Derivative Instruments and Hedging - Fair Value of Derivative Contract (Detail) - Renewables - USD ($) $ in Millions	Sep. 30, 2020	Dec. 31, 2019
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative fair value	$ 89	$ 85
Financial Power Contracts
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative fair value	62	73
Long | Wholesale Electricity Contract
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative fair value	8	10
Long | Natural Gas and Other fuel Contracts
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative fair value	6	(2)
Short | Wholesale Electricity Contract
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative fair value	$ 1	$ 4