Fair Value Measurements (Net Change in the Assets and Liabilities Measured at Fair Value on a Recurring Basis and Included in the Level 3 Fair Value Category) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Included in earnings	$ (77)	$ 43	$ (148)
The amount of total gains (losses) for the period included in earnings attributable to the change in unrealized gains (losses) relating to assets still held at the reporting date	22	(4)	(3)
Fair Value, Measurements, Recurring [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	(50)	(66)	99
Included in earnings	(77)	43	(148)
Included in other comprehensive income (loss)	14	(49)	(188)
Included in regulatory assets/liabilities	(42)	24	52
Settlements	88	(38)	126
Transfers out of Level 3	4	(36)	7
Ending balance	(71)	(50)	(66)
The amount of total gains (losses) for the period included in earnings attributable to the change in unrealized gains (losses) relating to assets still held at the reporting date	$ 22	$ (4)	$ (3)