Segment Information - Schedule of Operating Results of Alcoa's Reportable Segments (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Segment Reporting Information [Line Items]
Segment Adjusted EBITDA	$ 359	$ 400	$ 919	$ 1,286
Depreciation, depletion, and amortization	154	177	464	509
Equity income (loss)	(10)	(5)	(34)	(29)
Operating Segments [Member]
Segment Reporting Information [Line Items]
Total sales	2,927	3,172	8,557	9,908
Intersegment Eliminations [Member]
Segment Reporting Information [Line Items]
Intersegment sales	567	624	1,677	1,975
Third-Party Sales [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Third-party sales	2,360	2,548	6,880	7,933
Bauxite [Member]
Segment Reporting Information [Line Items]
Segment Adjusted EBITDA	124	134	375	372
Depreciation, depletion, and amortization	33	35	97	90
Bauxite [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total sales	292	351	909	965
Bauxite [Member] | Intersegment Eliminations [Member]
Segment Reporting Information [Line Items]
Intersegment sales	236	251	716	733
Bauxite [Member] | Third-Party Sales [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Third-party sales	56	100	193	232
Alumina [Member]
Segment Reporting Information [Line Items]
Segment Adjusted EBITDA	119	223	400	964
Depreciation, depletion, and amortization	41	54	127	157
Equity income (loss)	(4)		(21)	15
Alumina [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total sales	1,026	1,140	2,961	3,763
Alumina [Member] | Intersegment Eliminations [Member]
Segment Reporting Information [Line Items]
Intersegment sales	329	369	954	1,231
Alumina [Member] | Third-Party Sales [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Third-party sales	697	771	2,007	2,532
Aluminum [Member]
Segment Reporting Information [Line Items]
Segment Adjusted EBITDA	116	43	144	(50)
Depreciation, depletion, and amortization	80	88	240	262
Equity income (loss)	(6)	(5)	(13)	(44)
Aluminum [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total sales	1,609	1,681	4,687	5,180
Aluminum [Member] | Intersegment Eliminations [Member]
Segment Reporting Information [Line Items]
Intersegment sales	2	4	7	11
Aluminum [Member] | Third-Party Sales [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Third-party sales	$ 1,607	$ 1,677	$ 4,680	$ 5,169