Derivative Instruments and Hedging - Fair Value of Derivative Contract (Detail) - Renewables and Gas Activities - USD ($) $ in Millions	Jun. 30, 2019	Dec. 31, 2018
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative Fair Value, Net	$ 42	$ 46
Financial Power Contracts
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative Fair Value, Net	71	55
Long | Wholesale Electricity Contract
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative Fair Value, Net	28	11
Long | Natural Gas and Other fuel Contracts
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative Fair Value, Net	(3)	(2)
Long | Basis Swap
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative Fair Value, Net	(2)	(6)
Short | Wholesale Electricity Contract
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative Fair Value, Net	$ (52)	$ (12)