Investments - Summary of Unaudited Financial Information for Alcoa Corporation's Equity Investments (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Schedule of Equity Method Investments [Line Items]
Net (loss) income	$ (150)	$ (293)	$ (11)	$ (351)
Ma'aden Joint Venture [Member]
Schedule of Equity Method Investments [Line Items]
Equity in net (loss) income of affiliated companies, before reconciling adjustments	(19)	(14)	(22)	(31)
Other	(1)	(2)	(4)	4
Alcoa Corporation’s equity in net (loss) income of affiliated companies	(20)	(16)	(26)	(27)
Ma'aden Joint Venture [Member] | Equity Method Investment, Nonconsolidated Investee or Group of Investees [Member]
Schedule of Equity Method Investments [Line Items]
Sales	507	976	1,092	1,934
Cost of goods sold	446	821	908	1,673
Net (loss) income	(75)	(57)	(88)	(125)
Mining [Member]
Schedule of Equity Method Investments [Line Items]
Equity in net (loss) income of affiliated companies, before reconciling adjustments	3	3	9	8
Other	2	(2)	(1)	6
Alcoa Corporation’s equity in net (loss) income of affiliated companies	5	1	8	14
Mining [Member] | Equity Method Investment, Nonconsolidated Investee or Group of Investees [Member]
Schedule of Equity Method Investments [Line Items]
Sales	209	187	427	415
Cost of goods sold	130	132	278	281
Net (loss) income	(5)	2	4
Energy [Member]
Schedule of Equity Method Investments [Line Items]
Equity in net (loss) income of affiliated companies, before reconciling adjustments	7	10	18	21
Other	1	1	(1)	1
Alcoa Corporation’s equity in net (loss) income of affiliated companies	8	11	17	22
Energy [Member] | Equity Method Investment, Nonconsolidated Investee or Group of Investees [Member]
Schedule of Equity Method Investments [Line Items]
Sales	47	64	106	127
Cost of goods sold	24	35	50	64
Net (loss) income	20	26	47	54
Other [Member]
Schedule of Equity Method Investments [Line Items]
Equity in net (loss) income of affiliated companies, before reconciling adjustments	(3)		(7)	(3)
Other	5		10	2
Alcoa Corporation’s equity in net (loss) income of affiliated companies	2		3	(1)
Other [Member] | Equity Method Investment, Nonconsolidated Investee or Group of Investees [Member]
Schedule of Equity Method Investments [Line Items]
Sales	85	25	158	39
Cost of goods sold	75	$ 23	142	38
Net (loss) income	$ (6)		$ (15)	$ (6)