Derivative Instruments and Hedging - Effect of Trading and Non-trading Derivatives (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Revenues	$ 1,470	$ 1,487	$ 4,651	$ 4,729
Utilities operating expense, purchased power	259	279	999	1,101
Renewables
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative, gain (loss) on derivative, net				43
Revenues	276	347	876	896
Renewables | Trading
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative, gain (loss) on derivative, net	0	(1)	1	(1)
Renewables | Non-trading
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative, gain (loss) on derivative, net	(5)	43	10	66
Operating revenues | Renewables | Trading
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative, gain (loss) on derivative, net	0	(1)	1	(1)
Operating revenues | Renewables | Trading | Wholesale Electricity Contract | Long
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative, gain (loss) on derivative, net	3	(1)	1	(2)
Operating revenues | Renewables | Trading | Wholesale Electricity Contract | Short
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative, gain (loss) on derivative, net	(7)	0	(1)	2
Operating revenues | Renewables | Trading | Financial Power Contracts
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative, gain (loss) on derivative, net	4	0	1	0
Operating revenues | Renewables | Trading | Financial and Natural Gas Contracts
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative, gain (loss) on derivative, net	0	0	0	(1)
Operating revenues | Renewables | Non-trading
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative, gain (loss) on derivative, net	(31)	56	(7)	60
Operating revenues | Renewables | Non-trading | Wholesale Electricity Contract | Long
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative, gain (loss) on derivative, net	0	0	0	0
Operating revenues | Renewables | Non-trading | Wholesale Electricity Contract | Short
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative, gain (loss) on derivative, net	(11)	42	4	37
Operating revenues | Renewables | Non-trading | Financial Power Contracts
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative, gain (loss) on derivative, net	(12)	13	2	22
Operating revenues | Renewables | Non-trading | Financial and Natural Gas Contracts
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative, gain (loss) on derivative, net	(8)	1	(13)	1
Operating Expense | Renewables | Trading
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative, gain (loss) on derivative, net	0	0	0	0
Operating Expense | Renewables | Trading | Wholesale Electricity Contract | Long
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative, gain (loss) on derivative, net	0	0	0	0
Operating Expense | Renewables | Trading | Financial Power Contracts
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative, gain (loss) on derivative, net		0		0
Operating Expense | Renewables | Trading | Financial and Natural Gas Contracts
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative, gain (loss) on derivative, net	0	0	0	0
Operating Expense | Renewables | Non-trading
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative, gain (loss) on derivative, net	26	(13)	17	6
Operating Expense | Renewables | Non-trading | Wholesale Electricity Contract | Long
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative, gain (loss) on derivative, net	9	(19)	(2)	(2)
Operating Expense | Renewables | Non-trading | Financial Power Contracts
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative, gain (loss) on derivative, net		0		(2)
Operating Expense | Renewables | Non-trading | Financial and Natural Gas Contracts
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative, gain (loss) on derivative, net	$ 16	$ 6	$ 19	$ 10