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Retirement Plans (Details 2) (USD $)
3 Months Ended 9 Months Ended
Sep. 30, 2012
Sep. 30, 2011
Sep. 30, 2012
Sep. 30, 2011
Employees' savings plan        
Matching contribution by the company (as a percent)     100.00%  
Matching contribution by the company prior to September 1, 2012 (as a percent)     50.00%  
Expenses recognized under 401(K) plan $ 418,000 $ 240,000 $ 1,100,000 $ 720,000
Minimum
       
Employees' savings plan        
Age of employees to be eligible to participate in the defined contribution plan (in years)     21 years  
Percentage of annual base compensation that participants may elect to contribute     1.00%  
Maximum
       
Employees' savings plan        
Percentage of annual base compensation that participants may elect to contribute     50.00%  
Percentage of employees salary for which the company contributes a matching contribution     5.00%  
Percentage of employees salary for which the company contributes a matching contribution, prior to September 1, 2012     6.00%