Borrowings and Other Financing Instruments, Short-Term Borrowings (Details) (USD $)	3 Months Ended	12 Months Ended
	Sep. 30, 2013	Dec. 31, 2012
Short-term Debt [Line Items]
Amount outstanding at period end	$ 0	$ 154,000,000
Commercial Paper
Short-term Debt [Line Items]
Borrowing limit	700,000,000	700,000,000
Amount outstanding at period end	0	154,000,000
Average amount outstanding	0	8,000,000
Maximum amount outstanding	0	165,000,000
Weighted average interest rate, computed on a daily basis (in hundredths)		0.33%
Weighted average interest rate at period end (in hundredths)		0.35%
Money Pool
Short-term Debt [Line Items]
Borrowing limit	250,000,000	250,000,000
Amount outstanding at period end	0	0
Average amount outstanding	0	300,000
Maximum amount outstanding	$ 0	$ 8,000,000
Weighted average interest rate, computed on a daily basis (in hundredths)		0.33%