Investments - Summary of Unaudited Financial Information for Alcoa Corporation's Equity Investments (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Schedule of Equity Method Investments [Line Items]
Net income (loss)	$ 370	$ (20)	$ 939	$ (31)
Ma'aden Joint Venture [Member]
Schedule of Equity Method Investments [Line Items]
Equity in net income (loss) of affiliated companies, before reconciling adjustments	38	(10)	75	(32)
Other	(3)	(1)	(7)	(5)
Alcoa Corporation’s equity in net income (loss) of affiliated companies	35	(11)	68	(37)
Ma'aden Joint Venture [Member] | Equity Method Investment, Nonconsolidated Investee or Group of Investees [Member]
Schedule of Equity Method Investments [Line Items]
Sales	776	552	2,199	1,644
Cost of goods sold	479	435	1,490	1,343
Net income (loss)	152	(38)	299	(126)
Mining [Member]
Schedule of Equity Method Investments [Line Items]
Equity in net income (loss) of affiliated companies, before reconciling adjustments	6	5	16	14
Other	(1)		(2)	(1)
Alcoa Corporation’s equity in net income (loss) of affiliated companies	5	5	14	13
Mining [Member] | Equity Method Investment, Nonconsolidated Investee or Group of Investees [Member]
Schedule of Equity Method Investments [Line Items]
Sales	198	195	568	622
Cost of goods sold	145	131	414	409
Net income (loss)	11	16	31	20
Energy [Member]
Schedule of Equity Method Investments [Line Items]
Equity in net income (loss) of affiliated companies, before reconciling adjustments	8	11	29	29
Other		1
Alcoa Corporation’s equity in net income (loss) of affiliated companies	8	12	29	29
Energy [Member] | Equity Method Investment, Nonconsolidated Investee or Group of Investees [Member]
Schedule of Equity Method Investments [Line Items]
Sales	73	51	188	157
Cost of goods sold	51	28	108	78
Net income (loss)	22	28	74	75
Other [Member]
Schedule of Equity Method Investments [Line Items]
Equity in net income (loss) of affiliated companies, before reconciling adjustments	(5)	(4)	(8)	(11)
Other	4	3	15	13
Alcoa Corporation’s equity in net income (loss) of affiliated companies	(1)	(1)	7	2
Other [Member] | Equity Method Investment, Nonconsolidated Investee or Group of Investees [Member]
Schedule of Equity Method Investments [Line Items]
Sales	100	72	291	230
Cost of goods sold	90	65	261	207
Net income (loss)	$ (12)	$ (10)	$ (18)	$ (25)