Segment Information - Schedule of Operating Results of Alcoa's Reportable Segments (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Segment Reporting Information [Line Items]
Segment Adjusted EBITDA	$ 185	$ 484	$ 560	$ 886
Depreciation, depletion, and amortization	146	167	310	332
Equity income (loss)	(20)	(14)	(24)	(24)
Operating Segments [Member]
Segment Reporting Information [Line Items]
Total sales	2,680	3,383	5,630	6,736
Intersegment Eliminations [Member]
Segment Reporting Information [Line Items]
Intersegment sales	536	695	1,110	1,351
Third-Party Sales [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Third-party sales	2,144	2,688	4,520	5,385
Bauxite [Member]
Segment Reporting Information [Line Items]
Segment Adjusted EBITDA	131	112	251	238
Depreciation, depletion, and amortization	30	27	64	55
Bauxite [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total sales	311	313	617	614
Bauxite [Member] | Intersegment Eliminations [Member]
Segment Reporting Information [Line Items]
Intersegment sales	245	246	480	482
Bauxite [Member] | Third-Party Sales [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Third-party sales	66	67	137	132
Alumina [Member]
Segment Reporting Information [Line Items]
Segment Adjusted EBITDA	88	369	281	741
Depreciation, depletion, and amortization	37	55	86	103
Equity income (loss)	(8)	3	(17)	15
Alumina [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total sales	892	1,309	1,935	2,623
Alumina [Member] | Intersegment Eliminations [Member]
Segment Reporting Information [Line Items]
Intersegment sales	289	445	625	862
Alumina [Member] | Third-Party Sales [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Third-party sales	603	864	1,310	1,761
Aluminum [Member]
Segment Reporting Information [Line Items]
Segment Adjusted EBITDA	(34)	3	28	(93)
Depreciation, depletion, and amortization	79	85	160	174
Equity income (loss)	(12)	(17)	(7)	(39)
Aluminum [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total sales	1,477	1,761	3,078	3,499
Aluminum [Member] | Intersegment Eliminations [Member]
Segment Reporting Information [Line Items]
Intersegment sales	2	4	5	7
Aluminum [Member] | Third-Party Sales [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Third-party sales	$ 1,475	$ 1,757	$ 3,073	$ 3,492