Derivatives and Other Financial Instruments - Schedule of Fair Values of Level 3 Derivative Instruments Recorded as Liabilities (Detail) - Level 3 [Member] - USD ($) $ in Millions	Sep. 30, 2021	Dec. 31, 2020
Derivative Instruments Gain Loss [Line Items]
Fair value liability derivatives	$ 1,555	$ 838
Derivatives Designated as Hedging Instruments [Member]
Derivative Instruments Gain Loss [Line Items]
Fair value liability derivatives	1,530	815
Derivatives Designated as Hedging Instruments [Member] | Fair Value of Derivative Contracts - Current [Member] | Power Contract [Member]
Derivative Instruments Gain Loss [Line Items]
Fair value liability derivatives	273	94
Derivatives Designated as Hedging Instruments [Member] | Fair Value of Derivative Contracts - Current [Member] | Financial Contracts [Member]
Derivative Instruments Gain Loss [Line Items]
Fair value liability derivatives		1
Derivatives Designated as Hedging Instruments [Member] | Fair Value of Derivative Contracts - Noncurrent [Member] | Power Contract [Member]
Derivative Instruments Gain Loss [Line Items]
Fair value liability derivatives	1,257	720
Derivatives Not Designated as Hedging Instruments [Member]
Derivative Instruments Gain Loss [Line Items]
Fair value liability derivatives	25	23
Derivatives Not Designated as Hedging Instruments [Member] | Fair Value of Derivative Contracts - Current [Member] | Embedded Credit Derivative [Member]
Derivative Instruments Gain Loss [Line Items]
Fair value liability derivatives	3	4
Derivatives Not Designated as Hedging Instruments [Member] | Fair Value of Derivative Contracts - Current [Member] | Financial Contracts [Member]
Derivative Instruments Gain Loss [Line Items]
Fair value liability derivatives	8
Derivatives Not Designated as Hedging Instruments [Member] | Fair Value of Derivative Contracts - Noncurrent [Member] | Embedded Credit Derivative [Member]
Derivative Instruments Gain Loss [Line Items]
Fair value liability derivatives	$ 14	$ 19