Investments - Summary of Unaudited Financial Information for Alcoa Corporation's Equity Investments (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Schedule of Equity Method Investments [Line Items]
Net income (loss)	$ 219	$ 139
Ma'aden Joint Venture [Member]
Schedule of Equity Method Investments [Line Items]
Equity in net income (loss) of affiliated companies, before reconciling adjustments	11	(3)
Other	(4)	(3)
Alcoa Corporation’s equity in net income of affiliated companies	7	(6)
Ma'aden Joint Venture [Member] | Equity Method Investment, Nonconsolidated Investee or Group of Investees [Member]
Schedule of Equity Method Investments [Line Items]
Sales	682	585
Cost of goods sold	503	462
Net income (loss)	45	(13)
Mining [Member]
Schedule of Equity Method Investments [Line Items]
Equity in net income (loss) of affiliated companies, before reconciling adjustments	1	6
Other		(3)
Alcoa Corporation’s equity in net income of affiliated companies	1	3
Mining [Member] | Equity Method Investment, Nonconsolidated Investee or Group of Investees [Member]
Schedule of Equity Method Investments [Line Items]
Sales	173	218
Cost of goods sold	123	148
Net income (loss)	(5)	9
Energy [Member]
Schedule of Equity Method Investments [Line Items]
Equity in net income (loss) of affiliated companies, before reconciling adjustments	10	11
Other		(2)
Alcoa Corporation’s equity in net income of affiliated companies	10	9
Energy [Member] | Equity Method Investment, Nonconsolidated Investee or Group of Investees [Member]
Schedule of Equity Method Investments [Line Items]
Sales	55	59
Cost of goods sold	25	26
Net income (loss)	25	27
Other [Member]
Schedule of Equity Method Investments [Line Items]
Equity in net income (loss) of affiliated companies, before reconciling adjustments	(1)	(4)
Other	3	5
Alcoa Corporation’s equity in net income of affiliated companies	2	1
Other [Member] | Equity Method Investment, Nonconsolidated Investee or Group of Investees [Member]
Schedule of Equity Method Investments [Line Items]
Sales	95	73
Cost of goods sold	63	67
Net income (loss)	$ (2)	$ (9)