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
	Mar. 31, 2013	Mar. 31, 2012
Total gains (losses) included in earnings	$ 14	$ (35)
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	(5)	1
Operating revenue
Total gains (losses) included in earnings	12	(9)
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	(5)	1
Electric fuel and other energy-related purchases
Total gains (losses) included in earnings	2	(26)
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