Segment Information - Schedule of Operating Results, Capital Expenditures and Assets of Alcoa's Reportable Segments (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2025	Jun. 30, 2024	Jun. 30, 2025	Jun. 30, 2024	Dec. 31, 2024
Segment Reporting Information [Line Items]
Adjusted operating costs	$ 2,348	$ 2,156	$ 4,645	$ 4,231
Other segment items	895	790	1,880	1,524
Segment Adjusted EBITDA	236	419	1,034	608
Depreciation, depletion, and amortization	146	158	289	313
Equity income (loss)	(6)	23	3	14
Capital expenditures	127	158	217	256
Equity investments	1,012		1,012		$ 966
Segment assets	12,408		12,408		12,267
Operating Segments [Member]
Segment Reporting Information [Line Items]
Total sales	3,479	3,365	7,559	6,363
Intersegment Eliminations [Member]
Segment Reporting Information [Line Items]
Intersegment sales	472	460	1,188	859
Third-Party Sales [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Third-party sales	3,007	2,905	6,371	5,504
Alumina [Member]
Segment Reporting Information [Line Items]
Adjusted operating costs	770	814	1,493	1,610
Other segment items	609	467	1,397	888
Segment Adjusted EBITDA	139	186	803	325
Depreciation, depletion, and amortization	80	90	156	177
Equity income (loss)	(9)	2	6	(9)
Capital expenditures	73	109	126	166
Equity investments	444		444		420
Segment assets	6,119		6,119		6,138
Alumina [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total sales	1,518	1,467	3,693	2,823
Alumina [Member] | Intersegment Eliminations [Member]
Segment Reporting Information [Line Items]
Intersegment sales	467	457	1,179	852
Alumina [Member] | Third-Party Sales [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Third-party sales	1,051	1,010	2,514	1,971
Aluminum [Member]
Segment Reporting Information [Line Items]
Adjusted operating costs	1,578	1,342	3,152	2,621
Other segment items	286	323	483	636
Segment Adjusted EBITDA	97	233	231	283
Depreciation, depletion, and amortization	66	68	133	136
Equity income (loss)	3	21	(3)	23
Capital expenditures	54	49	91	90
Equity investments	568		568		546
Segment assets	6,289		6,289		$ 6,129
Aluminum [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total sales	1,961	1,898	3,866	3,540
Aluminum [Member] | Intersegment Eliminations [Member]
Segment Reporting Information [Line Items]
Intersegment sales	5	3	9	7
Aluminum [Member] | Third-Party Sales [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Third-party sales	$ 1,956	$ 1,895	$ 3,857	$ 3,533