Borrowings and Other Financing Instruments (Details) (USD $)	1 Months Ended	6 Months Ended	9 Months Ended		1 Months Ended			3 Months Ended		12 Months Ended		3 Months Ended		12 Months Ended
	Jul. 31, 2012	Jun. 30, 2012	Sep. 30, 2012	Dec. 31, 2011	Jun. 30, 2012 First Mortgage Bonds [Member] Series Due Aug. 15, 2041 [Member]	Sep. 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]	Sep. 30, 2012 Commercial Paper [Member]		Dec. 31, 2011 Commercial Paper [Member]		Sep. 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								0		54,000,000		0		12,000,000
Maximum amount outstanding								16,000,000		161,000,000		9,000,000		71,000,000
Weighted average interest rate, computed on a daily basis (in hundredths)								0.33%		0.37%		0.33%		0.35%
Weighted average interest rate at period end (in hundredths)									[1]		[1]		[1]	0.35%
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%
Maturity date					Aug. 15, 2041
Principal outstanding						$ 300,000,000

[1] N/A