Restructuring and Other Charges, Net - Additional Information (Detail) $ in Millions		1 Months Ended	3 Months Ended					9 Months Ended			12 Months Ended
		Sep. 30, 2025 USD ($) Employees	Sep. 30, 2025 USD ($) Employees	Dec. 31, 2024 USD ($)	Sep. 30, 2024 USD ($)	Jun. 30, 2024 USD ($)	Mar. 31, 2024 USD ($)	Sep. 30, 2025 USD ($) Employees	Dec. 31, 2024 USD ($)	Sep. 30, 2024 USD ($)	Dec. 31, 2024 USD ($)
Restructuring Cost and Reserve [Line Items]
Restructuring And Other Charges			$ 885		$ 30			$ 904		$ 250
Cash payment for restructuring								159			$ 183
Restructuring and other charges, net			$ 885		30			$ 904		250
Kwinana Refinery [Member]
Restructuring Cost and Reserve [Line Items]
Severance costs							$ 41
Number of employees | Employees		220	220					220
Asset impairments							5
Payments related to other costs			$ 46					$ 132
Restructuring and other charges, net				$ 82		$ 8	197
Kwinana Alumina Refinery [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring And Other Charges										205
Asset impairments		$ 265
Accrue for other costs		86
Remaining cash outlays expected through 2025								75
Other costs								39
Cash outlays expected over next six years								600
Charges for write-off remaining net book value of assets.		75
Reserves for asset retirement obligations and environmental remediation		430
Restructuring and other charges, net		856	856					856
Pension Benefits [Member]
Restructuring Cost and Reserve [Line Items]
Settlement of certain other postretirement benefits	[1]		(1)					(1)		(1)
Alcoa Corporation [Member]
Restructuring Cost and Reserve [Line Items]
Noncurrent portion of the reserve		$ 30	30	$ 8				30	$ 8		8
Intalco [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring and other charges, net					12					12
2024 Restructuring Plans Action [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring and other charges, net					30					250
2025 Restructuring Plans Action [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring and other charges, net			885					904
Environmental and Asset Retirement Obligation Reserves [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring and other charges, net			17		14			37		15
Certain Employee Obligations [Member] | San Ciprian Aluminum Smelter [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring And Other Charges			2					11
Severance and Employee Termination Costs [Member]
Restructuring Cost and Reserve [Line Items]
Cash payment for restructuring								2			38
Other Exit Costs [Member]
Restructuring Cost and Reserve [Line Items]
Cash payment for restructuring								157			$ 145
Water Management Costs [Member] | Kwinana Refinery [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring And Other Charges							(123)
Other Restructuring [Member] | Kwinana Refinery [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring And Other Charges							136		96
Asset Retirement Obligations [Member] | Kwinana Refinery [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring And Other Charges							15		$ (6)
Take Or-Pay Contracts [Member] | Kwinana Refinery [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring And Other Charges							$ (13)
Contract Costs [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring and other charges, net			$ 7		$ 4			$ (3)		$ 16

[1] These amounts were reported in Restructuring and other charges, net on the accompanying Statement of Consolidated Operations and Cash Flows.