Borrowings and Other Financing Instruments (Details) (USD $)	1 Months Ended	6 Months Ended			3 Months Ended	12 Months Ended		3 Months Ended	12 Months Ended
	Jul. 31, 2012	Jun. 30, 2012		Dec. 31, 2011	Jun. 30, 2012 Commercial Paper [Member]	Dec. 31, 2011 Commercial Paper [Member]		Jun. 30, 2012 Money Pool [Member]	Dec. 31, 2011 Money Pool [Member]
Short-term Debt [Line Items]
Borrowing limit					$ 700,000,000	$ 700,000,000		$ 250,000,000	$ 250,000,000
Amount outstanding at period end		5,000,000		0	5,000,000	0
Amount outstanding at period end		5,000,000		0				5,000,000	0
Average amount outstanding					3,000,000	73,000,000		0	3,000,000
Maximum amount outstanding					31,000,000	304,000,000		5,000,000	53,000,000
Weighted average interest rate, computed on a daily basis (in hundredths)					0.31%	0.37%		0.33%	0.35%
Weighted average interest rate at period end (in hundredths)					0.30%		[1]	0.34%		[1]
Committed Credit Facilities [Abstract]
Credit facility		700,000,000
Drawn		8,000,000	[2]
Available		692,000,000
Credit facility bank borrowings outstanding		0		0
Amended Credit Agreement [Abstract]
Amended term (in years)	5Y
Original term (in years)		4Y
Minimum borrowing margin based on long-term credit ratings (in hundredths)	0.875%	1.00%
Maximum borrowing margin based on long-term credit ratings (in hundredths)	1.75%	2.00%
Minimum commitment fees calculated on unused portion of lines of credit (in hundredths)	0.075%	0.10%
Maximum commitment fees calculated on unused portion of lines of credit (in hundredths)	0.275%	0.35%
Number of additional periods the revolving termination date can be extended, subject to majority bank group approval	2
Term of each additional period the revolving termination date can be extended, subject to majority bank group approval (in years)	1Y
Letters of Credit [Abstract]
Terms of letters of credit (in years)		1Y
Letters of credit outstanding under credit facilities		$ 3,000,000		$ 4,900,000

[1] N/A

[2] Includes outstanding commercial paper and letters of credit.