Derivatives and Other Financial Instruments - Schedule of Fair Values of Level 3 Derivative Instruments Recorded as Liabilities (Detail) - Level 3 [Member] - USD ($) $ in Millions	Mar. 31, 2021	Dec. 31, 2020
Derivative Instruments Gain Loss [Line Items]
Fair value liability derivatives	$ 1,080	$ 838
Derivatives Designated as Hedging Instruments [Member]
Derivative Instruments Gain Loss [Line Items]
Fair value liability derivatives	1,061	815
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	14	1
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	145	94
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	902	720
Derivatives Not Designated as Hedging Instruments [Member]
Derivative Instruments Gain Loss [Line Items]
Fair value liability derivatives	19	23
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	1
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 - Noncurrent [Member] | Embedded Credit Derivative [Member]
Derivative Instruments Gain Loss [Line Items]
Fair value liability derivatives	$ 15	$ 19