Fair Value Measurements (Gains and Losses Included in Earnings in the Level 3 Fair Value Category) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011
Total gains (losses) included in earnings	$ 12	$ (22)	$ (23)	$ (8)
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	33	27	33	31
Operating revenue
Total gains (losses) included in earnings	32	2	23	0
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	33	27	33	31
Electric fuel and other energy-related purchases
Total gains (losses) included in earnings	(20)	(24)	(46)	(8)
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