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	3 Months Ended		9 Months Ended
	Sep. 30, 2011	Sep. 30, 2010	Sep. 30, 2011	Sep. 30, 2010
Fair Value Disclosures [Abstract]
Included in earnings	$ (16)	$ 27	$ (24)	$ 40
The amount of gains (losses) for the period included in earnings attributable to the change in unrealized gains (losses) relating to assets/liabilities still held at the reporting date	7	4	29	(3)
Fair Value, Measurements, Recurring [Member]
Fair Value Disclosures [Abstract]
Beginning balance	(122)	32	(50)	(66)
Included in earnings	(16)	27	(24)	40
Included in other comprehensive income (loss)	75	(65)	16	20
Included in regulatory assets/liabilities	(3)	(13)	(35)	1
Settlements	24	(23)	47	(41)
Transfers out of Level 3	0	25	4	29
Ending balance	(42)	(17)	(42)	(17)
The amount of gains (losses) for the period included in earnings attributable to the change in unrealized gains (losses) relating to assets/liabilities still held at the reporting date	$ 7	$ 4	$ 29	$ (3)