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, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Earnings Per Share [Abstract]
Net (loss) income, Alcoa Corporation	$ (197)	$ (402)	$ (117)	$ (601)
Average shares outstanding – basic	186	186	186	185
Effect of dilutive securities:
Average shares outstanding – diluted	186	186	186	185