Segment Information - Schedule of Operating Results of Alcoa's Reportable Segment (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Segment Reporting Information [Line Items]
Total sales	$ 5,573	$ 6,239	$ 17,289	$ 17,529
Equity loss	(24)	(15)	(66)	(83)
Depreciation, depletion, and amortization	305	328	918	978
Income taxes	153	245	644	554
After-tax operating income (ATOI)	410	581	1,633	1,309
Operating Segments [Member]
Segment Reporting Information [Line Items]
Total sales	6,459	7,498	20,420	21,320
Intersegment Eliminations [Member]
Segment Reporting Information [Line Items]
Intersegment sales	899	1,264	3,155	3,793
Third-Party Sales [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Third-party sales	5,560	6,234	17,265	17,527
Alumina [Member]
Segment Reporting Information [Line Items]
Equity loss	(9)	(7)	(27)	(19)
Depreciation, depletion, and amortization	71	100	228	297
Income taxes	85	26	264	78
After-tax operating income (ATOI)	212	62	648	192
Alumina [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total sales	1,303	1,368	4,046	3,964
Alumina [Member] | Intersegment Eliminations [Member]
Segment Reporting Information [Line Items]
Intersegment sales	391	482	1,323	1,472
Alumina [Member] | Third-Party Sales [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Third-party sales	912	886	2,723	2,492
Primary Metals [Member]
Segment Reporting Information [Line Items]
Equity loss	(7)		(15)	(45)
Depreciation, depletion, and amortization	106	124	324	377
Income taxes	(49)	95	14	114
After-tax operating income (ATOI)	(59)	245	195	327
Primary Metals [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total sales	1,728	2,595	6,088	7,130
Primary Metals [Member] | Intersegment Eliminations [Member]
Segment Reporting Information [Line Items]
Intersegment sales	479	730	1,733	2,182
Primary Metals [Member] | Third-Party Sales [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Third-party sales	1,249	1,865	4,355	4,948
Global Rolled Products [Member]
Segment Reporting Information [Line Items]
Equity loss	(8)	(8)	(24)	(19)
Depreciation, depletion, and amortization	56	62	168	178
Income taxes	28	26	89	73
After-tax operating income (ATOI)	62	69	192	193
Global Rolled Products [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total sales	1,556	1,978	4,915	5,602
Global Rolled Products [Member] | Intersegment Eliminations [Member]
Segment Reporting Information [Line Items]
Intersegment sales	29	52	99	139
Global Rolled Products [Member] | Third-Party Sales [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Third-party sales	1,527	1,926	4,816	5,463
Engineered Products and Solutions [Member]
Segment Reporting Information [Line Items]
Depreciation, depletion, and amortization	61	32	166	95
Income taxes	71	78	228	234
After-tax operating income (ATOI)	151	155	472	455
Engineered Products and Solutions [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total sales	1,397	1,034	3,933	3,103
Engineered Products and Solutions [Member] | Third-Party Sales [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Third-party sales	1,397	1,034	3,933	3,103
Transportation and Construction Solutions [Member]
Segment Reporting Information [Line Items]
Depreciation, depletion, and amortization	11	10	32	31
Income taxes	18	20	49	55
After-tax operating income (ATOI)	44	50	126	142
Transportation and Construction Solutions [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total sales	475	523	1,438	1,521
Transportation and Construction Solutions [Member] | Third-Party Sales [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Third-party sales	$ 475	$ 523	$ 1,438	$ 1,521