Investments - Summary of Unaudited Financial Information for Alcoa Corporation's Equity Investments (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2025	Mar. 31, 2024
Schedule of Equity Method Investments [Line Items]
Net income (loss)	$ 548	$ (307)
Ma'aden Joint Venture [Member] | Equity Method Investment, Nonconsolidated Investee or Group of Investees [Member]
Schedule of Equity Method Investments [Line Items]
Sales	956	711
Cost of goods sold	775	599
Net income (loss)	13	(8)
Equity in net income (loss) of affiliated companies, before reconciling adjustments	3	(2)
Other	6	(8)
Alcoa Corporation [Member] | Ma'aden Joint Venture [Member]
Schedule of Equity Method Investments [Line Items]
Alcoa Corporations equity in net income (loss) of affiliated companies	9	(10)
Mining [Member] | Equity Method Investment, Nonconsolidated Investee or Group of Investees [Member]
Schedule of Equity Method Investments [Line Items]
Sales	168	115
Cost of goods sold	104	103
Net income (loss)	19	(5)
Equity in net income (loss) of affiliated companies, before reconciling adjustments	9	(2)
Other	0	0
Mining [Member] | Alcoa Corporation [Member]
Schedule of Equity Method Investments [Line Items]
Alcoa Corporations equity in net income (loss) of affiliated companies	9	(2)
Energy [Member] | Equity Method Investment, Nonconsolidated Investee or Group of Investees [Member]
Schedule of Equity Method Investments [Line Items]
Sales	58	63
Cost of goods sold	22	25
Net income (loss)	29	31
Equity in net income (loss) of affiliated companies, before reconciling adjustments	11	12
Other	0	(1)
Energy [Member] | Alcoa Corporation [Member]
Schedule of Equity Method Investments [Line Items]
Alcoa Corporations equity in net income (loss) of affiliated companies	11	11
Other [Member] | Equity Method Investment, Nonconsolidated Investee or Group of Investees [Member]
Schedule of Equity Method Investments [Line Items]
Sales	111	115
Cost of goods sold	100	105
Net income (loss)	(10)	(16)
Equity in net income (loss) of affiliated companies, before reconciling adjustments	(5)	(8)
Other	(7)	(5)
Other [Member] | Alcoa Corporation [Member]
Schedule of Equity Method Investments [Line Items]
Alcoa Corporations equity in net income (loss) of affiliated companies	$ (12)	$ (13)