Segment and Related Information - Schedule of Operating Results, Capital Expenditures and Assets of Alcoa's Reportable Segments (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Segment Reporting Information [Line Items]
Adjusted EBITDA	$ 3,203	$ 2,725	$ 1,453
Depreciation, depletion, and amortization	702	708	677
Equity income (loss)	(6)	(24)	(64)
Capital expenditures	366	375
Equity investments	1,335	1,383
Total assets	13,813	14,798
Operating Segments [Member]
Segment Reporting Information [Line Items]
Total sales	16,378	14,112	11,246
Intersegment Eliminations [Member]
Segment Reporting Information [Line Items]
Intersegment sales	3,063	2,619	2,100
Third-Party Sales [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Third-party sales	13,315	11,493	9,146
Bauxite [Member]
Segment Reporting Information [Line Items]
Adjusted EBITDA	426	424	374
Depreciation, depletion, and amortization	111	82	77
Capital expenditures	47	53
Equity investments	189	191
Total assets	1,448	1,609
Bauxite [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total sales	1,215	1,208	1,066
Bauxite [Member] | Intersegment Eliminations [Member]
Segment Reporting Information [Line Items]
Intersegment sales	944	875	751
Bauxite [Member] | Third-Party Sales [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Third-party sales	271	333	315
Alumina [Member]
Segment Reporting Information [Line Items]
Adjusted EBITDA	2,373	1,289	376
Depreciation, depletion, and amortization	197	207	186
Equity income (loss)	32	(5)	(40)
Capital expenditures	194	144
Equity investments	290	262
Total assets	4,643	5,129
Alumina [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total sales	6,316	4,856	3,607
Alumina [Member] | Intersegment Eliminations [Member]
Segment Reporting Information [Line Items]
Intersegment sales	2,101	1,723	1,307
Alumina [Member] | Third-Party Sales [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Third-party sales	4,215	3,133	2,300
Aluminum [Member]
Segment Reporting Information [Line Items]
Adjusted EBITDA	404	1,012	703
Depreciation, depletion, and amortization	394	419	414
Equity income (loss)	(38)	(19)	(24)
Capital expenditures	125	178
Equity investments	856	930
Total assets	7,722	8,060
Aluminum [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total sales	8,847	8,048	6,573
Aluminum [Member] | Intersegment Eliminations [Member]
Segment Reporting Information [Line Items]
Intersegment sales	18	21	42
Aluminum [Member] | Third-Party Sales [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Third-party sales	$ 8,829	$ 8,027	$ 6,531