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Derivatives and Other Financial Instruments - Additional Information (Detail)
In Millions, unless otherwise specified
 1 Months Ended 9 Months Ended 1 Months Ended
Jul. 31, 2012
Contract
Plant
Jan. 31, 2012
Sep. 30, 2013
USD ($)
t
Contract
Sep. 30, 2013
CAD
t
Contract
Dec. 31, 2012
USD ($)
Sep. 30, 2012
USD ($)
Sep. 30, 2013
Level 3 [Member]
USD ($)
Dec. 31, 2012
Level 3 [Member]
USD ($)
Sep. 30, 2013
Level 1 [Member]
USD ($)
Dec. 31, 2012
Level 1 [Member]
USD ($)
Sep. 30, 2013
Level 2 [Member]
USD ($)
Dec. 31, 2012
Level 2 [Member]
USD ($)
Jul. 31, 2012
Power Contracts One [Member]
Jul. 31, 2012
Power Contracts Two [Member]
Jan. 31, 2012
Interest Rate Swap [Member]
USD ($)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]                              
Net unrealized loss on derivative contracts             $ 94                
Other derivative contracts estimated term of quoted market prices, in years     10 years 10 years                      
Number of contracts 2                            
Power contracts maturity date                         Mar. 31, 2014 Mar. 31, 2016  
Number of smelters 2                            
Derivative asset amount     422   596 596 385 550 8 27 36 45      
Power purchase period forecasted by derivative instrument     Through December 2036 Through December 2036                      
Period of natural gas supply contract     6 years 6 years                      
Number of commodities involved for valuation of supply contract     2 2                      
Number of contracts ceased to qualify as fair value hedge     0 0                      
Amount of aluminum kmt designated as fair value hedges     370,000 370,000                      
Derivative designated as fair value hedge start period     2013 2013                      
Derivative designated as fair value hedge end period     2017 2017                      
Face amount of debt the hedges effectively convert from fixed to floating through 2018     200                        
Notional amount of interest rate swaps                             315
Long-term debt, interest rate   6.00%                          
Long-term debt, maturity date   2012                          
Interest rate cash flow hedges     0                        
Forward contract to purchase to mitigate the foreign currency risk related to a Canadian-denominated loan due in 2014     56 58                      
Long-term debt, fair value                 $ 8,085 $ 8,936 $ 501 $ 569