Discontinued Operations - Derivative Gain (Loss) for Retail Electric and Natural Gas Supply Businesses (Detail) - Discontinued Operations [Member] - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Derivative Instruments, Gain (Loss) [Line Items]
Reclassification of mark-to-market to realized on settlement of contracts			$ 10
Unrealized mark-to-market loss	$ 0	$ 0	0
Total net gain			$ 10