Annual report pursuant to Section 13 and 15(d)

Stock Based Compensation - Summary of Stock Option Activity (Detail)

v3.8.0.1
Stock Based Compensation - Summary of Stock Option Activity (Detail) - USD ($)
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Number of Options, Granted 0 0 0
Employee Stock Option [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Number of Options, Outstanding 1,309,444    
Number of Options, Granted 0    
Number of Options, Exercised (272,205)    
Number of Options, Forfeited 0    
Number of Options, Outstanding 1,037,239 1,309,444  
Number of Options, Exercisable 1,037,239    
Weighted Average Exercise Price, Outstanding $ 4.55    
Weighted Average Exercise Price, Granted 0    
Weighted Average Exercise Price, Exercised 3.60    
Weighted Average Exercise Price, Forfeited 0    
Weighted Average Exercise Price, Outstanding 4.80 $ 4.55  
Weighted Average Exercise Price, Exercisable $ 4.80    
Weighted Average Remaining Life In Years, Outstanding 6 years 4 months 14 days 7 years 1 month 6 days  
Weighted Average Remaining Life In Years, Exercisable 6 years 4 months 24 days    
Intrinsic Value, Outstanding $ 15,198,044 $ 16,107,929  
Intrinsic Value, Exercisable $ 15,198,044