Discontinued Operations - Derivative Gain (Loss) for Retail Electric and Natural Gas Supply Businesses (Detail) (Discontinued Operations [Member], USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012
Discontinued Operations [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Reclassification of mark-to-market to realized on settlement of contracts	$ 10	$ 27
Unrealized mark-to-market loss		(3)
Total net gain	$ 10	$ 24