Investments - Summary of Profit and Loss Information for Alcoa Corporation's Equity Investments (Detail) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Schedule of Equity Method Investments [Line Items]
Net (loss) income	$ 17	$ (20)	$ (150)	$ 139	$ (355)	$ (147)	$ (293)	$ (58)	$ (14)	$ (853)	$ 893
Ma'aden Joint Venture [Member]
Schedule of Equity Method Investments [Line Items]
Sales									2,279	3,185	3,986
Cost of goods sold									1,829	2,722	3,334
Net (loss) income									(108)	(198)	9
Equity in net (loss) income of affiliated companies, before reconciling adjustments									(27)	(50)	2
Other									(7)	3	(13)
Alcoa Corporation’s equity in net (loss) income of affiliated companies									(34)	(47)	(11)
Mining [Member]
Schedule of Equity Method Investments [Line Items]
Sales									841	846	802
Cost of goods sold									543	580	522
Net (loss) income									46	35	71
Equity in net (loss) income of affiliated companies, before reconciling adjustments									23	16	23
Other									(1)	5	(10)
Alcoa Corporation’s equity in net (loss) income of affiliated companies									22	21	13
Energy [Member]
Schedule of Equity Method Investments [Line Items]
Sales									238	269	283
Cost of goods sold									107	143	146
Net (loss) income									74	107	114
Equity in net (loss) income of affiliated companies, before reconciling adjustments									31	42	46
Other									2	1	(4)
Alcoa Corporation’s equity in net (loss) income of affiliated companies									33	43	42
Other [Member]
Schedule of Equity Method Investments [Line Items]
Sales									316	159	120
Cost of goods sold									283	151	110
Net (loss) income									(24)	(28)	16
Equity in net (loss) income of affiliated companies, before reconciling adjustments									(11)	(13)	8
Other									14	16	(1)
Alcoa Corporation’s equity in net (loss) income of affiliated companies									$ 3	$ 3	$ 7