Aluminum futures designated as fair value hedges in kilo metric ton.

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Number of refineries.

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Number of smelters.

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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.

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Fair value, before 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 elected not to be offset. Excludes assets not subject to a master netting arrangement.

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Fair value, before 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 elected not to be offset. Excludes liabilities not subject to a master netting arrangement.

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Nominal energy measure used to calculate payments on a derivative instrument.

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The number of derivative instruments of a particular group held by the entity.

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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.

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Description of the arrangement to purchase electricity from a production plant constructed and financed by a public utility district, cooperative, or other organization with an expiration date longer than one year from the balance sheet date, including its form and substance.

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Amount after tax of reclassification adjustment from accumulated other comprehensive income of accumulated gain (loss) realized from derivative instruments designated and qualifying as the effective portion of cash flow hedges and an entity's share of an equity investee's deferred hedging gain (loss).

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The net change in the difference between the fair value and the carrying value, or in the comparative fair values, of open derivatives, commodity, or energy contracts, held at each balance sheet date, that was included in earnings for the period.

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Derivatives and Other Financial Instruments - Additional Information (Detail) $ in Millions	1 Months Ended	3 Months Ended	12 Months Ended
	Feb. 28, 2019 Smelter	Mar. 31, 2019 USD ($) MWh Smelter Refinery Derivative kt	Dec. 31, 2018 USD ($) MWh kt
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Number of smelters \| Smelter	2
Number of derivative instruments \| Derivative		1
Embedded Aluminum Derivative [Member] \| Derivatives Designated as Hedging Instruments [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Aluminum forecast sales \| kt		2,496	2,508
Energy Contracts [Member] \| Derivatives Designated as Hedging Instruments [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Electricity purchases		At March 31, 2019 and December 31, 2018, the financial contract hedges forecasted electricity purchases of 5,742,396 and 6,348,276 megawatt hours, respectively.
Energy Contracts [Member] \| Derivatives Designated as Hedging Instruments [Member] \| Cost of Goods Sold [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Amount of gain (loss) expected to be recognized into earnings over the next 12 months		$ 69
Level 3 [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Number of smelters \| Smelter		9
Number of refineries \| Refinery		3
Derivative [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Fair value of derivative contracts recorded as assets		$ 2	$ 49
Fair value of derivative contracts recorded as liabilities		$ 2	$ 54
Other Comprehensive Loss [Member] \| Energy Contracts [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Forecasted energy purchases in megawatt hours \| MWh		5,742,396	6,348,276
Cash Flow Hedging [Member] \| Embedded Aluminum Derivative [Member] \| Derivatives Designated as Hedging Instruments [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Amount of gain (loss) expected to be recognized into earnings over the next 12 months		$ 59
Cash Flow Hedging [Member] \| Other Comprehensive Loss [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Unrealized gain (loss) recognized		8	$ 68
Realized gain (loss) on derivatives		$ 4	$ 1