Investments - Summary of Unaudited Financial Information for Alcoa Corporation's Equity Investments (Detail) - USD ($) $ in Millions	3 Months Ended				6 Months Ended
	Jun. 30, 2022	Mar. 31, 2022	Jun. 30, 2021	Mar. 31, 2021	Jun. 30, 2022	Jun. 30, 2021
Schedule of Equity Method Investments [Line Items]
Net income (loss)	$ 674	$ 553	$ 350	$ 219	$ 1,227	$ 569
Ma'aden Joint Venture [Member]
Schedule of Equity Method Investments [Line Items]
Equity in net income (loss) of affiliated companies, before reconciling adjustments	39		26		78	37
Other	(4)				(4)	(4)
Alcoa Corporation’s equity in net income of affiliated companies	35		26		74	33
Ma'aden Joint Venture [Member] | Equity Method Investment, Nonconsolidated Investee or Group of Investees [Member]
Schedule of Equity Method Investments [Line Items]
Sales	1,016		741		1,913	1,423
Cost of goods sold	712		508		1,328	1,011
Net income (loss)	156		102		311	147
Mining [Member]
Schedule of Equity Method Investments [Line Items]
Equity in net income (loss) of affiliated companies, before reconciling adjustments	12		9		26	10
Other	(2)		(1)		(2)	(1)
Alcoa Corporation’s equity in net income of affiliated companies	10		8		24	9
Mining [Member] | Equity Method Investment, Nonconsolidated Investee or Group of Investees [Member]
Schedule of Equity Method Investments [Line Items]
Sales	191		197		422	370
Cost of goods sold	123		146		269	269
Net income (loss)	32		25		81	20
Energy [Member]
Schedule of Equity Method Investments [Line Items]
Equity in net income (loss) of affiliated companies, before reconciling adjustments	12		11		21	21
Other	(2)				(1)
Alcoa Corporation’s equity in net income of affiliated companies	10		11		20	21
Energy [Member] | Equity Method Investment, Nonconsolidated Investee or Group of Investees [Member]
Schedule of Equity Method Investments [Line Items]
Sales	65		60		127	115
Cost of goods sold	23		32		53	57
Net income (loss)	29		27		52	52
Other [Member]
Schedule of Equity Method Investments [Line Items]
Equity in net income (loss) of affiliated companies, before reconciling adjustments	(15)		(2)		(28)	(3)
Other	19		8		15	11
Alcoa Corporation’s equity in net income of affiliated companies	4		6		(13)	8
Other [Member] | Equity Method Investment, Nonconsolidated Investee or Group of Investees [Member]
Schedule of Equity Method Investments [Line Items]
Sales	125		96		242	191
Cost of goods sold	115		78		222	171
Net income (loss)	$ (30)		$ (4)		$ (58)	$ (6)