Acquisitions and Divestitures - Additional Information (Detail) € in Millions, ر.س in Millions					3 Months Ended				6 Months Ended	9 Months Ended
	Jul. 01, 2025 USD ($) shares	Jul. 01, 2025 SAR (ر.س) shares	Aug. 01, 2024 USD ($) shares	Jul. 31, 2024 USD ($)	Sep. 30, 2025 USD ($)	Mar. 31, 2025 USD ($)	Mar. 31, 2025 EUR (€)	Sep. 30, 2024 USD ($)	Sep. 30, 2025 USD ($)	Sep. 30, 2025 USD ($)	Sep. 30, 2024 USD ($)	Mar. 31, 2025 EUR (€)	Jul. 31, 2024 $ / shares
Business Disposition [Line Items]
Market-to-market gain					$ 267,000,000			$ 0		$ 267,000,000	$ 0
Acquisition of cash			$ 9,000,000
Alumina Limited [Member]
Business Disposition [Line Items]
Business acquisition, shares outstanding | shares			2,760,056,014
Exchange ratio value per share | $ / shares													$ 1.45
Aggregate purchase consideration				$ 2,700,000,000
Noncontrolling interest			$ 1,472,000,000
Other current liabilities			1,000,000
Net assets acquired			1,216,000,000
Transaction costs			32,000,000
Acquisition of noncontrolling interest			$ 1,099,000,000
Alumina Limited [Member] | Aluminum Segment [Member]
Business Disposition [Line Items]
Ownership interest in joint venture			55.00%
Alumina Limited [Member] | Common Stock [Member]
Business Disposition [Line Items]
Business acquisition, shares issued | shares			78,772,422
Alumina Limited [Member] | Nonvoting Convertible Preferred Stock [Member]
Business Disposition [Line Items]
Business acquisition, shares issued | shares			4,041,989
Business acquisition, shares outstanding | shares			141,625,403
Alumina Limited [Member] | Scheme Implementation Deed [Member]
Business Disposition [Line Items]
Business acquision, number of shares | shares			4
Assumption of Indebtedness			$ 385,000,000
Deferred tax assets			$ 121,000,000
Alumina Limited [Member] | Scheme Implementation Deed [Member] | New Alcoa Preferred Stock [Member]
Business Disposition [Line Items]
Business acquisition, shares issued | shares			0.02854
Alumina Limited [Member] | Scheme Implementation Deed [Member] | Alcoa Common Stock [Member]
Business Disposition [Line Items]
Business acquisition, shares issued | shares			0.02854
Alumina Limited [Member] | AWAC [Member]
Business Disposition [Line Items]
Non-controlling interest, ownership percentage			40.00%
Alumina Limited [Member] | CHESS Depositary Interests [Member] | Scheme Implementation Deed [Member]
Business Disposition [Line Items]
Business acquisition, shares issued | shares			0.02854
Saudi Arabia Joint Venture [Member]
Business Disposition [Line Items]
Ownership interest in joint venture	25.10%	25.10%
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					786,000,000					786,000,000
Transaction costs					18,000,000					18,000,000
Market-to-market gain					267,000,000					267,000,000
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											15,000,000
Payments against the reserve					1,000,000			$ 7		6,000,000	$ 19
Remaining reserve					$ 2,000,000				$ 2,000,000	$ 2,000,000