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		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Derivative Instruments And Hedging Activities Disclosures [Line Items]
(Loss) Gain on Derivative, Net	$ (12)	$ 9	$ 5	$ 4
Wholesale Electricity Contract [Member] | Long [Member]
Derivative Instruments And Hedging Activities Disclosures [Line Items]
(Loss) Gain on Derivative, Net	4	11	(2)	8
Wholesale Electricity Contract [Member] | Short [Member]
Derivative Instruments And Hedging Activities Disclosures [Line Items]
(Loss) Gain on Derivative, Net	(5)	(20)	6	(14)
Financial Power Contracts [Member]
Derivative Instruments And Hedging Activities Disclosures [Line Items]
(Loss) Gain on Derivative, Net	(10)	(17)	6	(16)
Financial and Natural Gas Contracts [Member]
Derivative Instruments And Hedging Activities Disclosures [Line Items]
(Loss) Gain on Derivative, Net	$ (1)	$ 35	$ (5)	$ 26