Borrowings and Other Financing Instruments, Short-Term Borrowings (Details) (USD $)	3 Months Ended	12 Months Ended
	Mar. 31, 2013	Dec. 31, 2012
Short-term Debt [Line Items]
Amount outstanding at period end	$ 16,000,000	$ 9,000,000
Money Pool [Member]
Short-term Debt [Line Items]
Borrowing limit	100,000,000	100,000,000
Amount outstanding at period end	20,000,000	0
Average amount outstanding	2,000,000	10,000,000
Maximum amount outstanding	30,000,000	63,000,000
Weighted average interest rate, computed on a daily basis (in hundredths)	0.34%	0.33%
Weighted average interest rate at end of period (in hundredths)	0.34%		[1]
Commercial Paper [Member]
Short-term Debt [Line Items]
Borrowing limit	300,000,000	300,000,000
Amount outstanding at period end	16,000,000	9,000,000
Average amount outstanding	28,000,000	18,000,000
Maximum amount outstanding	$ 49,000,000	$ 106,000,000
Weighted average interest rate, computed on a daily basis (in hundredths)	0.34%	0.39%
Weighted average interest rate at end of period (in hundredths)	0.34%	0.36%

[1] N/A