Segment Information - Schedule of Operating Results of Alcoa's Reportable Segments (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Segment Reporting Information [Line Items]
Segment Adjusted EBITDA	$ 402	$ 689
Depreciation, depletion, and amortization	165	188
Equity income (loss)	(10)	(1)
Operating Segments [Member]
Segment Reporting Information [Line Items]
Total sales	3,353	3,779
Intersegment Eliminations [Member]
Segment Reporting Information [Line Items]
Intersegment sales	656	707
Third-Party Sales [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Third-party sales	2,697	3,072
Bauxite [Member]
Segment Reporting Information [Line Items]
Segment Adjusted EBITDA	126	110
Depreciation, depletion, and amortization	28	29
Bauxite [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total sales	301	296
Bauxite [Member] | Intersegment Eliminations [Member]
Segment Reporting Information [Line Items]
Intersegment sales	236	249
Bauxite [Member] | Third-Party Sales [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Third-party sales	65	47
Alumina [Member]
Segment Reporting Information [Line Items]
Segment Adjusted EBITDA	372	392
Depreciation, depletion, and amortization	48	53
Equity income (loss)	12	(1)
Alumina [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total sales	1,314	1,368
Alumina [Member] | Intersegment Eliminations [Member]
Segment Reporting Information [Line Items]
Intersegment sales	417	454
Alumina [Member] | Third-Party Sales [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Third-party sales	897	914
Aluminum [Member]
Segment Reporting Information [Line Items]
Segment Adjusted EBITDA	(96)	187
Depreciation, depletion, and amortization	89	106
Equity income (loss)	(22)
Aluminum [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total sales	1,738	2,115
Aluminum [Member] | Intersegment Eliminations [Member]
Segment Reporting Information [Line Items]
Intersegment sales	3	4
Aluminum [Member] | Third-Party Sales [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Third-party sales	$ 1,735	$ 2,111