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Segment Information - Schedule of Segment ATOI to Consolidated Net Income Attributable to Alcoa (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 segment ATOI	$ 410	$ 581	$ 1,633	$ 1,309
Interest expense	(123)	(126)	(369)	(351)
Noncontrolling interests	(62)	18	(189)	46
Consolidated net income attributable to Alcoa	44	149	379	109
Other [Member]
Segment Reporting Information [Line Items]
Impact of LIFO	50	(18)	93	(33)
Metal price lag	(48)	38	(110)	56
Interest expense	(80)	(81)	(240)	(228)
Noncontrolling interests	(62)	18	(189)	46
Corporate expense	(72)	(72)	(199)	(204)
Restructuring and other charges	(48)	(189)	(368)	(587)
Other	$ (106)	$ (128)	$ (241)	$ (250)

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