Summary of Significant Accounting Policies (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2011	Dec. 31, 2010
Schedule of Regulatory Assets and Liabilities [Abstract]
Total assets (liabilities), net	$ 3,865	$ 2,598
Asset retirement obligations-liability [Member]
Schedule of Regulatory Assets and Liabilities [Abstract]
Total assets (liabilities), net	(42)	(82)
Other cost of removal obligations [Member]
Schedule of Regulatory Assets and Liabilities [Abstract]
Total assets (liabilities), net	(1,196)	(1,188)
Deferred income tax credits [Member]
Schedule of Regulatory Assets and Liabilities [Abstract]
Total assets (liabilities), net	(225)	(237)
State income tax credits [Member]
Schedule of Regulatory Assets and Liabilities [Abstract]
Total assets (liabilities), net	(62)	0
Over recovered regulatory clause revenues [Member]
Schedule of Regulatory Assets and Liabilities [Abstract]
Total assets (liabilities), net	(28)	(40)
Property damage reserves [Member]
Schedule of Regulatory Assets and Liabilities [Abstract]
Total assets (liabilities), net	(206)	(216)
Fuel hedging-liability [Member]
Schedule of Regulatory Assets and Liabilities [Abstract]
Total assets (liabilities), net	(13)	(7)
Environmental remediation-liability [Member]
Schedule of Regulatory Assets and Liabilities [Abstract]
Total assets (liabilities), net	(8)	(10)
Other liabilities [Member]
Schedule of Regulatory Assets and Liabilities [Abstract]
Total assets (liabilities), net	(30)	(13)
Deferred income tax charges [Member]
Schedule of Regulatory Assets and Liabilities [Abstract]
Total assets (liabilities), net	1,293	1,204
Deferred income tax charges - Medicare subsidy [Member]
Schedule of Regulatory Assets and Liabilities [Abstract]
Total assets (liabilities), net	77	82
Asset retirement obligations-asset [Member]
Schedule of Regulatory Assets and Liabilities [Abstract]
Total assets (liabilities), net	117	79
Loss on reacquired debt [Member]
Schedule of Regulatory Assets and Liabilities [Abstract]
Total assets (liabilities), net	285	274
Vacation pay [Member]
Schedule of Regulatory Assets and Liabilities [Abstract]
Total assets (liabilities), net	160	151
Under recovered regulatory clause revenues [Member]
Schedule of Regulatory Assets and Liabilities [Abstract]
Total assets (liabilities), net	50	27
Building leases [Member]
Schedule of Regulatory Assets and Liabilities [Abstract]
Total assets (liabilities), net	43	45
Generating plant outage costs [Member]
Schedule of Regulatory Assets and Liabilities [Abstract]
Total assets (liabilities), net	38	31
Under recovered storm damage costs [Member]
Schedule of Regulatory Assets and Liabilities [Abstract]
Total assets (liabilities), net	43	8
Fuel hedging-asset [Member]
Schedule of Regulatory Assets and Liabilities [Abstract]
Total assets (liabilities), net	249	211
Other assets [Member]
Schedule of Regulatory Assets and Liabilities [Abstract]
Total assets (liabilities), net	290	171
Environmental remediation-asset [Member]
Schedule of Regulatory Assets and Liabilities [Abstract]
Total assets (liabilities), net	71	67
Retiree benefit plans [Member]
Schedule of Regulatory Assets and Liabilities [Abstract]
Total assets (liabilities), net	$ 2,959	$ 2,041