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Derivatives and Other Financial Instruments - Additional Information (Detail)
In Millions, unless otherwise specified
 1 Months Ended 3 Months Ended 3 Months Ended 1 Months Ended
Jul. 31, 2012
Plant
Contract
Mar. 31, 2014
USD ($)
t
Contract
Mar. 31, 2014
CAD
Dec. 31, 2013
USD ($)
Sep. 30, 2012
USD ($)
Mar. 31, 2014
Forward Contracts [Member]
USD ($)
Mar. 31, 2014
Forward Contracts [Member]
BRL
Mar. 31, 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               $ 67      
Other derivative contracts estimated term of quoted market prices, in years   10 years                  
Number of contracts 2                    
Power contracts maturity date                   Mar. 31, 2014 Mar. 31, 2016
Number of smelters 2                    
Derivative asset amount   371   390 596     337 355    
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   347,000                  
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                  
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 58     231 543        
Maturity date of forward contract           Mar. 31, 2014 Mar. 31, 2014        
Loss recognized on forward contract   $ 4