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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

X

- Definition

Fair value, after the effects of master netting arrangements, of a financial asset or other contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes assets not subject to a master netting arrangement and not elected to be offset.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
-Section 55
-Paragraph 10
-URI http://asc.fasb.org/extlink&oid=99393222&loc=SL20226008-175313

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 50
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- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

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- Definition

Fair value, after the effects of master netting arrangements, of a financial liability or contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes liabilities not subject to a master netting arrangement and not elected to be offset.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
-Section 50
-Paragraph 3
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-URI http://asc.fasb.org/extlink&oid=51824906&loc=SL20225862-175312

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-Name Accounting Standards Codification
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