Earnings Per Share - Schedule of Computation of Basic and Diluted EPS Attributable to Alcoa Corporation Common Shareholders (Detail) - USD ($) shares in Millions, $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Earnings Per Share [Abstract]
Net income (loss) attributable to Alcoa Corporation	$ 309	$ (197)	$ 484	$ (117)
Average shares outstanding – basic	187	186	186	186
Effect of dilutive securities:
Stock units	3		3
Average shares outstanding – diluted	190	186	189	186