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Employees' Profit Sharing Plan (Details) (Deferred profit sharing plan, USD $)
 12 Months Ended
Dec. 31, 2012
Dec. 31, 2011
Dec. 31, 2010
Deferred profit sharing plan
      
Employees' profit sharing plan      
Minimum number of continuous employment 1 year    
Profit sharing costs $ 3,400,000 $ 3,900,000 $ 4,095,000