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	$ 0		$ 154,000,000
Commercial Paper [Member]
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	154,000,000		8,000,000
Maximum amount outstanding	332,000,000		165,000,000
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)		[1]	0.35%
Money Pool [Member]
Short-term Debt [Line Items]
Borrowing limit	250,000,000		250,000,000
Amount outstanding at period end	0		0
Average amount outstanding	1,000,000		300,000
Maximum amount outstanding	$ 12,000,000		$ 8,000,000
Weighted average interest rate, computed on a daily basis (in hundredths)	0.36%		0.33%
Weighted average interest rate at end of period (in hundredths)		[1]		[1]

[1] N/A