Investments - Summary of Unaudited Financial Information for Alcoa Corporation's Equity Investments (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2025	Jun. 30, 2024	Jun. 30, 2025	Jun. 30, 2024
Schedule of Equity Method Investments [Line Items]
Net income (loss)	$ 151	$ 31	$ 699	$ (276)
Ma'aden Joint Venture [Member] | Equity Method Investment, Nonconsolidated Investee or Group of Investees [Member]
Schedule of Equity Method Investments [Line Items]
Sales	824	804	1,780	1,515
Cost of goods sold	687	613	1,462	1,212
Net income (loss)	(8)	102	5	94
Equity in net income (loss) of affiliated companies, before reconciling adjustments	(2)	26	1	24
Other	(6)	(4)	0	(12)
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	(8)	22	1	12
Mining [Member] | Equity Method Investment, Nonconsolidated Investee or Group of Investees [Member]
Schedule of Equity Method Investments [Line Items]
Sales	192	158	360	273
Cost of goods sold	109	104	213	207
Net income (loss)	25	14	44	9
Equity in net income (loss) of affiliated companies, before reconciling adjustments	11	6	20	4
Other	(3)	0	(3)	0
Mining [Member] | Alcoa Corporation [Member]
Schedule of Equity Method Investments [Line Items]
Alcoa Corporations equity in net income (loss) of affiliated companies	8	6	17	4
Energy [Member] | Equity Method Investment, Nonconsolidated Investee or Group of Investees [Member]
Schedule of Equity Method Investments [Line Items]
Sales	60	59	118	122
Cost of goods sold	27	25	49	50
Net income (loss)	29	29	58	60
Equity in net income (loss) of affiliated companies, before reconciling adjustments	12	11	23	23
Other	2	0	2	(1)
Energy [Member] | Alcoa Corporation [Member]
Schedule of Equity Method Investments [Line Items]
Alcoa Corporations equity in net income (loss) of affiliated companies	14	11	25	22
Other [Member] | Equity Method Investment, Nonconsolidated Investee or Group of Investees [Member]
Schedule of Equity Method Investments [Line Items]
Sales	111	114	222	229
Cost of goods sold	101	104	201	209
Net income (loss)	(25)	(20)	(35)	(36)
Equity in net income (loss) of affiliated companies, before reconciling adjustments	(12)	(9)	(17)	(17)
Other	2	12	(5)	7
Other [Member] | Alcoa Corporation [Member]
Schedule of Equity Method Investments [Line Items]
Alcoa Corporations equity in net income (loss) of affiliated companies	$ (10)	$ 3	$ (22)	$ (10)