Derivatives and Other Financial Instruments - Schedule of Fair Values of Level 3 Derivative Instruments Recorded as Assets and Liabilities (Detail) - USD ($) $ in Millions	Mar. 31, 2025	Dec. 31, 2024
Derivative Instruments Gain Loss [Line Items]
Total asset derivatives	$ 38	$ 25
Total liability derivatives	258	263
Total liability derivatives	850	836
Level 3 [Member] | Energy Contracts [Member]
Derivative Instruments Gain Loss [Line Items]
Total asset derivatives	30	24
Total liability derivatives	1,096	1,079
Level 3 [Member] | Derivatives Not Designated as Hedging Instruments [Member] | Energy Contracts [Member]
Derivative Instruments Gain Loss [Line Items]
Total asset derivatives	30	24
Total liability derivatives	3	2
Level 3 [Member] | Derivatives Not Designated as Hedging Instruments [Member] | Embedded Credit Derivative [Member] | Energy Contracts [Member]
Derivative Instruments Gain Loss [Line Items]
Total liability derivatives	1	1
Total liability derivatives	2	1
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	30	24
Level 3 [Member] | Derivatives Designated as Hedging Instruments [Member] | Energy Contracts [Member]
Derivative Instruments Gain Loss [Line Items]
Total liability derivatives	1,093	1,077
Level 3 [Member] | Derivatives Designated as Hedging Instruments [Member] | Power Contract [Member] | Energy Contracts [Member]
Derivative Instruments Gain Loss [Line Items]
Total liability derivatives	249	251
Total liability derivatives	$ 844	$ 826