weighted average interest rate at period end (percentage)		0.47%