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Share-based compensation - Schedule of Fair Values of Stock Options Granted (Detail)
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Risk-free interest rate, Minimum [1] 0.77% 0.77%
Risk-free interest rate, Maximum [1] 1.76% 1.76%
Dividend yield [2] 0.00% 0.00%
Volatility rate, Minimum [3] 40.07% 40.07%
Volatility rate, Maximum [3] 43.30% 43.30%
Minimum [Member]    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Expected term (in years) [4] 4 years 22 days 4 years 22 days
Maximum [Member]    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Expected term (in years) [4] 5 years 6 months 22 days 5 years 6 months 22 days
[1] The risk-free interest rate of periods within the contractual life of the share option is based on the USD denominated China Government Bond yield as at the valuation dates.
[2] The Company has no history or expectation of paying dividends on its common shares.
[3] Expected volatility is estimated based on the average of historical volatilities of the comparable companies in the same industry as at the valuation dates.
[4] The expected term is developed by assuming the share options will be exercised in the middle point between the vesting dates and maturity dates.