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Stock Option Plans - Summary of Option Activity (Detail) - USD ($)
 6 Months Ended 12 Months Ended
Jun. 30, 2015
Dec. 31, 2008
Dec. 31, 2007
Number of Shares      
Outstanding at beginning of period (in shares) 748,208    
Granted (in shares) 225,000 20,000 73,403
Exercised (in shares) (69,809)    
Expired/Forfeited (in shares) 0    
Outstanding at end of period (in shares) 903,399    
Exercisable at end of period (in shares) 479,024    
Weighted Average Exercise Price per share      
Outstanding at beginning of period (in usd per share) $ 44.79    
Granted (in usd per share) 51.07    
Exercised (in usd per share) 42.80    
Expired/Forfeited (in usd per share) 0.00    
Outstanding at end of period (in usd per share) 46.51    
Exercisable at end of period (in usd per share) $ 45.31    
Aggregate Intrinsic Value      
Outstanding at beginning of period $ 9,274,028    
Granted 0    
Exercised $ 1,019,890    
Expired/Forfeited      
Outstanding at end of period $ 3,226,939    
Exercisable at end of period $ 2,310,289