Investments - Summary of Unaudited Financial Information for Alcoa Corporation's Equity Investments (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Schedule of Equity Method Investments [Line Items]
Net income (loss)	$ 350	$ (150)	$ 569	$ (11)
Ma'aden Joint Venture [Member]
Schedule of Equity Method Investments [Line Items]
Equity in net income (loss) of affiliated companies, before reconciling adjustments	26	(19)	37	(22)
Other		(1)	(4)	(4)
Alcoa Corporation’s equity in net (loss) income of affiliated companies	26	(20)	33	(26)
Ma'aden Joint Venture [Member] | Equity Method Investment, Nonconsolidated Investee or Group of Investees [Member]
Schedule of Equity Method Investments [Line Items]
Sales	741	507	1,423	1,092
Cost of goods sold	508	446	1,011	908
Net income (loss)	102	(75)	147	(88)
Mining [Member]
Schedule of Equity Method Investments [Line Items]
Equity in net income (loss) of affiliated companies, before reconciling adjustments	9	3	10	9
Other	(1)	2	(1)	(1)
Alcoa Corporation’s equity in net (loss) income of affiliated companies	8	5	9	8
Mining [Member] | Equity Method Investment, Nonconsolidated Investee or Group of Investees [Member]
Schedule of Equity Method Investments [Line Items]
Sales	197	209	370	427
Cost of goods sold	146	130	269	278
Net income (loss)	25	(5)	20	4
Energy [Member]
Schedule of Equity Method Investments [Line Items]
Equity in net income (loss) of affiliated companies, before reconciling adjustments	11	7	21	18
Other		1		(1)
Alcoa Corporation’s equity in net (loss) income of affiliated companies	11	8	21	17
Energy [Member] | Equity Method Investment, Nonconsolidated Investee or Group of Investees [Member]
Schedule of Equity Method Investments [Line Items]
Sales	60	47	115	106
Cost of goods sold	32	24	57	50
Net income (loss)	27	20	52	47
Other [Member]
Schedule of Equity Method Investments [Line Items]
Equity in net income (loss) of affiliated companies, before reconciling adjustments	(2)	(3)	(3)	(7)
Other	8	5	11	10
Alcoa Corporation’s equity in net (loss) income of affiliated companies	6	2	8	3
Other [Member] | Equity Method Investment, Nonconsolidated Investee or Group of Investees [Member]
Schedule of Equity Method Investments [Line Items]
Sales	96	85	191	158
Cost of goods sold	78	75	171	142
Net income (loss)	$ (4)	$ (6)	$ (6)	$ (15)