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Stock-Based Compensation - Assumptions Used to Value Options Granted (Detail) - Employees [Member]
9 Months Ended
Sep. 30, 2017
Sep. 30, 2016
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
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% 0.85%
Expected term 4 years 6 months  
Expected volatility 53.15% 78.08%
Maximum [Member]    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Risk free interest rate 1.59% 1.14%
Expected term 4 years 9 months  
Expected volatility 55.20% 79.29%