Property, Plant and Equipment - Capital Lease Assets Recorded within Property, Plant and Equipment (Detail) - USD ($) $ in Millions	Dec. 31, 2015	Dec. 31, 2014
Capital Leased Assets [Line Items]
Original Cost	$ 152	$ 152
Accumulated Amortization	102	92
Net Book Value	50	60
Transmission [Member]
Capital Leased Assets [Line Items]
Original Cost	76	76
Accumulated Amortization	51	46
Net Book Value	25	30
Distribution [Member]
Capital Leased Assets [Line Items]
Original Cost	76	76
Accumulated Amortization	51	46
Net Book Value	$ 25	$ 30