Segment Information - Schedule of Segment Adjusted EBITDA to Consolidated Net Loss Attributable to Alcoa Corporation (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Segment Reporting Information [Line Items]
Total Segment Adjusted EBITDA	$ 185	$ 484	$ 560	$ 886
Transformation	(10)	3	(26)	5
Intersegment eliminations	30	(1)	22	85
Corporate expenses	(21)	(28)	(48)	(52)
Provision for depreciation, depletion, and amortization	(152)	(174)	(322)	(346)
Restructuring and other charges, net (D)	(37)	(370)	(39)	(483)
Interest expense	(32)	(30)	(62)	(60)
Other (expenses) income, net (Q)	(51)	(50)	81	(91)
Consolidated (loss) income before income taxes	(105)	(177)	114	(85)
Provision for income taxes	(45)	(116)	(125)	(266)
Net income attributable to noncontrolling interest	(47)	(109)	(106)	(250)
Consolidated net loss attributable to Alcoa Corporation	(197)	(402)	(117)	(601)
Other [Member]
Segment Reporting Information [Line Items]
Restructuring and other charges, net (D)	(37)	(370)	(39)	(483)
Interest expense	(32)	(30)	(62)	(60)
Other	$ (17)	$ (11)	$ (52)	$ (29)