Segment Information - Schedule of Operating Results of Alcoa's Reportable Segments (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2024	Sep. 30, 2023
Segment Reporting Information [Line Items]
Segment Adjusted EBITDA	$ 547	$ 132	$ 1,155	$ 562
Depreciation, depletion, and amortization	153	158	466	453
Equity (loss) income	(5)	(24)	9	(125)
Operating Segments [Member]
Segment Reporting Information [Line Items]
Total sales	3,468	2,986	9,831	9,160
Intersegment Eliminations [Member]
Segment Reporting Information [Line Items]
Intersegment sales	570	385	1,429	1,210
Third-Party Sales [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Third-party sales	2,898	2,601	8,402	7,950
Alumina [Member]
Segment Reporting Information [Line Items]
Segment Adjusted EBITDA	367	53	692	189
Depreciation, depletion, and amortization	85	89	262	246
Equity (loss) income	6	(9)	(3)	(37)
Alumina [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total sales	1,661	1,338	4,484	3,907
Alumina [Member] | Intersegment Eliminations [Member]
Segment Reporting Information [Line Items]
Intersegment sales	565	381	1,417	1,199
Alumina [Member] | Third-Party Sales [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Third-party sales	1,096	957	3,067	2,708
Aluminum [Member]
Segment Reporting Information [Line Items]
Segment Adjusted EBITDA	180	79	463	373
Depreciation, depletion, and amortization	68	69	204	207
Equity (loss) income	(11)	(15)	12	(88)
Aluminum [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total sales	1,807	1,648	5,347	5,253
Aluminum [Member] | Intersegment Eliminations [Member]
Segment Reporting Information [Line Items]
Intersegment sales	5	4	12	11
Aluminum [Member] | Third-Party Sales [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Third-party sales	$ 1,802	$ 1,644	$ 5,335	$ 5,242