Borrowings and Other Financing Instruments (Details) (USD $)	1 Months Ended	6 Months Ended		1 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 First Mortgage Bonds [Member] Series Due Aug. 15, 2041 [Member]	Aug. 31, 2011 First Mortgage Bonds [Member] Series Due Aug. 15, 2041 [Member]	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						$ 300,000,000		$ 300,000,000		$ 100,000,000		$ 100,000,000
Amount outstanding at period end						0		0
Amount outstanding at period end		0	5,000,000							0		5,000,000
Average amount outstanding						55,000,000		54,000,000		14,000,000		12,000,000
Maximum amount outstanding						106,000,000		161,000,000		65,000,000		71,000,000
Weighted average interest rate, computed on a daily basis (in hundredths)						0.39%		0.37%		0.35%		0.35%
Weighted average interest rate at period end (in hundredths)							[1]		[1]		[1]	0.35%
Committed Credit Facilities [Abstract]
Credit facility		300,000,000
Drawn		0
Available		300,000,000
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		0	0
Long-Term Borrowings [Abstract]
Face amount				100,000,000	200,000,000
Interest rate, stated percentage (in hundredths)				4.50%	4.50%
Maturity date				Aug. 15, 2041	Aug. 15, 2041
Unamortized premium				10,100,000
Principal outstanding				$ 300,000,000

[1] N/A