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Stock Repurchases (Tables)
6 Months Ended
Jun. 30, 2011
Stock Repurchases  
Schedule of Common Stock Repurchased
     Three Months Ended
June 30,
     Six Months Ended
June 30,
 
     2011      2010      2011      2010  

Number of shares

     1,100,000           1,000,000           3,100,000           1,952,500     

Total cost

     $79,423           $105,034           $218,520           $200,059     

Average cost per share

     $72.20           $105.03           $70.49           $102.46