Investments - Summary of Unaudited Financial Information for Alcoa Corporation's Equity Investments (Detail) - USD ($) $ in Millions	3 Months Ended						9 Months Ended
	Sep. 30, 2024	Jun. 30, 2024	Mar. 31, 2024	Sep. 30, 2023	Jun. 30, 2023	Mar. 31, 2023	Sep. 30, 2024	Sep. 30, 2023
Schedule Of Equity Method Investments [Line Items]
Net (loss) income	$ 98	$ 31	$ (307)	$ (193)	$ (121)	$ (232)	$ (178)	$ (546)
Ma'aden Joint Venture [Member] | Equity Method Investment, Nonconsolidated Investee or Group of Investees [Member]
Schedule Of Equity Method Investments [Line Items]
Sales	838			725			2,353	2,025
Cost of goods sold	696			645			1,908	1,947
Net (loss) income	(5)			(39)			89	(390)
Equity in net (loss) income of affiliated companies, before reconciling adjustments	(2)			(10)			22	(98)
Other	(4)			(15)			(16)	(30)
Alcoa Corporation [Member] | Ma'aden Joint Venture [Member]
Schedule Of Equity Method Investments [Line Items]
Alcoa Corporations equity in net (loss) income of affiliated companies	(6)			(25)			6	(128)
Mining [Member] | Equity Method Investment, Nonconsolidated Investee or Group of Investees [Member]
Schedule Of Equity Method Investments [Line Items]
Sales	146			157			419	516
Cost of goods sold	113			121			320	325
Net (loss) income	7			6			16	44
Equity in net (loss) income of affiliated companies, before reconciling adjustments	3			3			7	20
Other	0			0			0	1
Mining [Member] | Alcoa Corporation [Member]
Schedule Of Equity Method Investments [Line Items]
Alcoa Corporations equity in net (loss) income of affiliated companies	3			3			7	21
Energy [Member] | Equity Method Investment, Nonconsolidated Investee or Group of Investees [Member]
Schedule Of Equity Method Investments [Line Items]
Sales	61			61			183	178
Cost of goods sold	27			28			77	87
Net (loss) income	28			31			88	77
Equity in net (loss) income of affiliated companies, before reconciling adjustments	12			12			35	30
Other	0			(2)			(1)	(1)
Energy [Member] | Alcoa Corporation [Member]
Schedule Of Equity Method Investments [Line Items]
Alcoa Corporations equity in net (loss) income of affiliated companies	12			10			34	29
Other [Member] | Equity Method Investment, Nonconsolidated Investee or Group of Investees [Member]
Schedule Of Equity Method Investments [Line Items]
Sales	116			117			345	354
Cost of goods sold	104			105			313	324
Net (loss) income	(19)			(21)			(55)	(70)
Equity in net (loss) income of affiliated companies, before reconciling adjustments	(9)			(11)			(26)	(34)
Other	1			(6)			8	(6)
Other [Member] | Alcoa Corporation [Member]
Schedule Of Equity Method Investments [Line Items]
Alcoa Corporations equity in net (loss) income of affiliated companies	$ (8)			$ (17)			$ (18)	$ (40)