Summary Of Significant Accounting Policies (Equity Changes Attributable To Noncontrolling Interest) (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended		3 Months Ended	6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012 Genco [Member]	Jun. 30, 2012 Genco [Member]	Jun. 30, 2011 Genco [Member]	Mar. 31, 2011 Genco [Member]
Noncontrolling Interest [Line Items]
Noncontrolling interest, beginning of period	$ 147	$ 155	$ 149	$ 154	$ 5	$ 7	$ 11	$ 12
Net income (loss) attributable to noncontrolling interests	(1)	1	(1)	4	(2)	(4)	1
Dividends paid to noncontrolling interest holders	(1)	(1)	(3)	(3)
Noncontrolling interest, end of period	$ 145	$ 155	$ 145	$ 155	$ 3	$ 3	$ 12	$ 12