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Gain on sale of ownership interest.

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Liabilities recorded from disposal group including discontinued operation.

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Ownership interests in joint venture.

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Payments for financial contributions for divestiture of businesses.

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Acquisitions and Divestitures - Additional Information (Detail) € in Millions, ر.س in Millions			3 Months Ended				6 Months Ended
	Jul. 01, 2025 USD ($) shares	Jul. 01, 2025 SAR (ر.س) shares	Jun. 30, 2025 USD ($)	Mar. 31, 2025 USD ($)	Mar. 31, 2025 EUR (€)	Jun. 30, 2024 USD ($)	Jun. 30, 2025 USD ($)	Jun. 30, 2024 USD ($)	Mar. 31, 2025 EUR (€)	Aug. 01, 2024
Alcoa Corporation [Member]
Business Disposition [Line Items]
Carrying value of investment			$ 546,000,000				$ 546,000,000
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]
Ownership interest in joint venture	25.10%	25.10%
Saudi Arabia Joint Venture [Member] \| Subsequent Event [Member]
Business Disposition [Line Items]
Minimum holding period	3 years	3 years
Percentage of shares transferable	0.3333%	0.3333%
Total consideration	$ 1,350,000,000
Sale of ownership interest under agreement exchange for issuance shares \| shares	85,977,547	85,977,547
Sale of ownership interest under agreement exchange for issuance shares value	$ 1,200,000,000	ر.س 4,501
Sale of ownership interest under agreement exchange for cash	150,000,000	ر.س 562
Gain on sale of ownership interest	$ 780,000,000
San Ciprian Joint Venture [Member] \| Alcoa Corporation [Member]
Business Disposition [Line Items]
Ownership interest in joint venture				75.00%					75.00%
Contribution to joint venture				$ 81,000,000	€ 75
Additional fund for operations				$ 117,000,000					€ 100
Percentage of shared fund				75.00%					75.00%
San Ciprian Joint Venture [Member] \| IGNIS EQT [Member]
Business Disposition [Line Items]
Foreign currency translation adjustments reclassified to AOCI				$ 31,000,000
Net assets included in joint venture formation				103,000,000
Contribution to joint venture				27,000,000	€ 25
Contribution to form the joint venture				$ 108,000,000
Percentage of shared fund				25.00%					25.00%
Purchase percentage of interest			25.00%	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						$ 4,000,000		$ 15,000,000
Payments against the reserve			$ 3,000,000			$ 5	5,000,000	$ 12
Remaining reserve			$ 3,000,000				$ 3,000,000