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Share-Based Compensation - Summary of Fair Value of Share Options Granted to Employees (Detail)
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]      
Expected dividend yield 0.00%    
Employee Stock Option [Member]      
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]      
Risk-free interest rate, minimum 1.49% 1.15% 1.56%
Risk-free interest rate, maximum 2.23% 2.18% 1.89%
Expected volatility, minimum 68.95% 60.89% 62.14%
Expected volatility, maximum 72.24% 68.76% 71.02%
Expected dividend yield 0.00% 0.00% 0.00%
Employee Stock Option [Member] | Minimum [Member]      
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]      
Expected term (in years) 3 years 3 years 5 years 6 months 7 days
Employee Stock Option [Member] | Maximum [Member]      
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]      
Expected term (in years) 6 years 3 months 6 years 3 months 6 years 1 month 13 days