Fair Value Measurements (Gains and Losses Included in Earnings 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
Total gains (losses) included in earnings	$ (16)	$ 27	$ (24)	$ 40
The amount of total 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)
Operating Revenue [Member]
Total gains (losses) included in earnings	(8)	5	(8)	(5)
The amount of total 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)
Electric Fuel and Other Energy-Related Purchases [Member]
Total gains (losses) included in earnings	(8)	22	(16)	49
The amount of total 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	0	0	0	0
Purchased Gas [Member]
Total gains (losses) included in earnings	0	0	0	(4)
The amount of total 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	$ 0	$ 0	$ 0	$ 0