Segment Information - Schedule of Operating Results of Alcoa's Reportable Segment (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015
Segment Reporting Information [Line Items]
Total sales	$ 4,947	$ 5,819
Equity (loss) income	(21)	(19)
Depreciation, depletion, and amortization	297	306
Income taxes	115	275
After-tax operating income (ATOI)	291	656
Operating Segments [Member]
Segment Reporting Information [Line Items]
Total sales	5,739	7,037
Intersegment Eliminations [Member]
Segment Reporting Information [Line Items]
Intersegment sales	796	1,229
Third-Party Sales [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Third-party sales	4,943	5,808
Alumina [Member]
Segment Reporting Information [Line Items]
Equity (loss) income	(14)	(7)
Depreciation, depletion, and amortization	63	80
Income taxes	5	92
After-tax operating income (ATOI)	8	221
Alumina [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total sales	837	1,388
Alumina [Member] | Intersegment Eliminations [Member]
Segment Reporting Information [Line Items]
Intersegment sales	292	501
Alumina [Member] | Third-Party Sales [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Third-party sales	545	887
Primary Metals [Member]
Segment Reporting Information [Line Items]
Equity (loss) income	4	(3)
Depreciation, depletion, and amortization	102	109
Income taxes	(16)	57
After-tax operating income (ATOI)	14	187
Primary Metals [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total sales	1,598	2,264
Primary Metals [Member] | Intersegment Eliminations [Member]
Segment Reporting Information [Line Items]
Intersegment sales	475	692
Primary Metals [Member] | Third-Party Sales [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Third-party sales	1,123	1,572
Global Rolled Products [Member]
Segment Reporting Information [Line Items]
Equity (loss) income	(11)	(9)
Depreciation, depletion, and amortization	56	56
Income taxes	34	36
After-tax operating income (ATOI)	68	54
Global Rolled Products [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total sales	1,426	1,657
Global Rolled Products [Member] | Intersegment Eliminations [Member]
Segment Reporting Information [Line Items]
Intersegment sales	29	36
Global Rolled Products [Member] | Third-Party Sales [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Third-party sales	1,397	1,621
Engineered Products and Solutions [Member]
Segment Reporting Information [Line Items]
Depreciation, depletion, and amortization	65	51
Income taxes	78	76
After-tax operating income (ATOI)	162	156
Engineered Products and Solutions [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total sales	1,449	1,257
Engineered Products and Solutions [Member] | Third-Party Sales [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Third-party sales	1,449	1,257
Transportation and Construction Solutions [Member]
Segment Reporting Information [Line Items]
Depreciation, depletion, and amortization	11	10
Income taxes	14	14
After-tax operating income (ATOI)	39	38
Transportation and Construction Solutions [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total sales	429	471
Transportation and Construction Solutions [Member] | Third-Party Sales [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Third-party sales	$ 429	$ 471