Quarterly report pursuant to Section 13 or 15(d)

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

v3.8.0.1
Stock Based Compensation - Summary of Stock Option Activity (Detail) - Employee Stock Option [Member] - USD ($)
3 Months Ended 12 Months Ended
Mar. 31, 2018
Dec. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Number of Options, Outstanding 1,037,239  
Number of Options, Granted 0  
Number of Options, Exercised (179,732)  
Number of Options, Forfeited 0  
Number of Options, Outstanding 857,507 1,037,239
Number of Options, Exercisable 1,037,239  
Number of Options, Vested 0  
Number of Options, Exercised (179,732)  
Number of Options, Forfeited 0  
Number of Options, Exercisable 857,507 1,037,239
Weighted Average Exercise Price, Outstanding $ 4.80  
Weighted Average Exercise Price, Granted 0  
Weighted Average Exercise Price, Exercised 5.46  
Weighted Average Exercise Price, Forfeited 0  
Weighted Average Exercise Price, Outstanding 4.66 $ 4.80
Weighted Average Exercise Price, Exercisable 4.80  
Weighted Average Exercise Price, Vested 0  
Weighted Average Exercise Price, Exercised 5.46  
Weighted Average Exercise Price, Forfeited 0  
Weighted Average Exercise Price, Exercisable $ 4.66 $ 4.80
Weighted Average Remaining Life In Years, Outstanding 6 years 2 months 12 days 6 years 4 months 24 days
Weighted Average Remaining Life In Years, Exercisable 6 years 2 months 12 days 6 years 4 months 24 days
Intrinsic Value, Outstanding $ 9,750,635 $ 15,198,044
Intrinsic Value, Exercisable $ 9,750,635 $ 15,198,044