Borrowings and Other Financing Instruments, Short-Term Borrowings (Details) (USD $)	3 Months Ended	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Short-term Debt [Line Items]
Amount outstanding at period end	$ 37,000,000	$ 37,000,000	$ 84,000,000
Money Pool
Short-term Debt [Line Items]
Borrowing limit	100,000,000	100,000,000	100,000,000	100,000,000
Amount outstanding at period end	16,000,000	16,000,000	38,000,000	0
Average amount outstanding	0	9,000,000	46,000,000	10,000,000
Maximum amount outstanding	16,000,000	100,000,000	100,000,000	63,000,000
Weighted average interest rate, computed on a daily basis (in hundredths)	0.35%	0.22%	0.29%	0.33%
Weighted average interest rate at period end (percentage)	0.45%	0.45%	0.25%
Commercial Paper
Short-term Debt [Line Items]
Borrowing limit	400,000,000	400,000,000	300,000,000	300,000,000
Amount outstanding at period end	37,000,000	37,000,000	84,000,000	9,000,000
Average amount outstanding	22,000,000	83,000,000	32,000,000	18,000,000
Maximum amount outstanding	$ 54,000,000	$ 241,000,000	$ 140,000,000	$ 106,000,000
Weighted average interest rate, computed on a daily basis (in hundredths)	0.31%	0.26%	0.30%	0.39%
Weighted average interest rate at period end (percentage)	0.47%	0.47%	0.27%	0.36%