Fair Value Measurements (Gains and Losses Included in Earnings 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
Total gains (losses) 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)
Operating Revenue [Member]
Total gains (losses) included in earnings	(32)	(4)	29
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)	1
Electric Fuel and Other Energy Related Purchases
Total gains (losses) included in earnings	(45)	51	(165)
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	0	0	0
Purchased Gas [Member]
Total gains (losses) included in earnings	0	(4)	(12)
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	$ 0	$ 0	$ (4)