Derivative Instruments and Hedging - Effect of Non-Trading Derivatives (Detail) - Renewables and Gas Activities [Member] - Non-Trading Derivatives [Member] - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Derivative Instruments And Hedging Activities Disclosures [Line Items]
(Loss) Gain on Derivative, Net	$ 17	$ (5)
Wholesale Electricity Contract [Member] | Long [Member]
Derivative Instruments And Hedging Activities Disclosures [Line Items]
(Loss) Gain on Derivative, Net	(6)	(3)
Wholesale Electricity Contract [Member] | Short [Member]
Derivative Instruments And Hedging Activities Disclosures [Line Items]
(Loss) Gain on Derivative, Net	11	6
Financial Power Contracts [Member]
Derivative Instruments And Hedging Activities Disclosures [Line Items]
(Loss) Gain on Derivative, Net	16	1
Financial and Natural Gas Contracts [Member]
Derivative Instruments And Hedging Activities Disclosures [Line Items]
(Loss) Gain on Derivative, Net	$ (4)	$ (9)