Segment Information - Schedule of Operating Results, Capital Expenditures and Assets of Alcoa's Reportable Segments (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2025	Mar. 31, 2024	Dec. 31, 2024
Segment Reporting Information [Line Items]
Adjusted operating costs	$ 2,297	$ 2,075
Other segment items	985	734
Segment Adjusted EBITDA	798	189
Depreciation, depletion, and amortization	143	155
Equity income (loss)	9	(9)
Capital expenditures	90	98
Equity investments	996		$ 966
Segment assets	12,564		12,267
Operating Segments [Member]
Segment Reporting Information [Line Items]
Total sales	4,080	2,998
Intersegment Eliminations [Member]
Segment Reporting Information [Line Items]
Intersegment sales	716	399
Third-Party Sales [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Third-party sales	3,364	2,599
Alumina [Member]
Segment Reporting Information [Line Items]
Adjusted operating costs	723	796
Other segment items	788	421
Segment Adjusted EBITDA	664	139
Depreciation, depletion, and amortization	76	87
Equity income (loss)	15	(11)
Capital expenditures	53	57
Equity investments	443		420
Segment assets	6,295		6,138
Alumina [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total sales	2,175	1,356
Alumina [Member] | Intersegment Eliminations [Member]
Segment Reporting Information [Line Items]
Intersegment sales	712	395
Alumina [Member] | Third-Party Sales [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Third-party sales	1,463	961
Aluminum [Member]
Segment Reporting Information [Line Items]
Adjusted operating costs	1,574	1,279
Other segment items	197	313
Segment Adjusted EBITDA	134	50
Depreciation, depletion, and amortization	67	68
Equity income (loss)	(6)	2
Capital expenditures	37	41
Equity investments	553		546
Segment assets	6,269		$ 6,129
Aluminum [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total sales	1,905	1,642
Aluminum [Member] | Intersegment Eliminations [Member]
Segment Reporting Information [Line Items]
Intersegment sales	4	4
Aluminum [Member] | Third-Party Sales [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Third-party sales	$ 1,901	$ 1,638