Segment Information - Schedule of Operating Results of Alcoa's Reportable Segments (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2022	Mar. 31, 2021
Segment Reporting Information [Line Items]
Segment Adjusted EBITDA	$ 1,013	$ 569
Depreciation, depletion, and amortization	154	176
Equity (loss) income	40	8
Operating Segments [Member]
Segment Reporting Information [Line Items]
Total sales	3,881	3,416
Intersegment Eliminations [Member]
Segment Reporting Information [Line Items]
Intersegment sales	595	551
Third-Party Sales [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Third-party sales	3,286	2,865
Bauxite [Member]
Segment Reporting Information [Line Items]
Segment Adjusted EBITDA	38	59
Depreciation, depletion, and amortization	35	57
Bauxite [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total sales	213	243
Bauxite [Member] | Intersegment Eliminations [Member]
Segment Reporting Information [Line Items]
Intersegment sales	170	185
Bauxite [Member] | Third-Party Sales [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Third-party sales	43	58
Alumina [Member]
Segment Reporting Information [Line Items]
Segment Adjusted EBITDA	262	227
Depreciation, depletion, and amortization	50	46
Equity (loss) income	1	(5)
Alumina [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total sales	1,273	1,124
Alumina [Member] | Intersegment Eliminations [Member]
Segment Reporting Information [Line Items]
Intersegment sales	418	364
Alumina [Member] | Third-Party Sales [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Third-party sales	855	760
Aluminum [Member]
Segment Reporting Information [Line Items]
Segment Adjusted EBITDA	713	283
Depreciation, depletion, and amortization	69	73
Equity (loss) income	39	13
Aluminum [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total sales	2,395	2,049
Aluminum [Member] | Intersegment Eliminations [Member]
Segment Reporting Information [Line Items]
Intersegment sales	7	2
Aluminum [Member] | Third-Party Sales [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Third-party sales	$ 2,388	$ 2,047