Restructuring and Other Charges, Net - Additional Information (Detail) $ in Millions		1 Months Ended	3 Months Ended		12 Months Ended
	Mar. 31, 2023 USD ($)	Mar. 31, 2023 USD ($) Employees	Mar. 31, 2024 USD ($) Employees	Mar. 31, 2023 USD ($) Employees	Dec. 31, 2023 USD ($)	Sep. 30, 2025 Employee	Sep. 30, 2024 Employee	Jan. 31, 2024 Employee
Restructuring Cost and Reserve [Line Items]
Restructuring And Other Charges			$ 202	$ 149
Restructuring and other charges, net			202	$ 149
Kwinana Refinery [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring And Other Charges			197
Severance costs			$ 41
Number of employees | Employee								780
Number of employees associated with employee termination and severance costs | Employees			580	150
Asset impairments			$ 5
Cash outlays includes existing employee related liabilities and asset retirement obligations expected through 2025			215
Cash outlays to be spent in 2024			140
Cash outlays			2
Kwinana Refinery [Member] | Forecast [Member]
Restructuring Cost and Reserve [Line Items]
Number of employees | Employee						50	250
Alcoa Corporation [Member]
Restructuring Cost and Reserve [Line Items]
Noncurrent portion of the reserve			45		$ 15
Intalco [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring And Other Charges	$ 101	$ 101
Severance costs		$ 1
Number of employees associated with employee termination and severance costs | Employees		12
Asset impairments		$ 50
Cash outlays			4
San Ciprian Facility [Member]
Restructuring Cost and Reserve [Line Items]
Cash outlays includes existing employee related liabilities and asset retirement obligations expected through 2025			38
Cash outlays to be spent in 2024			25
Cash outlays			8
Restart costs			33
Commitments For Capital Improvement Costs			78
Restricted cash			86
Capital improvement costs			115
Cost of Goods Sold [Member] | Intalco [Member]
Restructuring Cost and Reserve [Line Items]
Other costs		16
Restructuring and Other Charges [Member] | San Ciprian Facility [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring and other charges, net					$ 53
2023 Restructuring Plans Action [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring and other charges, net				$ 149
2024 Restructuring Plans Action [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring and other charges, net			202
Closure Cost [Member] | Intalco [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring And Other Charges		117
Cash outlays to be spent in 2024			45
Cash outlays expected over next three years			80
Certain Employee Obligations [Member] | San Ciprian Aluminum Smelter [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring And Other Charges				$ 47
Environmental and Demolition Obligation [Member] | Intalco [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring And Other Charges		$ 50
Water Management Costs [Member] | Kwinana Refinery [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring And Other Charges			123
Asset Retirement Obligations [Member] | Kwinana Refinery [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring And Other Charges			15
Take Or-Pay Contracts [Member] | Kwinana Refinery [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring And Other Charges			$ 13