Borrowings and Other Financing Instruments, Short-Term Borrowings (Details) (USD $)	3 Months Ended	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Short-term Debt [Line Items]
Amount outstanding at period end	$ 9,000,000	$ 9,000,000	$ 0
Money Pool [Member]
Short-term Debt [Line Items]
Borrowing limit	100,000,000	100,000,000	100,000,000	100,000,000
Amount outstanding at period end	0	0	5,000,000	0
Average amount outstanding	1,000,000	10,000,000	12,000,000	16,000,000
Maximum amount outstanding	9,000,000	63,000,000	71,000,000	77,000,000
Weighted average interest rate, computed on a daily basis (in hundredths)	0.34%	0.33%	0.35%	0.37%
Weighted average interest rate at end of period (in hundredths)			0.35%
Commercial Paper [Member]
Short-term Debt [Line Items]
Borrowing limit	300,000,000	300,000,000	300,000,000	248,000,000
Amount outstanding at period end	9,000,000	9,000,000	0	49,000,000
Average amount outstanding	4,000,000	18,000,000	54,000,000	8,000,000
Maximum amount outstanding	$ 28,000,000	$ 106,000,000	$ 161,000,000	$ 65,000,000
Weighted average interest rate, computed on a daily basis (in hundredths)	0.34%	0.39%	0.37%	0.37%
Weighted average interest rate at end of period (in hundredths)	0.36%	0.36%		0.37%