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Share-based compensation - Weighted Average Assumptions Used in Determining Fair Value of Share Based Awards at Grant Date by Using Black-Scholes Option Pricing Model (Detail)
9 Months Ended
Sep. 30, 2015
Sep. 30, 2014
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]    
Dividend yield 0.00% 0.00%
Risk free interest rate   1.69%
Expected volatility   59.00%
Expected lives 3 years 5 years
Minimum    
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]    
Risk free interest rate 0.83%  
Expected volatility 49.00%  
Maximum    
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]    
Risk free interest rate 1.03%  
Expected volatility 60.00%