Derivative Instruments and Hedging - Effect of Trading and Non-trading Derivatives (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Total gain included in operating revenues	$ 1,789	$ 1,842
Total loss included in purchased power, natural gas and fuel used	475	563
Renewables and Gas Activities | Trading
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Total Gain	3	(1)
Renewables and Gas Activities | Trading | Wholesale Electricity Contract
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Total Gain		1
Renewables and Gas Activities | Trading | Financial Power Contracts
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Total Gain		(1)
Renewables and Gas Activities | Trading | Financial and Natural Gas Contracts
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Total Gain		0
Renewables and Gas Activities | Non-trading
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Total Gain	14	4
Renewables and Gas Activities | Non-trading | Wholesale Electricity Contract
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Total Gain		0
Renewables and Gas Activities | Non-trading | Financial Power Contracts
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Total Gain		(13)
Renewables and Gas Activities | Non-trading | Financial and Natural Gas Contracts
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Total Gain		(9)
Operating revenues | Renewables and Gas Activities | Trading
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Total Gain	3	(1)
Operating revenues | Renewables and Gas Activities | Trading | Wholesale Electricity Contract | Long
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Total Gain	(1)
Operating revenues | Renewables and Gas Activities | Trading | Wholesale Electricity Contract | Short
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Total Gain	4
Operating revenues | Renewables and Gas Activities | Trading | Financial Power Contracts
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Total Gain	0
Operating revenues | Renewables and Gas Activities | Trading | Financial and Natural Gas Contracts
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Total Gain		(1)
Operating revenues | Renewables and Gas Activities | Non-trading
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Total Gain	33	(24)
Operating revenues | Renewables and Gas Activities | Non-trading | Wholesale Electricity Contract | Long
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Total Gain	0
Operating revenues | Renewables and Gas Activities | Non-trading | Wholesale Electricity Contract | Short
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Total Gain	11
Operating revenues | Renewables and Gas Activities | Non-trading | Financial Power Contracts
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Total Gain	22
Operating revenues | Renewables and Gas Activities | Non-trading | Financial and Natural Gas Contracts
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Total Gain		(2)
Purchased power, natural gas and fuel used | Renewables and Gas Activities | Trading
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Total Gain	0	0
Purchased power, natural gas and fuel used | Renewables and Gas Activities | Trading | Wholesale Electricity Contract | Long
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Total Gain	0	0
Purchased power, natural gas and fuel used | Renewables and Gas Activities | Trading | Financial Power Contracts
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Total Gain	0	0
Purchased power, natural gas and fuel used | Renewables and Gas Activities | Trading | Financial and Natural Gas Contracts
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Total Gain	0	0
Purchased power, natural gas and fuel used | Renewables and Gas Activities | Non-trading
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Total Gain	(19)	28
Purchased power, natural gas and fuel used | Renewables and Gas Activities | Non-trading | Wholesale Electricity Contract | Long
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Total Gain	(11)	20
Purchased power, natural gas and fuel used | Renewables and Gas Activities | Non-trading | Financial Power Contracts
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Total Gain	(6)	1
Purchased power, natural gas and fuel used | Renewables and Gas Activities | Non-trading | Financial and Natural Gas Contracts
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Total Gain	$ (2)	$ 7