Segment Information - Schedule of Operating Results of Alcoa's Reportable Segment (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Segment Reporting Information [Line Items]
Equity (loss) income		$ (23)	$ (38)	$ (42)
Total sales	$ 5,295	5,897	10,242	11,716
Depreciation, depletion, and amortization	296	307	593	613
Income taxes	173	216	288	491
After-tax operating income (ATOI)	444	567	735	1,223
Operating Segments [Member]
Segment Reporting Information [Line Items]
Total sales	6,097	6,924	11,836	13,961
Intersegment Eliminations [Member]
Segment Reporting Information [Line Items]
Intersegment sales	802	1,027	1,598	2,256
Third-Party Sales [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Third-party sales	5,295	5,897	10,238	11,705
Alumina [Member]
Segment Reporting Information [Line Items]
Equity (loss) income		(11)	(21)	(18)
Depreciation, depletion, and amortization	66	77	129	157
Income taxes	40	87	45	179
After-tax operating income (ATOI)	109	215	117	436
Alumina [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total sales	994	1,355	1,831	2,743
Alumina [Member] | Intersegment Eliminations [Member]
Segment Reporting Information [Line Items]
Intersegment sales	300	431	592	932
Alumina [Member] | Third-Party Sales [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Third-party sales	694	924	1,239	1,811
Primary Metals [Member]
Segment Reporting Information [Line Items]
Equity (loss) income		(5)	4	(8)
Depreciation, depletion, and amortization	101	109	203	218
Income taxes		6	(16)	63
After-tax operating income (ATOI)	41	67	55	254
Primary Metals [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total sales	1,592	2,096	3,190	4,360
Primary Metals [Member] | Intersegment Eliminations [Member]
Segment Reporting Information [Line Items]
Intersegment sales	473	562	948	1,254
Primary Metals [Member] | Third-Party Sales [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Third-party sales	1,119	1,534	2,242	3,106
Global Rolled Products [Member]
Segment Reporting Information [Line Items]
Equity (loss) income		(7)	(21)	(16)
Depreciation, depletion, and amortization	55	56	111	112
Income taxes	28	25	62	61
After-tax operating income (ATOI)	68	76	136	130
Global Rolled Products [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total sales	1,579	1,702	3,005	3,359
Global Rolled Products [Member] | Intersegment Eliminations [Member]
Segment Reporting Information [Line Items]
Intersegment sales	29	34	58	70
Global Rolled Products [Member] | Third-Party Sales [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Third-party sales	1,550	1,668	2,947	3,289
Engineered Products and Solutions [Member]
Segment Reporting Information [Line Items]
Depreciation, depletion, and amortization	62	54	127	105
Income taxes	87	81	165	157
After-tax operating income (ATOI)	180	165	342	321
Engineered Products and Solutions [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total sales	1,465	1,279	2,914	2,536
Engineered Products and Solutions [Member] | Third-Party Sales [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Third-party sales	1,465	1,279	2,914	2,536
Transportation and Construction Solutions [Member]
Segment Reporting Information [Line Items]
Depreciation, depletion, and amortization	12	11	23	21
Income taxes	18	17	32	31
After-tax operating income (ATOI)	46	44	85	82
Transportation and Construction Solutions [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total sales	467	492	896	963
Transportation and Construction Solutions [Member] | Third-Party Sales [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Third-party sales	$ 467	$ 492	$ 896	$ 963