Segment Information - Schedule of Operating Results of Alcoa's Reportable Segments (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Segment Reporting Information [Line Items]
Segment Adjusted EBITDA	$ 400	$ 850	$ 1,286	$ 2,507
Depreciation, depletion, and amortization	177	166	509	538
Equity income (loss)	(5)	5	(29)	10
Operating Segments [Member]
Segment Reporting Information [Line Items]
Total sales	3,172	4,140	9,908	12,243
Intersegment Eliminations [Member]
Segment Reporting Information [Line Items]
Intersegment sales	624	774	1,975	2,247
Third-Party Sales [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Third-party sales	2,548	3,366	7,933	9,996
Bauxite [Member]
Segment Reporting Information [Line Items]
Segment Adjusted EBITDA	134	106	372	316
Depreciation, depletion, and amortization	35	27	90	83
Bauxite [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total sales	351	291	965	890
Bauxite [Member] | Intersegment Eliminations [Member]
Segment Reporting Information [Line Items]
Intersegment sales	251	224	733	699
Bauxite [Member] | Third-Party Sales [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Third-party sales	100	67	232	191
Alumina [Member]
Segment Reporting Information [Line Items]
Segment Adjusted EBITDA	223	660	964	1,690
Depreciation, depletion, and amortization	54	48	157	150
Equity income (loss)		10	15	23
Alumina [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total sales	1,140	1,645	3,763	4,617
Alumina [Member] | Intersegment Eliminations [Member]
Segment Reporting Information [Line Items]
Intersegment sales	369	544	1,231	1,534
Alumina [Member] | Third-Party Sales [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Third-party sales	771	1,101	2,532	3,083
Aluminum [Member]
Segment Reporting Information [Line Items]
Segment Adjusted EBITDA	43	84	(50)	501
Depreciation, depletion, and amortization	88	91	262	305
Equity income (loss)	(5)	(5)	(44)	(13)
Aluminum [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total sales	1,681	2,204	5,180	6,736
Aluminum [Member] | Intersegment Eliminations [Member]
Segment Reporting Information [Line Items]
Intersegment sales	4	6	11	14
Aluminum [Member] | Third-Party Sales [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Third-party sales	$ 1,677	$ 2,198	$ 5,169	$ 6,722