Borrowings and Other Financing Instruments (Details) (USD $) In Millions, unless otherwise specified	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	Sep. 30, 2012 First Mortgage Bonds [Member] Series Due Sept. 15, 2022 [Member]	Sep. 30, 2012 First Mortgage Bonds [Member] Series Due Sept. 15, 2042 [Member]	Sep. 30, 2012 First Mortgage Bonds [Member] Series Due Oct. 1, 2012 [Member]	Oct. 31, 2012 First Mortgage Bonds [Member] Series Due Jan. 1, 2019 [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										$ 700		$ 700		$ 250		$ 250
Amount outstanding at period end										0		0
Amount outstanding at period end														0		0
Average amount outstanding										6		73		1		3
Maximum amount outstanding										46		304		8		53
Weighted average interest rate, computed on a daily basis (in hundredths)										0.30%		0.37%		0.32%		0.35%
Weighted average interest rate at period end (in hundredths)											[1]		[1]		[1]		[1]
Credit Facilities [Abstract]
Credit facility			700.0
Drawn			3.9	[2]
Available			696.1
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			3.9		4.9
Long-Term Borrowings and Other Financing Instruments [Abstract]
Face amount						300	500
Interest rate, stated percentage (in hundredths)						2.25%	3.60%	7.875%	5.10%
Maturity date						Sep. 15, 2022	Sep. 15, 2042	Oct. 01, 2012	Jan. 01, 2019
Redemption of debt								$ 600.00	$ 48.75

[1] N/A

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