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	$ 15,000	$ 15,000	$ 37,000
Money Pool
Short-term Debt [Line Items]
Borrowing limit	100,000	100,000	100,000	$ 100,000
Amount outstanding at period end	0	0	16,000	38,000
Average amount outstanding	29,000	21,000	9,000	46,000
Maximum amount outstanding	$ 89,000	$ 100,000	$ 100,000	$ 100,000
Weighted average interest rate, computed on a daily basis (percentage)	0.36%	0.40%	0.22%	0.29%
Weighted average interest rate at period end (percentage)			0.45%	0.25%
Commercial Paper
Short-term Debt [Line Items]
Borrowing limit	$ 400,000	$ 400,000	$ 400,000	$ 300,000
Amount outstanding at period end	15,000	15,000	37,000	84,000
Average amount outstanding	4,000	100,000	83,000	32,000
Maximum amount outstanding	$ 55,000	$ 246,000	$ 241,000	$ 140,000
Weighted average interest rate, computed on a daily basis (percentage)	0.45%	0.46%	0.26%	0.30%
Weighted average interest rate at period end (percentage)	0.60%	0.60%	0.47%	0.27%