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
	Mar. 31, 2021	Mar. 31, 2020
Earnings Per Share [Abstract]
Net income attributable to Alcoa Corporation	$ 175	$ 80
Average shares outstanding – basic	186	186
Effect of dilutive securities:
Stock units	3	1
Average shares outstanding – diluted	189	187