Annual report pursuant to Section 13 and 15(d)

Stock-Based Compensation - Stock Options - Additional Information (Detail)

v3.8.0.1
Stock-Based Compensation - Stock Options - Additional Information (Detail) - USD ($)
$ / shares in Units, $ in Millions
1 Months Ended 12 Months Ended
Dec. 31, 2017
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Weighted average grant date fair value for stock options granted   $ 2.42 $ 5.03 $ 1.02
Total fair value of stock options vested   $ 6.7 $ 0.0 $ 0.0
Aggregate intrinsic value of stock options exercised   1.3 $ 0.0 $ 0.0
Total unrecognized stock-based compensation expense related to non-vested stock options $ 5.0 $ 5.0    
Unrecognized stock-based compensation expense related to non-vested stock options period of recognition   4 years 2 months 12 days    
Dividend yield percentage   0.00% 0.00% 0.00%
Employee Termination [Member]        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Compensation expense $ 0.7      
Stock-based compensation purchase 51,622      
IPO [Member]        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Number of stock options vested   1,159,914    
Compensation expense   $ 5.6    
Minimum [Member]        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Stock option, vesting period   2 years    
Maximum [Member]        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Stock option, vesting period   6 years    
Acceleration Vesting [Member]        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Percentage of vesting   25.00%    
Acceleration Vesting [Member] | Employee Termination [Member]        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Percentage of vesting   100.00%