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, 2025	Dec. 31, 2024
Derivative Instruments Gain Loss [Line Items]
Total asset derivatives	$ 44	$ 25
Total asset derivatives	47	0
Total liability derivatives	329	263
Total liability derivatives	929	836
Level 3 [Member] | Energy Contracts [Member]
Derivative Instruments Gain Loss [Line Items]
Total asset derivatives	17	24
Total liability derivatives	1,165	1,079
Level 3 [Member] | Derivatives Not Designated as Hedging Instruments [Member] | Energy Contracts [Member]
Derivative Instruments Gain Loss [Line Items]
Total asset derivatives	17	24
Total liability derivatives	0	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	0	1
Total liability derivatives	0	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	9	24
Total asset derivatives	8	0
Level 3 [Member] | Derivatives Designated as Hedging Instruments [Member] | Energy Contracts [Member]
Derivative Instruments Gain Loss [Line Items]
Total liability derivatives	1,165	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	285	251
Total liability derivatives	$ 880	$ 826