Discontinued Operations - Derivative Gain for Retail Electric and Natural Gas Supply Businesses (Detail) (Discontinued Operations [Member], USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013
Discontinued Operations [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Reclassification of mark-to-market to realized on settlement of contracts				$ 10
Unrealized mark-to-market loss
Total net gain				$ 10