Segment Information - Schedule of Operating Results of Alcoa's Reportable Segment (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013
Segment Reporting Information [Line Items]
Total sales	$ 6,239	$ 5,765	$ 17,529	$ 17,447
Equity loss	(15)	(18)	(83)	(40)
Depreciation, depletion, and amortization	326	327	973	1,009
Income taxes	263	124	584	377
After-tax operating income (ATOI)	619	338	1,362	993
Operating Segments [Member]
Segment Reporting Information [Line Items]
Total sales	7,436	6,939	21,136	21,186
Intersegment Eliminations [Member]
Segment Reporting Information [Line Items]
Intersegment sales	1,264	1,251	3,793	3,925
Third-Party Sales [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Third-party sales	6,172	5,688	17,343	17,261
Alumina [Member]
Segment Reporting Information [Line Items]
Equity loss	(7)	(2)	(19)	(2)
Depreciation, depletion, and amortization	100	100	297	324
Income taxes	26	17	78	45
After-tax operating income (ATOI)	62	67	192	189
Alumina [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total sales	1,368	1,359	3,964	4,183
Alumina [Member] | Intersegment Eliminations [Member]
Segment Reporting Information [Line Items]
Intersegment sales	482	513	1,472	1,689
Alumina [Member] | Third-Party Sales [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Third-party sales	886	846	2,492	2,494
Primary Metals [Member]
Segment Reporting Information [Line Items]
Equity loss		(13)	(45)	(29)
Depreciation, depletion, and amortization	124	131	377	398
Income taxes	95	(16)	114	(40)
After-tax operating income (ATOI)	245	8	327	15
Primary Metals [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total sales	2,595	2,291	7,130	7,073
Primary Metals [Member] | Intersegment Eliminations [Member]
Segment Reporting Information [Line Items]
Intersegment sales	730	691	2,182	2,095
Primary Metals [Member] | Third-Party Sales [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Third-party sales	1,865	1,600	4,948	4,978
Global Rolled Products [Member]
Segment Reporting Information [Line Items]
Equity loss	(8)	(3)	(19)	(9)
Depreciation, depletion, and amortization	62	56	178	168
Income taxes	42	32	99	103
After-tax operating income (ATOI)	103	71	241	231
Global Rolled Products [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total sales	1,978	1,852	5,602	5,602
Global Rolled Products [Member] | Intersegment Eliminations [Member]
Segment Reporting Information [Line Items]
Intersegment sales	52	47	139	141
Global Rolled Products [Member] | Third-Party Sales [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Third-party sales	1,926	1,805	5,463	5,461
Engineered Products and Solutions [Member]
Segment Reporting Information [Line Items]
Depreciation, depletion, and amortization	40	40	121	119
Income taxes	100	91	293	269
After-tax operating income (ATOI)	209	192	602	558
Engineered Products and Solutions [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total sales	1,495	1,437	4,440	4,328
Engineered Products and Solutions [Member] | Third-Party Sales [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Third-party sales	$ 1,495	$ 1,437	$ 4,440	$ 4,328