Investments - Summary of Unaudited Financial Information for Alcoa Corporation's Equity Investments (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2022	Mar. 31, 2021
Schedule of Equity Method Investments [Line Items]
Net income (loss)	$ 553	$ 219
Ma'aden Joint Venture [Member]
Schedule of Equity Method Investments [Line Items]
Equity in net income (loss) of affiliated companies, before reconciling adjustments	39	11
Other		(4)
Alcoa Corporation’s equity in net income (loss) of affiliated companies	39	7
Ma'aden Joint Venture [Member] | Equity Method Investment, Nonconsolidated Investee or Group of Investees [Member]
Schedule of Equity Method Investments [Line Items]
Sales	897	682
Cost of goods sold	616	503
Net income (loss)	155	45
Mining [Member]
Schedule of Equity Method Investments [Line Items]
Equity in net income (loss) of affiliated companies, before reconciling adjustments	14	1
Alcoa Corporation’s equity in net income (loss) of affiliated companies	14	1
Mining [Member] | Equity Method Investment, Nonconsolidated Investee or Group of Investees [Member]
Schedule of Equity Method Investments [Line Items]
Sales	231	173
Cost of goods sold	146	123
Net income (loss)	49	(5)
Energy [Member]
Schedule of Equity Method Investments [Line Items]
Equity in net income (loss) of affiliated companies, before reconciling adjustments	9	10
Other	1
Alcoa Corporation’s equity in net income (loss) of affiliated companies	10	10
Energy [Member] | Equity Method Investment, Nonconsolidated Investee or Group of Investees [Member]
Schedule of Equity Method Investments [Line Items]
Sales	62	55
Cost of goods sold	30	25
Net income (loss)	23	25
Other [Member]
Schedule of Equity Method Investments [Line Items]
Equity in net income (loss) of affiliated companies, before reconciling adjustments	(13)	(1)
Other	(4)	3
Alcoa Corporation’s equity in net income (loss) of affiliated companies	(17)	2
Other [Member] | Equity Method Investment, Nonconsolidated Investee or Group of Investees [Member]
Schedule of Equity Method Investments [Line Items]
Sales	117	95
Cost of goods sold	107	63
Net income (loss)	$ (28)	$ (2)