Assessed income tax amount exclusive of interest and penalties.

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

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Net cash tax impact.

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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 assessment deposit and interest receivable.

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Tax credit previously claimed and disallowed by tax authority.

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Value added tax refund received.

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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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Carrying value as of the balance sheet date of obligations incurred and payable for statutory income, sales, use, payroll, excise, real, property and other taxes. Used to reflect the current portion of the liabilities (due within one year or within the normal operating cycle if longer).

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Carrying amount as of the balance sheet date of value added taxes due either from customers arising from sales on credit terms, or as previously overpaid to tax authorities.

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Contingencies - Additional Information - 1 (Detail) R$ in Millions, $ in Millions, $ in Millions			1 Months Ended															3 Months Ended	6 Months Ended		9 Months Ended		12 Months Ended
	Sep. 17, 2020 USD ($)	Sep. 17, 2020 AUD ($)	Dec. 31, 2024 USD ($)	Dec. 31, 2024 BRL (R$)	Sep. 30, 2024 USD ($)	Sep. 30, 2024 BRL (R$)	Aug. 31, 2022 USD ($)	Aug. 31, 2022 BRL (R$)	Jul. 31, 2022 USD ($)	Jul. 31, 2022 BRL (R$)	Mar. 31, 2022 USD ($)	Mar. 31, 2022 BRL (R$)	Feb. 28, 2022 USD ($)	Feb. 28, 2022 BRL (R$)	Mar. 31, 2013 USD ($)	May 31, 2012 USD ($)	May 31, 2012 BRL (R$)	Sep. 30, 2020 USD ($)	Jun. 30, 2025 USD ($)	Jun. 30, 2025 BRL (R$)	Sep. 30, 2025 USD ($)	Sep. 30, 2025 AUD ($)	Dec. 31, 2012 USD ($)	Dec. 31, 2012 BRL (R$)	Jul. 31, 2025 USD ($)	Jul. 31, 2025 AUD ($)	Jun. 30, 2025 AUD ($)	Dec. 31, 2024 AUD ($)	Jul. 31, 2022 BRL (R$)	Feb. 28, 2022 BRL (R$)	Sep. 30, 2020 AUD ($)	Jul. 07, 2020 USD ($)	Jul. 07, 2020 AUD ($)	Mar. 31, 2013 BRL (R$)	Dec. 31, 2012 BRL (R$)
Loss Contingencies [Line Items]
Accured tax liabilty			$ 102																		$ 339
Australian Taxation Office [Member] \| Foreign Jurisdiction [Member] \| AofA [Member]
Loss Contingencies [Line Items]
Additional income tax payable, exclusive of interest and penalties																																$ 152	$ 214
Notices include claims for compounded interest on the tax amount																																$ 502	$ 707
Proposed administrative penalties	$ 91	$ 128
Payment of dispute resolution practices income tax percentage																		50.00%
Assessed income tax amount exclusive of interest and penalties																		$ 74													$ 107
Tax assessment deposit			66																						$ 69	$ 107		$ 107
Tax assessment deposit and interest refunds																									$ 9	$ 13
Net cash tax impact																					$ 149	$ 226
Accured tax liabilty			206																$ 225								$ 346	$ 332
Alcoa World Alumina Brasil [Member] \| Brazilian Federal Revenue Office [Member]
Loss Contingencies [Line Items]
Disallowed tax credits									$ 13				$ 4		$ 110														R$ 66	R$ 19				R$ 220
Percentage of penalty of the gross disallowed amount															50.00%
Value added tax receivable			$ 1	R$ 6	$ 1	R$ 5	$ 6	R$ 31	$ 10	R$ 53	$ 9	R$ 44	$ 14	R$ 65		$ 41	R$ 82
Federal value added tax credits																							$ 136												R$ 273
Value added tax refund received																							$ 68	R$ 136
Minimum [Member] \| Alcoa World Alumina Brasil [Member] \| Brazilian Federal Revenue Office [Member]
Loss Contingencies [Line Items]
Charge recorded in provision for income taxes to establish liability for estimated loss																			0
Maximum [Member] \| Alcoa World Alumina Brasil [Member] \| Brazilian Federal Revenue Office [Member]
Loss Contingencies [Line Items]
Charge recorded in provision for income taxes to establish liability for estimated loss																			$ 56	R$ 300