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, unless otherwise specified	3 Months Ended
	Mar. 31, 2012	Mar. 31, 2011
Total realized and unrealized gains (losses):
Included in earnings	$ (35)	$ 14
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	1	4
Fair Value, Measurements, Recurring
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	(71)	(50)
Total realized and unrealized gains (losses):
Included in earnings	(35)	14
Included in other comprehensive income (loss)	5	(94)
Included in regulatory assets/liabilities	11	(21)
Settlements	30	(16)
Transfers out of Level 3	(1)	4
Ending balance	(61)	(163)
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	$ 1	$ 4