Segment Information - Schedule of Operating Results of Alcoa's Reportable Segments (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Segment Reporting Information [Line Items]
Segment Adjusted EBITDA	$ 132	$ 230	$ 562	$ 2,199
Depreciation, depletion, and amortization	158	144	453	453
Equity (loss) income	(24)	(23)	(125)	52
Operating Segments [Member]
Segment Reporting Information [Line Items]
Total sales	2,986	3,289	9,160	11,136
Intersegment Eliminations [Member]
Segment Reporting Information [Line Items]
Intersegment sales	385	422	1,210	1,333
Third-Party Sales [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Third-party sales	2,601	2,867	7,950	9,803
Alumina [Member]
Segment Reporting Information [Line Items]
Segment Adjusted EBITDA	53	78	189	738
Depreciation, depletion, and amortization	89	74	246	243
Equity (loss) income	(9)	(18)	(37)	(22)
Alumina [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total sales	1,338	1,303	3,907	4,208
Alumina [Member] | Intersegment Eliminations [Member]
Segment Reporting Information [Line Items]
Intersegment sales	381	412	1,199	1,308
Alumina [Member] | Third-Party Sales [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Third-party sales	957	891	2,708	2,900
Aluminum [Member]
Segment Reporting Information [Line Items]
Segment Adjusted EBITDA	79	152	373	1,461
Depreciation, depletion, and amortization	69	70	207	210
Equity (loss) income	(15)	(5)	(88)	74
Aluminum [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total sales	1,648	1,986	5,253	6,928
Aluminum [Member] | Intersegment Eliminations [Member]
Segment Reporting Information [Line Items]
Intersegment sales	4	10	11	25
Aluminum [Member] | Third-Party Sales [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Third-party sales	$ 1,644	$ 1,976	$ 5,242	$ 6,903