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		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Earnings Per Share [Abstract]
Net income (loss) attributable to Alcoa Corporation	$ 337	$ (49)	$ 821	$ (166)
Average shares outstanding – basic	187	186	187	186
Effect of dilutive securities:
Stock units	4		3
Average shares outstanding – diluted	191	186	190	186