Discontinued Operations - Derivative Gain (Loss) for Retail Electric and Natural Gas Supply Businesses (Detail) (Discontinued Operations [Member], USD $) In Millions, unless otherwise specified	3 Months Ended	6 Months Ended
	Jun. 30, 2013	Jun. 30, 2013
Discontinued Operations [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Reclassification of mark-to-market to realized on settlement of contracts	$ 5	$ 10
Unrealized mark-to-market loss	0	0
Total net gain (loss)	$ 5	$ 10