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	$ 253,000,000	$ 0
Money Pool
Short-term Debt [Line Items]
Borrowing limit	250,000,000	250,000,000
Amount outstanding at period end	11,000,000	0
Average amount outstanding	10,000,000	100,000
Maximum amount outstanding	97,000,000	12,000,000
Weighted average interest rate, computed on a daily basis (percentage)	0.25%	0.36%
Weighted average interest rate at period end (percentage)	0.24%
Commercial Paper
Short-term Debt [Line Items]
Borrowing limit	700,000,000	700,000,000
Amount outstanding at period end	253,000,000	0
Average amount outstanding	225,000,000	38,000,000
Maximum amount outstanding	$ 315,000,000	$ 332,000,000
Weighted average interest rate, computed on a daily basis (percentage)	0.28%	0.34%
Weighted average interest rate at period end (percentage)	0.28%