Derivatives and Other Financial Instruments - Additional Information (Detail)	1 Months Ended	12 Months Ended							12 Months Ended				12 Months Ended						12 Months Ended										12 Months Ended				12 Months Ended				1 Months Ended
	Jul. 31, 2012 Plant Contract	Dec. 31, 2014 USD ($) Agency	Dec. 31, 2013 USD ($)	Dec. 31, 2012 USD ($)	Dec. 31, 2014 Level 1 [Member] USD ($) kt	Dec. 31, 2013 Level 1 [Member] USD ($)	Dec. 31, 2012 Level 1 [Member] USD ($)	Dec. 31, 2014 Level 3 [Member] Derivative	Dec. 31, 2014 Embedded Aluminum Derivative [Member] USD ($)	Dec. 31, 2013 Embedded Aluminum Derivative [Member] USD ($)	Dec. 31, 2012 Embedded Aluminum Derivative [Member] USD ($)	Dec. 31, 2014 Embedded Aluminum Derivative [Member] Level 3 [Member] Derivative	Dec. 31, 2014 Embedded Aluminum Derivative [Member] Scenario, Forecast [Member] kt	Dec. 31, 2013 Embedded Aluminum Derivative [Member] Scenario, Forecast [Member] kt	Dec. 31, 2014 Embedded Aluminum Derivative [Member] Cash Flow Hedging [Member] USD ($)	Dec. 31, 2013 Embedded Aluminum Derivative [Member] Cash Flow Hedging [Member] USD ($)	Dec. 31, 2012 Embedded Aluminum Derivative [Member] Cash Flow Hedging [Member] USD ($)	Dec. 31, 2014 Embedded Aluminum Derivative [Member] Cash Flow Hedging [Member] Level 3 [Member] Derivative	Dec. 31, 2014 Embedded Aluminum Derivative [Member] Cash Flow Hedging [Member] Nonoperating Income (Expense) [Member] USD ($)	Dec. 31, 2013 Embedded Aluminum Derivative [Member] Cash Flow Hedging [Member] Nonoperating Income (Expense) [Member] USD ($)	Dec. 31, 2012 Embedded Aluminum Derivative [Member] Cash Flow Hedging [Member] Nonoperating Income (Expense) [Member] USD ($)	Dec. 31, 2014 Foreign Exchange Contracts One [Member] USD ($)	Dec. 31, 2014 Foreign Exchange Contracts One [Member] CAD	Dec. 31, 2014 Foreign Exchange Contracts One [Member] Forward Contracts [Member] Canadian Denominated Loan [Member] USD ($)	Dec. 31, 2014 Foreign Exchange Contracts Two [Member] USD ($)	Dec. 31, 2014 Foreign Exchange Contracts Two [Member] Forward Contracts [Member]	Dec. 31, 2013 Foreign Exchange Contracts Two [Member] Forward Contracts [Member] USD ($)	Dec. 31, 2013 Foreign Exchange Contracts Two [Member] Forward Contracts [Member] BRL	Dec. 31, 2014 Energy Contracts [Member] USD ($)	Dec. 31, 2013 Energy Contracts [Member] USD ($)	Dec. 31, 2012 Energy Contracts [Member] USD ($)	Dec. 31, 2014 Energy Contracts [Member] Cash Flow Hedging [Member] Level 3 [Member] Derivative	Dec. 31, 2014 Embedded Credit Derivative [Member] USD ($)	Dec. 31, 2013 Embedded Credit Derivative [Member] USD ($)	Dec. 31, 2012 Embedded Credit Derivative [Member] USD ($)	Dec. 31, 2014 Embedded Credit Derivative [Member] Level 3 [Member] Derivative	Jul. 31, 2012 Power Contract One [Member]	Jul. 31, 2012 Power Contract Two [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Forward contract to purchase to mitigate the foreign currency risk related to a Canadian/Brazilian-denominated loan due in 2014																						$ 53,000,000	58,000,000				$ 231,000,000	543,000,000
Maturity date of forward contract																						Aug. 05, 2014	Aug. 05, 2014			Mar. 31, 2014
Gain recognized on forward contract																								1,000,000	4,000,000
Fair value of derivatives recorded as assets					2,000,000	31,000,000
Fair value of derivatives recorded as liabilities					4,000,000	45,000,000
Net gain (loss) of derivative instruments					(15,000,000)	4,000,000	88,000,000		(15,000,000)	28,000,000	16,000,000
Net gain of cash flow hedges						9,000,000	40,000,000
Aluminium forcast sales					343								3,610	841
Number of derivative instruments								9				7						5														1				1
Other derivative contracts estimated term of quoted market prices, in years		10 years
Number of contracts	2
Power contracts maturity date																																					Jun. 30, 2014	Jun. 30, 2016
Number of smelters	2
Number of credit rating agencies		3
Recognized an unrealized loss															78,000,000	190,000,000	(40,000,000)												4,000,000	3,000,000	1,000,000
Unrealized gain loss on derivatives															(28,000,000)	(29,000,000)	(41,000,000)
Amount of loss expected to be recognized into earnings over the next 12 months															(13,000,000)
Other expenses (income), net																			4,000,000	1,000,000	1,000,000
Derivative instruments ineffectiveness															0	0	0												0	0	0
Electricity purchasd of megawatt hours																													59,409,328
Derivative gain		$ (13,000,000)	$ 36,000,000	$ 13,000,000																													$ 2,000,000	$ 8,000,000	$ (3,000,000)