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Income Taxes - Narrative (Detail) - USD ($)
 3 Months Ended 6 Months Ended
Jun. 30, 2017
Jun. 30, 2016
Jun. 30, 2017
Jun. 30, 2016
Dec. 31, 2016
Income Tax Disclosure [Abstract]          
Income tax expense (benefit) $ 6,024,458 $ 5,624,808 $ 12,521,728 $ 10,796,584  
Effective tax rate 31.70% 36.50% 32.40% 36.20%  
Federal statutory income tax rate, percent 35.00% 35.00% 35.00% 35.00%  
Effective income tax rate reconciliation, share-based compensation, excess tax benefit (tax shortfall), Amount $ 800,000   $ 1,300,000    
Effective income tax rate reconciliation, tax benefit due to donation of appreciated securities, Amount     100,000 $ 100,000  
Income tax effects allocated directly to equity   $ (100,000)   $ (5,300,000)  
Valuation allowance $ 0   $ 0   $ 0