Restructuring and Other Charges, Net - Additional Information (Detail) $ in Millions		3 Months Ended				6 Months Ended			9 Months Ended	12 Months Ended
		Jun. 30, 2025 USD ($) Employees	Dec. 31, 2024 USD ($)	Jun. 30, 2024 USD ($)	Mar. 31, 2024 USD ($)	Dec. 31, 2025 USD ($)	Jun. 30, 2025 USD ($) Employees	Jun. 30, 2024 USD ($)	Dec. 31, 2024 USD ($)	Dec. 31, 2024 USD ($)
Restructuring Cost and Reserve [Line Items]
Restructuring And Other Charges		$ 14		$ 18			$ 19	$ 220
Cash payment for restructuring							103			$ 183
Restructuring and other charges, net		$ 14		18			$ 19	220
Kwinana Refinery [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring And Other Charges				8				205
Severance costs					$ 41
Number of employees | Employees		230					230
Asset impairments					5
Payments related to other costs		$ 45					$ 86
Restructuring and other charges, net			$ 82	8	197
Kwinana Refinery [Member] | Forecast [Member]
Restructuring Cost and Reserve [Line Items]
Remaining cash outlays expected through 2025						$ 60
Pension Benefits [Member]
Restructuring Cost and Reserve [Line Items]
Settlement of certain other postretirement benefits	[1]			(1)				(1)
Alcoa Corporation [Member]
Restructuring Cost and Reserve [Line Items]
Noncurrent portion of the reserve		2	$ 8				2		$ 8	8
2024 Restructuring Plans Action [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring and other charges, net				18				220
2025 Restructuring Plans Action [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring and other charges, net		14					19
Environmental and Asset Retirement Obligation Reserves [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring and other charges, net		20					20
Certain Employee Obligations [Member] | San Ciprian Aluminum Smelter [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring And Other Charges		6					9
Severance and Employee Termination Costs [Member]
Restructuring Cost and Reserve [Line Items]
Cash payment for restructuring							1			38
Other Exit Costs [Member]
Restructuring Cost and Reserve [Line Items]
Cash payment for restructuring							102			$ 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		$ (12)		$ 8			$ (10)	$ 12

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