Derivatives and Other Financial Instruments - Additional Information (Detail) kt in Thousands, $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2025 USD ($) kt	Jun. 30, 2024 USD ($)	Jun. 30, 2025 USD ($) Member kt	Jun. 30, 2024 USD ($)	Dec. 31, 2024 kt
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Minimum members required for strategic risk management committee | Member			3
Realized gain (loss) reclassified from Accumulated other comprehensive loss to earnings	$ (80)	$ (74)	$ (175)	$ (127)
Firming contract description			In June 2025, Alcoa entered into a firming contract to manage the variability and intermittency of renewable energy sources and reduce exposure to the spot energy market for the Mosjøen smelter (Norway) for the period from July 2025 through December 2028. The firming contract converts a certain pay-as-produced wind contract into baseload power. The contract does not qualify for hedge accounting treatment.
Level 1 and 2 Derivative Instruments [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Realized gain (loss) reclassified from Accumulated other comprehensive loss to earnings	$ (2)	$ 0	$ (4)	4
Commodity Contract | Level 1 [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Derivative instruments, expiration month and year			2025-12
Commodity Contract | Level 1 [Member] | Spain [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Derivative instruments, expiration month and year			2027-12
Foreign Exchange Forward | Spain [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Derivative instruments, expiration month and year			2027-12
Foreign Exchange Forward | Norway [Member] | Euro Power Purchases [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Derivative instruments, expiration month and year			2028-06
Foreign Exchange Forward | Brazil [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Derivative instruments, expiration month and year			2026-10
Foreign Exchange Forward | Canada [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Derivative instruments, expiration month and year			2025-03
Foreign Exchange Forward | Australia [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Derivative instruments, expiration month and year			2032-12
Energy Contracts [Member] | Spain [Member] | Natural Gas Purchases [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Derivative instruments, expiration month and year			2027-12
Energy Contracts [Member] | Spain [Member] | Electricity Purchases [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Derivative instruments, expiration month and year			2027-12
Derivatives Designated as Hedging Instruments [Member] | Level 1 and 2 Derivative Instruments [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Realized gain (loss) reclassified from Accumulated other comprehensive loss to earnings			$ (4)
Derivatives Designated as Hedging Instruments [Member] | Power Contract [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Aluminum forecast sales | kt	1,118		1,118		1,230
Derivatives Designated as Hedging Instruments [Member] | Power Contract [Member] | Cash Flow Hedging [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Amount of (loss) gain expected to be recognized into earnings over the next 12 months			$ 264
Derivatives Designated as Hedging Instruments [Member] | Sales [Member] | Level 1 and 2 Derivative Instruments [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Realized gain (loss) reclassified from Accumulated other comprehensive loss to earnings			(2)	$ 4
Derivatives Designated as Hedging Instruments [Member] | Cost of Goods Sold [Member] | Electricity Financial Related Derivative [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Realized gain (loss) reclassified from Accumulated other comprehensive loss to earnings			3
Derivatives Designated as Hedging Instruments [Member] | Cost of Goods Sold [Member] | Level 1 and 2 Derivative Instruments [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Realized gain (loss) reclassified from Accumulated other comprehensive loss to earnings	$ (2)		$ (2)