Assessed income tax amount exclusive of interest and penalties.

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Income tax examination reversal of provision for income taxes reserve.

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Income tax payable, possible unrecorded assessed amount from taxing authority.

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Notices include claims for compounded interest on the tax amount.

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Payment amount refund percentage.

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Payment of disputed tax amount, percentage.

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Percentage of penalty on amount of additional taxes due.

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Proposed administrative penalties.

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Tax assessment deposit.

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Tax credit previously claimed and disallowed by tax authority.

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Carrying amount as of the balance sheet date of unearned revenue or income, not otherwise specified in the taxonomy, which is expected to be taken into income after one year or beyond the normal operating cycle, if longer and noncurrent obligations not separately disclosed in the balance sheet (other liabilities, noncurrent).

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A credit or adjustment for government or taxing authority authorized decrease in taxes owed as a result of meeting certain tax policy conditions.

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Estimated amount of loss resulting from an adverse tax position.

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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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The equity interest of noncontrolling shareholders, partners or other equity holders in consolidated entity.

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The parent entity's interest in net assets of the subsidiary, expressed as a percentage.

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Amount of increase (decrease) to previously recorded tax expense. Includes, but is not limited to, significant settlements of income tax disputes, and unusual tax positions or infrequent actions taken by the entity, including tax assessment reversal, and IRS tax settlement.

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Contingencies - Additional Information - 1 (Detail) € in Millions, $ in Millions			1 Months Ended			3 Months Ended
	Sep. 17, 2020 USD ($)	Sep. 17, 2020 AUD ($)	Mar. 31, 2013 USD ($)	May 31, 2012 USD ($)	May 31, 2012 BRL (R$)	Mar. 31, 2021 USD ($)	Mar. 31, 2021 BRL (R$)	Mar. 31, 2021 AUD ($)	Dec. 31, 2020 USD ($)	Dec. 31, 2020 EUR (€)	Sep. 30, 2020 USD ($)	Sep. 30, 2020 AUD ($)	Sep. 30, 2018 USD ($)	Sep. 30, 2018 EUR (€)	Mar. 31, 2021 AUD ($)	Sep. 30, 2020 AUD ($)	Jul. 07, 2020 USD ($)	Jul. 07, 2020 AUD ($)	Mar. 31, 2013 BRL (R$)
Loss Contingencies [Line Items]
Other noncurrent liabilities and deferred credits						$ 558,000,000			$ 515,000,000
AWAC [Member] \| Alumina Limited [Member]
Loss Contingencies [Line Items]
Non-controlling interest, ownership percentage						40.00%									40.00%
Alcoa Corporation [Member] \| AWAC [Member]
Loss Contingencies [Line Items]
Ownership interest percentage						60.00%									60.00%
Tax Authority, Spain [Member]
Loss Contingencies [Line Items]
Charge recorded in provision for income taxes to establish liability for estimated loss													$ 30,000,000	€ 26
Reversal of reserve recorded in provision for income taxes to establish liability for estimated loss									$ 32,000,000	€ 26
Brazilian Federal Revenue Office [Member] \| Alcoa World Alumina Brasil [Member]
Loss Contingencies [Line Items]
Disallowed tax credits			$ 110,000,000																R$ 220,000,000
Percentage of penalty of the gross disallowed amount			50.00%
Value added tax receivable				$ 41,000,000	R$ 82,000,000
Brazilian Federal Revenue Office [Member] \| Minimum [Member] \| Alcoa World Alumina Brasil [Member]
Loss Contingencies [Line Items]
Charge recorded in provision for income taxes to establish liability for estimated loss						$ 0
Brazilian Federal Revenue Office [Member] \| Maximum [Member] \| Alcoa World Alumina Brasil [Member]
Loss Contingencies [Line Items]
Charge recorded in provision for income taxes to establish liability for estimated loss						38,000,000	R$ 220,000,000
Australian Taxation Office [Member] \| Foreign Jurisdiction [Member] \| AofA [Member]
Loss Contingencies [Line Items]
Additional income tax payable, exclusive of interest and penalties																	$ 162,000,000	$ 214
Notices include claims for compounded interest on the tax amount																	$ 537,000,000	$ 707
Proposed administrative penalties	$ 97,000,000	$ 128
Payment of dispute resolution practices income tax percentage											50.00%	50.00%
Assessed income tax amount exclusive of interest and penalties											$ 74,000,000					$ 107
Payment amount refund percentage											50.00%	50.00%
Reduction in current year cash tax payment						4,000,000		$ 5			$ 166,000,000	$ 219
Other noncurrent liabilities and deferred credits						170,000,000									$ 224
Tax assessment deposit						$ 81,000,000									$ 107