Segment and Geographic Area Information - Schedule of Operating Results of Alcoa's Reportable Segment (Detail) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Segment Reporting Information [Line Items]
Total sales	$ 5,245	$ 5,573	$ 5,897	$ 5,819	$ 6,377	$ 6,239	$ 5,836	$ 5,454	$ 22,534	$ 23,906	$ 23,032
Equity loss									(85)	(90)	(68)
Depreciation, depletion, and amortization									1,228	1,295	1,344
Income taxes									726	812	468
ATOI									1,906	1,968	1,267
Capital expenditures									1,107	1,132
Equity investments	1,518				1,785				1,518	1,785
Goodwill	4,925				4,762				4,925	4,762
Total assets	29,666				31,341				29,666	31,341
Operating Segments [Member]
Segment Reporting Information [Line Items]
Total sales									26,490	28,955	28,067
Intersegment Eliminations [Member]
Segment Reporting Information [Line Items]
Intersegment sales									3,982	5,057	5,034
Total sales									(3,982)	(5,057)	(5,034)
Third-Party Sales [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Third-party sales									22,508	23,898	23,033
Alumina [Member]
Segment Reporting Information [Line Items]
Equity loss									(41)	(29)	(4)
Depreciation, depletion, and amortization									296	387	426
Income taxes									300	153	66
ATOI									746	370	259
Capital expenditures									184	246
Equity investments	667				669				667	669
Goodwill	6				8				6	8
Total assets	6,165				7,350				6,165	7,350
Alumina [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total sales									5,142	5,450	5,561
Alumina [Member] | Intersegment Eliminations [Member]
Segment Reporting Information [Line Items]
Intersegment sales									1,687	1,941	2,235
Alumina [Member] | Third-Party Sales [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Third-party sales									3,455	3,509	3,326
Primary Metals [Member]
Segment Reporting Information [Line Items]
Equity loss									(12)	(34)	(51)
Depreciation, depletion, and amortization									429	494	526
Income taxes									(28)	203	(74)
ATOI									155	594	(20)
Capital expenditures									156	176
Equity investments	634				890				634	890
Total assets	7,324				9,308				7,324	9,308
Primary Metals [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total sales									7,761	9,731	9,217
Primary Metals [Member] | Intersegment Eliminations [Member]
Segment Reporting Information [Line Items]
Intersegment sales									2,170	2,931	2,621
Primary Metals [Member] | Third-Party Sales [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Third-party sales									5,591	6,800	6,596
Global Rolled Products [Member]
Segment Reporting Information [Line Items]
Equity loss									(32)	(27)	(13)
Depreciation, depletion, and amortization									227	235	226
Income taxes									109	89	123
ATOI									244	245	292
Capital expenditures									307	389
Equity investments	217				226				217	226
Goodwill	201				210				201	210
Total assets	4,498				4,908				4,498	4,908
Global Rolled Products [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total sales									6,363	7,536	7,284
Global Rolled Products [Member] | Intersegment Eliminations [Member]
Segment Reporting Information [Line Items]
Intersegment sales									125	185	178
Global Rolled Products [Member] | Third-Party Sales [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Third-party sales									6,238	7,351	7,106
Engineered Products and Solutions [Member]
Segment Reporting Information [Line Items]
Depreciation, depletion, and amortization									233	137	124
Income taxes									282	298	286
ATOI									595	579	569
Capital expenditures									383	249
Goodwill	4,660				4,458				4,660	4,458
Total assets	10,732				8,800				10,732	8,800
Engineered Products and Solutions [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total sales									5,342	4,217	4,054
Engineered Products and Solutions [Member] | Third-Party Sales [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Third-party sales									5,342	4,217	4,054
Transportation and Construction Solutions [Member]
Segment Reporting Information [Line Items]
Depreciation, depletion, and amortization									43	42	42
Income taxes									63	69	67
ATOI									166	180	167
Capital expenditures									77	72
Goodwill	58				86				58	86
Total assets	$ 947				$ 975				947	975
Transportation and Construction Solutions [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total sales									1,882	2,021	1,951
Transportation and Construction Solutions [Member] | Third-Party Sales [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Third-party sales									$ 1,882	$ 2,021	$ 1,951