Derivatives and Other Financial Instruments - Additional Information (Detail)	1 Months Ended	9 Months Ended				9 Months Ended					1 Months Ended
	Jul. 31, 2012 Plant Contract	Sep. 30, 2014 USD ($) t Contract	Sep. 30, 2014 CAD	Dec. 31, 2013 USD ($)	Sep. 30, 2012 USD ($)	Sep. 30, 2014 Forward Contracts [Member] USD ($)	Sep. 30, 2014 Forward Contracts [Member] BRL	Sep. 30, 2014 Forward Contracts [Member] Canadian Denominated Loan [Member] USD ($)	Sep. 30, 2014 Level 3 [Member] USD ($)	Dec. 31, 2013 Level 3 [Member] USD ($)	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]
Net unrealized loss on derivative contracts									$ 220,000,000
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
Derivative asset amount		249,000,000		390,000,000	596,000,000				217,000,000	355,000,000
Power purchase period forecasted by derivative instrument		Through December 2036
Period of natural gas supply contract		6 years
Number of commodities involved for valuation of supply contract		2
Number of contracts ceased to qualify as fair value hedge		0
Amount of aluminum kmt designated as fair value hedges		264,000
Derivative designated as fair value hedge start period		2014
Derivative designated as fair value hedge end period		2018
Face amount of debt the hedges effectively convert from fixed to floating through 2018		200,000,000
Interest rate cash flow hedges		0
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. 31, 2014				Mar. 31, 2014	Mar. 31, 2014	Oct. 08, 2014
Gain recognized on forward contract		$ 4,000,000						$ 1,000,000