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Derivative Instruments and Risk Management (Details) (USD $)
In Millions, unless otherwise specified
Jun. 30, 2012
Dec. 31, 2011
Carrying amount [Member]
   
Summary of commodity swap    
Commodity swaps $ 0 $ 0.2
Notional quantity- 2075 tonnes [Member] | Carrying amount [Member]
   
Summary of commodity swap    
Commodity swaps   0.2
Notional quantity- 2075 tonnes [Member] | Fair value [Member]
   
Summary of commodity swap    
Commodity swaps   0.2
Notional quantity- 525 tonnes [Member] | Carrying amount [Member]
   
Summary of commodity swap    
Commodity swaps (0.1)  
Notional quantity- 525 tonnes [Member] | Fair value [Member]
   
Summary of commodity swap    
Commodity swaps $ (0.1)