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Stock-Based Compensation - Assumptions Used to Value Options Granted (Detail) - Employees [Member]
6 Months Ended
Jun. 30, 2018
Jun. 30, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Risk free interest rate 2.15%  
Expected term 4 years 9 months  
Dividend yield 0.00% 0.00%
Minimum [Member]    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Risk free interest rate   1.45%
Expected term   4 years 6 months
Expected volatility 47.79% 53.15%
Maximum [Member]    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Risk free interest rate   1.59%
Expected term   4 years 9 months
Expected volatility 47.86% 55.20%