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Derivatives and Other Financial Instruments - Schedule of Fair Values of Level 3 Derivative Instruments Recorded as Liabilities (Detail) - Level 3 [Member] - USD ($) $ in Millions | Jun. 30, 2021 | Dec. 31, 2020
--- | --- | ---
Derivative Instruments Gain Loss [Line Items] |  |
Fair value asset derivatives | $ 5 |
Fair value liability derivatives | 1,415 | $ 838
Derivatives Designated as Hedging Instruments [Member] |  |
Derivative Instruments Gain Loss [Line Items] |  |
Fair value asset derivatives | 2 |
Fair value liability derivatives | 1,397 | 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 |  | 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 | 213 | 94
Derivatives Designated as Hedging Instruments [Member] | Current Assets | Financial Contracts [Member] |  |
Derivative Instruments Gain Loss [Line Items] |  |
Fair value asset derivatives | 2 |
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 | 1,184 | 720
Derivatives Not Designated as Hedging Instruments [Member] |  |
Derivative Instruments Gain Loss [Line Items] |  |
Fair value asset derivatives | 3 |
Fair value liability derivatives | 18 | 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] | Current Assets | Financial Contracts [Member] |  |
Derivative Instruments Gain Loss [Line Items] |  |
Fair value asset derivatives | 3 |
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

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