Contingencies - Additional Information - 1 (Detail) R$ in Millions, $ in Millions, $ in Millions			1 Months Ended															3 Months Ended	6 Months Ended		12 Months Ended		14 Months Ended
	Sep. 17, 2020 USD ($)	Sep. 17, 2020 AUD ($)	Dec. 31, 2024 USD ($)	Dec. 31, 2024 BRL (R$)	Sep. 30, 2024 USD ($)	Sep. 30, 2024 BRL (R$)	Aug. 31, 2022 USD ($)	Aug. 31, 2022 BRL (R$)	Jul. 31, 2022 USD ($)	Jul. 31, 2022 BRL (R$)	Mar. 31, 2022 USD ($)	Mar. 31, 2022 BRL (R$)	Feb. 28, 2022 USD ($)	Feb. 28, 2022 BRL (R$)	Mar. 31, 2013 USD ($)	May 31, 2012 USD ($)	May 31, 2012 BRL (R$)	Sep. 30, 2020 USD ($)	Jun. 30, 2025 USD ($)	Jun. 30, 2025 BRL (R$)	Dec. 31, 2012 USD ($)	Dec. 31, 2012 BRL (R$)	Jun. 30, 2026 USD ($)	Jun. 30, 2026 AUD ($)	Jun. 30, 2025 AUD ($)	Dec. 31, 2024 AUD ($)	Jul. 31, 2022 BRL (R$)	Feb. 28, 2022 BRL (R$)	Sep. 30, 2020 AUD ($)	Jul. 07, 2020 USD ($)	Jul. 07, 2020 AUD ($)	Mar. 31, 2013 BRL (R$)	Dec. 31, 2012 BRL (R$)
Loss Contingencies [Line Items]
Accured tax liabilty			$ 102																$ 246
Australian Taxation Office [Member] | Foreign Jurisdiction [Member] | AofA [Member]
Loss Contingencies [Line Items]
Additional income tax payable, exclusive of interest and penalties																														$ 139	$ 214
Notices include claims for compounded interest on the tax amount																														$ 460	$ 707
Proposed administrative penalties	$ 83	$ 128
Payment of dispute resolution practices income tax percentage																		50.00%
Assessed income tax amount exclusive of interest and penalties																		$ 74											$ 107
Tax assessment deposit			66																69						$ 107	$ 107
Tax assessment interest refund receivable																			9						13
Tax assessment deposit and interest refunds																			78						120
Accured tax liabilty			206																225						$ 346	$ 332
Australian Taxation Office [Member] | Foreign Jurisdiction [Member] | AofA [Member] | Forecast [Member]
Loss Contingencies [Line Items]
Net cash tax impact																							$ 147	$ 226
Alcoa World Alumina Brasil [Member] | Brazilian Federal Revenue Office [Member]
Loss Contingencies [Line Items]
Disallowed tax credits									$ 13				$ 4		$ 110												R$ 66	R$ 19				R$ 220
Percentage of penalty of the gross disallowed amount															50.00%
Value added tax receivable			$ 1	R$ 6	$ 1	R$ 5	$ 6	R$ 31	$ 10	R$ 53	$ 9	R$ 44	$ 14	R$ 65		$ 41	R$ 82
Federal value added tax credits																					$ 136												R$ 273
Value added tax refund received																					$ 68	R$ 136
Minimum [Member] | Alcoa World Alumina Brasil [Member] | Brazilian Federal Revenue Office [Member]
Loss Contingencies [Line Items]
Charge recorded in provision for income taxes to establish liability for estimated loss																			0
Maximum [Member] | Alcoa World Alumina Brasil [Member] | Brazilian Federal Revenue Office [Member]
Loss Contingencies [Line Items]
Charge recorded in provision for income taxes to establish liability for estimated loss																			$ 55	R$ 300