Segment Information - Schedule of Segment Adjusted EBITDA to Consolidated Net Income (Loss) Attributable to Alcoa Corporation (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Segment Reporting Information [Line Items]
Total Segment Adjusted EBITDA	$ 625	$ 185	$ 1,194	$ 560
Transformation	(13)	(10)	(24)	(26)
Intersegment eliminations	35	30	28	22
Corporate expenses	(28)	(21)	(54)	(48)
Provision for depreciation, depletion, and amortization	(161)	(152)	(343)	(322)
Restructuring and other charges, net (D)	(33)	(37)	(40)	(39)
Interest expense	(67)	(32)	(109)	(62)
Other income (expenses), net (Q)	105	(51)	129	81
Income (loss) before income taxes	461	(105)	773	114
Provision for income taxes	(111)	(45)	(204)	(125)
Net income attributable to noncontrolling interest	(41)	(47)	(85)	(106)
NET INCOME (LOSS) ATTRIBUTABLE TO ALCOA CORPORATION	309	(197)	484	(117)
Other [Member]
Segment Reporting Information [Line Items]
Restructuring and other charges, net (D)	(33)	(37)	(40)	(39)
Other	$ (2)	$ (17)	$ (8)	$ (52)