Annual report pursuant to Section 13 and 15(d)

Stock Based Compensation - Summary of Fair Values of Purchase Options Granted (Detail)

v3.8.0.1
Stock Based Compensation - Summary of Fair Values of Purchase Options Granted (Detail) - $ / shares
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Dividend yield   0.00%  
Expected volatility   75.00%  
Risk-free interest rate   1.04%  
Employee Stock Option [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Stock price     $ 7.41
Dividend yield 0.00% 0.00% 0.00%
Expected volatility     65.00%
Risk-free interest rate     0.13%
Expected life 6 months 6 months 6 months
Minimum [Member] | Employee Stock Option [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Stock price $ 16.08 $ 8.36  
Expected volatility 56.00% 56.00%  
Risk-free interest rate 0.62% 0.37%  
Maximum [Member] | Employee Stock Option [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Stock price $ 17.59 $ 12.16  
Expected volatility 66.00% 100.00%  
Risk-free interest rate 1.11% 0.49%