Derivatives and Other Financial Instruments - Schedule of Fair Values of Level 3 Derivative Instruments Recorded as Assets and Liabilities (Detail) - Level 3 [Member] - USD ($) $ in Millions	Sep. 30, 2020	Dec. 31, 2019
Derivative Instruments Gain Loss [Line Items]
Fair value asset derivatives	$ 2	$ 74
Fair value liability derivatives	516	615
Derivatives Designated as Hedging Instruments [Member]
Derivative Instruments Gain Loss [Line Items]
Fair value asset derivatives	2	74
Fair value liability derivatives	490	598
Derivatives Designated as Hedging Instruments [Member] | Fair Value of Derivative Contracts - Current [Member] | Financial Contracts [Member]
Derivative Instruments Gain Loss [Line Items]
Fair value asset derivatives	2	57
Derivatives Designated as Hedging Instruments [Member] | Fair Value of Derivative Contracts - Noncurrent [Member] | Financial Contracts [Member]
Derivative Instruments Gain Loss [Line Items]
Fair value asset derivatives		17
Derivatives Designated as Hedging Instruments [Member] | Fair Value of Derivative Contracts - Current [Member] | Power Related Derivative [Member]
Derivative Instruments Gain Loss [Line Items]
Fair value liability derivatives	45	47
Derivatives Designated as Hedging Instruments [Member] | Fair Value of Derivative Contracts - Noncurrent [Member] | Power Related Derivative [Member]
Derivative Instruments Gain Loss [Line Items]
Fair value liability derivatives	445	551
Derivatives Not Designated as Hedging Instruments [Member]
Derivative Instruments Gain Loss [Line Items]
Fair value liability derivatives	26	17
Derivatives Not Designated as Hedging Instruments [Member] | Fair Value of Derivative Contracts - Current [Member] | Power Related Derivative [Member]
Derivative Instruments Gain Loss [Line Items]
Fair value liability derivatives	5	3
Derivatives Not Designated as Hedging Instruments [Member] | Fair Value of Derivative Contracts - Noncurrent [Member] | Power Related Derivative [Member]
Derivative Instruments Gain Loss [Line Items]
Fair value liability derivatives	$ 21	$ 14