Segment Information - Schedule of Operating Results, Capital Expenditures and Assets of Alcoa's Reportable Segments (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2025	Sep. 30, 2024	Sep. 30, 2025	Sep. 30, 2024	Dec. 31, 2024
Segment Reporting Information [Line Items]
Adjusted operating costs	$ 2,185	$ 2,087	$ 6,830	$ 6,318
Other segment items	914	834	2,794	2,358
Segment Adjusted EBITDA	374	547	1,408	1,155
Depreciation, depletion, and amortization	155	153	444	466
Equity income (loss)	0	(5)	3	9
Capital expenditures	142	142	359	398
Equity investments	476		476		$ 966
Segment assets	11,818		11,818		12,267
Operating Segments [Member]
Segment Reporting Information [Line Items]
Total sales	3,473	3,468	11,032	9,831
Intersegment Eliminations [Member]
Segment Reporting Information [Line Items]
Intersegment sales	479	570	1,667	1,429
Third-Party Sales [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Third-party sales	2,994	2,898	9,365	8,402
Alumina [Member]
Segment Reporting Information [Line Items]
Adjusted operating costs	779	734	2,272	2,344
Other segment items	582	560	1,979	1,448
Segment Adjusted EBITDA	67	367	870	692
Depreciation, depletion, and amortization	88	85	244	262
Equity income (loss)	0	6	6	(3)
Capital expenditures	75	100	201	266
Equity investments	240		240		420
Segment assets	5,728		5,728		6,138
Alumina [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total sales	1,428	1,661	5,121	4,484
Alumina [Member] | Intersegment Eliminations [Member]
Segment Reporting Information [Line Items]
Intersegment sales	474	565	1,653	1,417
Alumina [Member] | Third-Party Sales [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Third-party sales	954	1,096	3,468	3,067
Aluminum [Member]
Segment Reporting Information [Line Items]
Adjusted operating costs	1,406	1,353	4,558	3,974
Other segment items	332	274	815	910
Segment Adjusted EBITDA	307	180	538	463
Depreciation, depletion, and amortization	67	68	200	204
Equity income (loss)	0	(11)	(3)	12
Capital expenditures	67	42	158	132
Equity investments	236		236		546
Segment assets	6,090		6,090		$ 6,129
Aluminum [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total sales	2,045	1,807	5,911	5,347
Aluminum [Member] | Intersegment Eliminations [Member]
Segment Reporting Information [Line Items]
Intersegment sales	5	5	14	12
Aluminum [Member] | Third-Party Sales [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Third-party sales	$ 2,040	$ 1,802	$ 5,897	$ 5,335