Segment Information - Schedule of Operating Results of Alcoa's Reportable Segments (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2024	Mar. 31, 2023
Segment Reporting Information [Line Items]
Segment Adjusted EBITDA	$ 189	$ 287
Depreciation, depletion, and amortization	155	147
Equity (loss) income	(9)	(74)
Operating Segments [Member]
Segment Reporting Information [Line Items]
Total sales	2,998	3,091
Intersegment Eliminations [Member]
Segment Reporting Information [Line Items]
Intersegment sales	399	424
Third-Party Sales [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Third-party sales	2,599	2,667
Alumina [Member]
Segment Reporting Information [Line Items]
Segment Adjusted EBITDA	139	103
Depreciation, depletion, and amortization	87	77
Equity (loss) income	(11)	(17)
Alumina [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total sales	1,356	1,278
Alumina [Member] | Intersegment Eliminations [Member]
Segment Reporting Information [Line Items]
Intersegment sales	395	421
Alumina [Member] | Third-Party Sales [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Third-party sales	961	857
Aluminum [Member]
Segment Reporting Information [Line Items]
Segment Adjusted EBITDA	50	184
Depreciation, depletion, and amortization	68	70
Equity (loss) income	2	(57)
Aluminum [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total sales	1,642	1,813
Aluminum [Member] | Intersegment Eliminations [Member]
Segment Reporting Information [Line Items]
Intersegment sales	4	3
Aluminum [Member] | Third-Party Sales [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Third-party sales	$ 1,638	$ 1,810