Segment Information - Schedule of Operating Results of Alcoa's Reportable Segments (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Segment Reporting Information [Line Items]
Segment Adjusted EBITDA	$ 784	$ 359	$ 1,978	$ 919
Depreciation, depletion, and amortization	149	154	480	464
Equity (loss) income	37	(10)	72	(34)
Operating Segments [Member]
Segment Reporting Information [Line Items]
Total sales	3,636	2,927	10,406	8,557
Intersegment Eliminations [Member]
Segment Reporting Information [Line Items]
Intersegment sales	529	567	1,605	1,677
Third-Party Sales [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Third-party sales	3,107	2,360	8,801	6,880
Bauxite [Member]
Segment Reporting Information [Line Items]
Segment Adjusted EBITDA	23	124	123	375
Depreciation, depletion, and amortization	30	33	119	97
Bauxite [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total sales	228	292	689	909
Bauxite [Member] | Intersegment Eliminations [Member]
Segment Reporting Information [Line Items]
Intersegment sales	172	236	536	716
Bauxite [Member] | Third-Party Sales [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Third-party sales	56	56	153	193
Alumina [Member]
Segment Reporting Information [Line Items]
Segment Adjusted EBITDA	148	119	499	400
Depreciation, depletion, and amortization	47	41	143	127
Equity (loss) income	(1)	(4)	(7)	(21)
Alumina [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total sales	1,105	1,026	3,260	2,961
Alumina [Member] | Intersegment Eliminations [Member]
Segment Reporting Information [Line Items]
Intersegment sales	349	329	1,056	954
Alumina [Member] | Third-Party Sales [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Third-party sales	756	697	2,204	2,007
Aluminum [Member]
Segment Reporting Information [Line Items]
Segment Adjusted EBITDA	613	116	1,356	144
Depreciation, depletion, and amortization	72	80	218	240
Equity (loss) income	38	(6)	79	(13)
Aluminum [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total sales	2,303	1,609	6,457	4,687
Aluminum [Member] | Intersegment Eliminations [Member]
Segment Reporting Information [Line Items]
Intersegment sales	8	2	13	7
Aluminum [Member] | Third-Party Sales [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Third-party sales	$ 2,295	$ 1,607	$ 6,444	$ 4,680