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Business disposition.

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Capital gain tax on implied shares value.

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Estimated liabilities for future site separation commitment charge.

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Liabilities recorded from disposal group including discontinued operation.

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Minimum holding period.

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Net assets included in joint venture formation.

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Ownership interest in joint venture.

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Payments for financial contributions for divestiture of businesses.

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Percentage of shared fund.

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Purchase percentage interest of put option.

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Fair value of joint venture at its formation date.

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

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Acquisitions and Divestitures - Additional Information (Detail) € in Millions, $ in Millions				3 Months Ended
	Sep. 15, 2024 USD ($) shares	Sep. 12, 2024 USD ($)	Aug. 01, 2024	Mar. 31, 2025 USD ($)	Mar. 31, 2025 EUR (€)	Mar. 31, 2024 USD ($)	Mar. 31, 2025 EUR (€)
Alcoa Corporation [Member]
Business Disposition [Line Items]
Carrying value of investment				$ 553
Alumina Limited [Member] \| Aluminum Segment [Member]
Business Disposition [Line Items]
Ownership interest in joint venture			55.00%
Alumina Limited [Member] \| AWAC [Member]
Business Disposition [Line Items]
Non-controlling interest, ownership percentage			40.00%
Saudi Arabia Joint Venture [Member]
Business Disposition [Line Items]
Sale of ownership interest under agreement exchange for issuance shares \| shares	85,977,547
Sale of ownership interest under agreement exchange for issuance shares value		$ 950
Minimum holding period	3 years
Percentage of shares transferable	33.33%
Sale of ownership interest under agreement exchange for cash	$ 150
Ownership interest in joint venture	25.10%
Capital gain tax on implied shares value		$ 65
San Ciprian Joint Venture [Member] \| Alcoa Corporation [Member]
Business Disposition [Line Items]
Contribution to joint venture				81	€ 75
Additional fund for operations				$ 108			€ 100
Percentage of shared fund				75.00%			75.00%
Ownership interest in joint venture				75.00%	75.00%
San Ciprian Joint Venture [Member] \| IGNIS EQT [Member]
Business Disposition [Line Items]
Foreign currency translation adjustments reclassified to AOCI				$ 31
Net assets included in joint venture formation				103
Contribution to joint venture				27	€ 25
Contribution to form the joint venture				$ 108
Percentage of shared fund				25.00%			25.00%
Purchase percentage of interest				25.00%	25.00%
San Ciprian Joint Venture [Member] \| IGNIS Equity Holdings SL [Member] \| IGNIS EQT [Member]
Business Disposition [Line Items]
Non-controlling interest, ownership percentage				25.00%			25.00%
Warrick Rolling Mill [Member] \| Disposal Group, Held-for-Sale, Not Discontinued Operations [Member]
Business Disposition [Line Items]
Estimated liabilities for future site separation commitment charge						$ 11
Payments against the reserve				$ 2		$ 7
Remaining reserve				$ 6