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Share-based compensation - Summary of Restricted Shares Activities (Detail) - Restricted shares [Member] - $ / shares
12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
2010 Plan        
Number of restricted shares        
Unvested at beginning of period 6,652,040 1,272,150 943,220  
Granted 800,000 6,750,520 2,050,000  
Vested (1,296,540) (267,630) (115,125)  
Forfeited (971,000) (1,103,000) (1,605,945)  
Unvested at end of period 5,184,500 6,652,040 1,272,150 943,220
Vested and expected to vest 4,406,825 5,654,234 1,081,327 801,737
Weighted-Average Grant-Date Fair Value        
Weighted-Average Grant Date Fair Value, Granted $ 0.81 $ 2.32 $ 0.69  
2013 Plan        
Number of restricted shares        
Unvested at beginning of period 34,175 587,760 1,714,535  
Vested (27,475) (525,140) (996,835)  
Forfeited (6,700) (28,445) (129,940)  
Unvested at end of period 34,175 34,175 587,760 1,714,535
Vested and expected to vest   29,049 499,596  
2014 Plan        
Number of restricted shares        
Unvested at beginning of period 3,476,650 5,806,350 10,276,300  
Vested (1,318,450) (2,086,450) (2,447,950)  
Forfeited (837,000) (243,250) (2,022,000)  
Unvested at end of period 1,321,200 3,476,650 5,806,350 10,276,300
Vested and expected to vest 1,123,020 2,955,153 4,935,398