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	$ 9,000,000
Money Pool
Short-term Debt [Line Items]
Borrowing limit	100,000,000	100,000,000
Amount outstanding at period end	80,000,000	0
Average amount outstanding	66,000,000	10,000,000
Maximum amount outstanding	99,000,000	63,000,000
Weighted average interest rate, computed on a daily basis (in hundredths)	0.28%	0.33%
Weighted average interest rate at period end (in hundredths)	0.26%
Commercial Paper
Short-term Debt [Line Items]
Borrowing limit	300,000,000	300,000,000
Amount outstanding at period end	0	9,000,000
Average amount outstanding	33,000,000	18,000,000
Maximum amount outstanding	$ 140,000,000	$ 106,000,000
Weighted average interest rate, computed on a daily basis (in hundredths)	0.28%	0.39%
Weighted average interest rate at period end (in hundredths)		0.36%