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Share-based compensation - Summary of Restricted Shares Activities (Detail) - Restricted shares - $ / shares
12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
2010 Plan        
Number of restricted shares        
Unvested at beginning of period 5,184,500 6,652,040 1,272,150  
Granted   800,000 6,750,520  
Vested (965,500) (1,296,540) (267,630)  
Forfeited (2,959,000) (971,000) (1,103,000)  
Unvested at end of period 1,260,000 5,184,500 6,652,040 1,272,150
Vested and expected to vest 1,071,000 4,406,825 5,654,234 1,081,327
Weighted-Average Grant-Date Fair Value        
Weighted-Average Grant Date Fair Value, Granted   $ 0.81 $ 2.32  
2013 Plan        
Number of restricted shares        
Unvested at beginning of period   34,175 587,760  
Vested   (27,475) (525,140)  
Forfeited   (6,700) (28,445)  
Unvested at end of period 34,175   34,175 587,760
Vested and expected to vest     29,049  
2014 Plan        
Number of restricted shares        
Unvested at beginning of period 1,321,200 3,476,650 5,806,350  
Vested (228,200) (1,318,450) (2,086,450)  
Forfeited (1,067,000) (837,000) (243,250)  
Unvested at end of period 26,000 1,321,200 3,476,650 5,806,350
Vested and expected to vest 22,100 1,123,020 2,955,153