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Summary of Significant Accounting Policies - Assumptions Used to Value Options Granted (Detail)
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%    
Employees [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Expected term   4 years 9 months  
Dividend yield 0.00% 0.00% 0.00%
Minimum [Member] | Employees [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Risk free interest rate 1.45% 0.85% 0.87%
Expected term 4 years 6 months   4 years 6 months 21 days
Expected volatility 53.15% 55.23% 76.86%
Maximum [Member] | Employees [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Risk free interest rate 1.59% 1.45% 1.20%
Expected term 4 years 9 months   4 years 9 months
Expected volatility 55.20% 79.29% 83.09%