Borrowings and Other Financing Instruments, Short-Term Borrowings (Details) (USD $)	3 Months Ended	12 Months Ended
	Sep. 30, 2014	Dec. 31, 2013
Short-term Debt [Line Items]
Amount outstanding at period end	$ 0	$ 84,000,000
Money Pool
Short-term Debt [Line Items]
Borrowing limit	100,000,000	100,000,000
Amount outstanding at period end	0	38,000,000
Average amount outstanding	1,000,000	46,000,000
Maximum amount outstanding	22,000,000	100,000,000
Weighted average interest rate, computed on a daily basis (percentage)	0.26%	0.15%
Weighted average interest rate at period end (percentage)		0.25%
Commercial Paper
Short-term Debt [Line Items]
Borrowing limit	300,000,000	300,000,000
Amount outstanding at period end	0	84,000,000
Average amount outstanding	43,000,000	32,000,000
Maximum amount outstanding	$ 106,000,000	$ 140,000,000
Weighted average interest rate, computed on a daily basis (percentage)	0.26%	0.30%
Weighted average interest rate at period end (percentage)		0.27%