Segment and Related Information - Schedule of Segment Adjusted EBITDA to Consolidated Net (Loss) Income Attributable to Alcoa Corporation (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
Segment Reporting Information [Line Items]
Total Segment Adjusted EBITDA									$ 1,317	$ 1,626	$ 3,250
Transformation									(45)	(7)	(3)
Intersegment eliminations									(8)	150	(8)
Corporate expenses									(102)	(101)	(96)
Provision for depreciation, depletion, and amortization									(653)	(713)	(733)
Restructuring and other charges, net (D)									(104)	(1,031)	(527)
Interest expense (U)									(146)	(121)	(122)
Other expenses, net (U)									(8)	(162)	(64)
Income (loss) before income taxes									173	(438)	1,625
Provision for income taxes (Q)									(187)	(415)	(732)
Net income attributable to noncontrolling interest									(156)	(272)	(643)
Net (loss) income attributable to Alcoa Corporation	$ (4)	$ (49)	$ (197)	$ 80	$ (303)	$ (221)	$ (402)	$ (199)	(170)	(1,125)	250
Other [Member]
Segment Reporting Information [Line Items]
Restructuring and other charges, net (D)									(104)	(1,031)	(527)
Other									$ (78)	$ (79)	$ (72)