Segment Information - Schedule of Segment Adjusted EBITDA to Consolidated Net Income (Loss) Attributable to Alcoa Corporation (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Segment Reporting Information [Line Items]
Total Segment Adjusted EBITDA	$ 784	$ 359	$ 1,978	$ 919
Transformation	(10)	(11)	(34)	(37)
Intersegment eliminations	(8)	(35)	20	(13)
Corporate expenses	(30)	(24)	(84)	(72)
Provision for depreciation, depletion, and amortization	(156)	(161)	(499)	(483)
Restructuring and other charges, net (D)	(33)	(5)	(73)	(44)
Interest expense	(58)	(41)	(167)	(103)
Other income (expenses), net (Q)	18	(45)	147	36
Income before income taxes	497	22	1,270	136
Provision for income taxes	(127)	(42)	(331)	(167)
Net income attributable to noncontrolling interest	(33)	(29)	(118)	(135)
NET INCOME (LOSS) ATTRIBUTABLE TO ALCOA CORPORATION	337	(49)	821	(166)
Other [Member]
Segment Reporting Information [Line Items]
Restructuring and other charges, net (D)	(33)	(5)	(73)	(44)
Other	$ (10)	$ (15)	$ (18)	$ (67)