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Derivatives and Other Financial Instruments - Additional Information (Detail) kt in Thousands, $ in Millions | 3 Months Ended | 6 Months Ended | 12 Months Ended |
--- | --- | --- | --- | ---
Jun. 30, 2021 USD ($) kt | Jun. 30, 2020 USD ($) | Jun. 30, 2021 USD ($) MWh kt | Jun. 30, 2020 USD ($) | Dec. 31, 2020 MWh kt | Mar. 31, 2021 Contract
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |  |  |  |  |
Realized (loss) gain reclassed from Other comprehensive (loss) income to earnings | $ (67) | $ (13) | $ (119) | $ (38) |  |
Level 1 and 2 Derivative Instruments [Member] |  |  |  |  |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |  |  |  |  |
Realized (loss) gain reclassed from Other comprehensive (loss) income to earnings | $ (3) | (1) | $ (3) | (15) |  |
Energy Contracts [Member] | Other Comprehensive Loss [Member] |  |  |  |  |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |  |  |  |  |
Forecasted energy purchases in megawatt hours | MWh |  |  | 208,692 |  | 1,427,184 |
Derivatives Designated as Hedging Instruments [Member] | Power Contract [Member] |  |  |  |  |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |  |  |  |  |
Aluminum forecast sales | kt | 2,018 |  | 2,018 |  | 2,130 |
Derivatives Designated as Hedging Instruments [Member] | Power Contract [Member] | Cash Flow Hedging [Member] |  |  |  |  |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |  |  |  |  |
Amount of (loss) gain expected to be recognized into earnings over the next 12 months |  |  | $ (213) |  |  |
Derivatives Designated as Hedging Instruments [Member] | Financial Contracts [Member] | Cash Flow Hedging [Member] |  |  |  |  |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |  |  |  |  |
Amount of (loss) gain expected to be recognized into earnings over the next 12 months |  |  | 2 |  |  |
Derivatives Designated as Hedging Instruments [Member] | Cost of Goods Sold [Member] | Level 1 and 2 Derivative Instruments [Member] |  |  |  |  |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |  |  |  |  |
Realized (loss) gain reclassed from Other comprehensive (loss) income to earnings | $ (1) | $ (1) | (1) | (8) |  |
Derivatives Designated as Hedging Instruments [Member] | Sales [Member] | Level 1 and 2 Derivative Instruments [Member] |  |  |  |  |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |  |  |  |  |
Realized (loss) gain reclassed from Other comprehensive (loss) income to earnings | $ (2) |  | $ (2) | $ (7) |  |
Derivatives Not Designated as Hedging Instruments [Member] |  |  |  |  |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |  |  |  |  |
Number of new financial contracts | Contract |  |  |  |  |  | 4

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- Definition The estimated net amount of existing gains or losses on cash flow hedges at the reporting date expected to be reclassified to earnings within the next 12 months. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 30 -Section 50 -Paragraph 1 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=121549185&loc=d3e80748-113994 + Details
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