Contingencies and Commitments - Additional Information - 1 (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended	12 Months Ended
	Sep. 30, 2015	Jun. 30, 2015	Sep. 30, 2015	Dec. 31, 2014	Dec. 31, 2012	Dec. 31, 2009
Loss Contingencies [Line Items]
Number of cleanup locations			More than 100
Remediation reserve balance	$ 558		$ 558	$ 543
Remediation reserve balance, classified as a current liability	63		63	70
Increase in remediation reserve	1
Payments related to remediation expenses applied against the reserve	14		30
Increase (Decrease) in reserves due to effects of foreign currency translation	10		$ 16
Actual remediation fieldwork period			4 years
Guarantee debt service expiration year			2019
Firth Rixson [Member]
Loss Contingencies [Line Items]
Increase (Decrease) in reserves due to acquisition	5		$ 5
Minimum [Member]
Loss Contingencies [Line Items]
Majority of the project funding period			2016
Maximum [Member]
Loss Contingencies [Line Items]
Planning and design phase period			3 years
Majority of the project funding period			2020
Recurring Costs of Managing Hazardous Substances and Environmental Programs [Member]
Loss Contingencies [Line Items]
Percentage of cost of goods sold			2.00%
Grasse River [Member]
Loss Contingencies [Line Items]
Increase in remediation reserve	3		$ 56
Mosjoen [Member]
Loss Contingencies [Line Items]
Increase in remediation reserve	1	$ 11	12		$ 20
Other Sites [Member]
Loss Contingencies [Line Items]
Remediation reserve adjustment	2		8
Pocos De Caldas Smelter and the Anglesea Power Station [Member]
Loss Contingencies [Line Items]
Remediation reserve adjustment	29
Portovesme [Member]
Loss Contingencies [Line Items]
Increase in remediation reserve				3		$ 3
Remediation reserve adjustment			7
Massena West, NY [Member]
Loss Contingencies [Line Items]
Remediation reserve balance	235		235	239
Sherwin, TX site [Member]
Loss Contingencies [Line Items]
Remediation reserve balance	31		31	32
Expected portion of funding through 2019	16		16
East St. Louis, IL Site [Member]
Loss Contingencies [Line Items]
Remediation reserve balance	$ 11		$ 11	$ 15