Segment Information - Schedule of Operating Results of Alcoa's Reportable Segments (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Segment Reporting Information [Line Items]
Segment Adjusted EBITDA	$ 625	$ 185	$ 1,194	$ 560
Depreciation, depletion, and amortization	155	146	331	310
Equity (loss) income	27	(20)	35	(24)
Operating Segments [Member]
Segment Reporting Information [Line Items]
Total sales	3,354	2,680	6,770	5,630
Intersegment Eliminations [Member]
Segment Reporting Information [Line Items]
Intersegment sales	525	536	1,076	1,110
Third-Party Sales [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Third-party sales	2,829	2,144	5,694	4,520
Bauxite [Member]
Segment Reporting Information [Line Items]
Segment Adjusted EBITDA	41	131	100	251
Depreciation, depletion, and amortization	32	30	89	64
Bauxite [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total sales	218	311	461	617
Bauxite [Member] | Intersegment Eliminations [Member]
Segment Reporting Information [Line Items]
Intersegment sales	179	245	364	480
Bauxite [Member] | Third-Party Sales [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Third-party sales	39	66	97	137
Alumina [Member]
Segment Reporting Information [Line Items]
Segment Adjusted EBITDA	124	88	351	281
Depreciation, depletion, and amortization	50	37	96	86
Equity (loss) income	(1)	(8)	(6)	(17)
Alumina [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total sales	1,031	892	2,155	1,935
Alumina [Member] | Intersegment Eliminations [Member]
Segment Reporting Information [Line Items]
Intersegment sales	343	289	707	625
Alumina [Member] | Third-Party Sales [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Third-party sales	688	603	1,448	1,310
Aluminum [Member]
Segment Reporting Information [Line Items]
Segment Adjusted EBITDA	460	(34)	743	28
Depreciation, depletion, and amortization	73	79	146	160
Equity (loss) income	28	(12)	41	(7)
Aluminum [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total sales	2,105	1,477	4,154	3,078
Aluminum [Member] | Intersegment Eliminations [Member]
Segment Reporting Information [Line Items]
Intersegment sales	3	2	5	5
Aluminum [Member] | Third-Party Sales [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Third-party sales	$ 2,102	$ 1,475	$ 4,149	$ 3,073