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Derivative Financial Instruments and Fair Value and Credit Risk of Financial Instruments - Additional information (Detail) (USD $)
In Millions, unless otherwise specified
 1 Months Ended 6 Months Ended 12 Months Ended
Aug. 31, 2012
bbl
Dec. 31, 2010
Dec. 31, 2012
bbl
Dec. 31, 2011
Jun. 30, 2010
Derivative [Line Items]          
Unrealized loss, included in Accumulated other comprehensive income/loss     $ 1.3 $ (0.5)  
Percentage of assets classified within level 3 of fair value hierarchy to total assets and liabilities measured at fair value     1.00%    
Commodity Contract
          
Derivative [Line Items]          
LME copper prices, average (per ton) 118.90 7,828.00 7,951.00 8,836.00 6,651.00
LME copper prices, average provisional price (per ton)     7,322    
Applicable forward copper price     7,930    
Commodity contract, volume 10,000        
Commodity contract outstanding,volume     20,000    
Commodity contract, mark-to-market value     0.1    
Commodity Contract | Sales
          
Derivative [Line Items]          
Gain (loss) on mark-to-market value of contracts     $ 15.6 $ 20.6