Employee Benefit Plans (Reconciliation of Assets Measured at Fair Value on a Recurring Basis) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2011		Dec. 31, 2010	Dec. 31, 2009
Beginning balance	$ 146		$ 223	$ 258
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Purchases, Sales, Issuances, Settlements			(60)	(23)
Purchases	16	[1]
Sales	(3)
Ending Balance	85		146	223
Master Trust [Member]
Beginning balance	185		256
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Purchases, Sales, Issuances, Settlements			(71)
Purchases	156
Sales	(29)
Total losses, (realized and unrealized) and other	10
Ending Balance	$ 322		$ 185

[1] Derivative amounts relate to financial transmission rights