Segment Information - Schedule of Operating Results of Alcoa's Reportable Segments (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Segment Reporting Information [Line Items]
Segment Adjusted EBITDA	$ 569	$ 375
Depreciation, depletion, and amortization	176	164
Equity (loss) income	8	(4)
Operating Segments [Member]
Segment Reporting Information [Line Items]
Total sales	3,416	2,950
Intersegment Eliminations [Member]
Segment Reporting Information [Line Items]
Intersegment sales	551	574
Third-Party Sales [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Third-party sales	2,865	2,376
Bauxite [Member]
Segment Reporting Information [Line Items]
Segment Adjusted EBITDA	59	120
Depreciation, depletion, and amortization	57	34
Bauxite [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total sales	243	306
Bauxite [Member] | Intersegment Eliminations [Member]
Segment Reporting Information [Line Items]
Intersegment sales	185	235
Bauxite [Member] | Third-Party Sales [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Third-party sales	58	71
Alumina [Member]
Segment Reporting Information [Line Items]
Segment Adjusted EBITDA	227	193
Depreciation, depletion, and amortization	46	49
Equity (loss) income	(5)	(9)
Alumina [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total sales	1,124	1,043
Alumina [Member] | Intersegment Eliminations [Member]
Segment Reporting Information [Line Items]
Intersegment sales	364	336
Alumina [Member] | Third-Party Sales [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Third-party sales	760	707
Aluminum [Member]
Segment Reporting Information [Line Items]
Segment Adjusted EBITDA	283	62
Depreciation, depletion, and amortization	73	81
Equity (loss) income	13	5
Aluminum [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total sales	2,049	1,601
Aluminum [Member] | Intersegment Eliminations [Member]
Segment Reporting Information [Line Items]
Intersegment sales	2	3
Aluminum [Member] | Third-Party Sales [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Third-party sales	$ 2,047	$ 1,598