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Share-based compensation - Summary of Restricted Shares Activities (Detail) - Restricted shares [Member] - $ / shares
12 Months Ended
Dec. 31, 2021
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
2010 share incentive plan [Member]        
Number of restricted shares        
Unvested at beginning of period 1,260,000 5,184,500 6,652,040  
Granted     800,000  
Vested (400,000) (965,500) (1,296,540)  
Forfeited (210,000) (2,959,000) (971,000)  
Unvested at end of period 650,000 1,260,000 5,184,500 6,652,040
Vested and expected to vest 552,500 1,071,000 4,406,825 5,654,234
Weighted-Average Grant-Date Fair Value        
Weighted-Average Grant Date Fair Value, Granted     $ 0.81  
2013 Plan [Member]        
Number of restricted shares        
Unvested at beginning of period   34,175    
Vested     (27,475)  
Forfeited     (6,700)  
Unvested at end of period     34,175  
2014 Plan [Member]        
Number of restricted shares        
Unvested at beginning of period 26,000 1,321,200 3,476,650  
Vested (26,000) (228,200) (1,318,450)  
Forfeited   (1,067,000) (837,000)  
Unvested at end of period 26,000 26,000 1,321,200 3,476,650
Vested and expected to vest   22,100 1,123,020  
2020 Incentive Plan        
Number of restricted shares        
Granted 31,091,840      
Vested (2,429,965)      
Forfeited (2,777,500)      
Unvested at end of period 25,884,375      
Vested and expected to vest 19,413,281      
Weighted-Average Grant-Date Fair Value        
Weighted-Average Grant Date Fair Value, Granted $ 0.83