Derivatives and Other Financial Instruments - Schedule of Fair Values of Level 3 Derivative Instruments Recorded as Assets and Liabilities (Detail) - USD ($) $ in Millions	Sep. 30, 2023	Dec. 31, 2022
Derivative Instruments Gain Loss [Line Items]
Total asset derivatives	$ 33	$ 134
Total liability derivatives	204	200
Total liability derivatives	927	1,026
Level 3 [Member] | Energy Contracts [Member]
Derivative Instruments Gain Loss [Line Items]
Total asset derivatives	7	52
Total liability derivatives	1,115	1,212
Level 3 [Member] | Derivatives Not Designated as Hedging Instruments [Member] | Energy Contracts [Member]
Derivative Instruments Gain Loss [Line Items]
Total asset derivatives	7	32
Level 3 [Member] | Derivatives Not Designated as Hedging Instruments [Member] | Financial Contracts [Member] | Energy Contracts [Member]
Derivative Instruments Gain Loss [Line Items]
Total asset derivatives	7	32
Level 3 [Member] | Derivatives Designated as Hedging Instruments [Member] | Energy Contracts [Member]
Derivative Instruments Gain Loss [Line Items]
Total asset derivatives	0	20
Total liability derivatives	1,115	1,212
Level 3 [Member] | Derivatives Designated as Hedging Instruments [Member] | Power Contract [Member] | Energy Contracts [Member]
Derivative Instruments Gain Loss [Line Items]
Total liability derivatives	196	195
Total liability derivatives	919	1,017
Level 3 [Member] | Derivatives Designated as Hedging Instruments [Member] | Financial Contracts [Member] | Energy Contracts [Member]
Derivative Instruments Gain Loss [Line Items]
Total asset derivatives	$ 0	$ 20