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Fair Value Measurements (Tables)
12 Months Ended
Dec. 31, 2011
Fair Value Measurements  
Schedule of assets and liabilities measured at fair value on a recurring basis

 

 

 
  Total   Quoted Prices
in Active
Markets for
Identical Assets
(Level 1)
  Significant
Other
Observable
Inputs
(Level 2)
  Significant
Unobservable
Inputs
(Level 3)
 

2011

                         

Investment in short-term commercial paper

  $ 84,830   $   $ 84,830   $  

Commodity futures—financial assets

    3,159         3,159      

Commodity futures—financial liabilities

    4,378         4,378      

2010

                         

Commodity futures—financial assets

    7,052         7,052      

Commodity futures—financial liabilities

    12,040         12,040