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Share-based compensation - Summary of Restricted Stock Units Activities Under All Incentive Plans (Detail) - Restricted Stock Units (RSUs) [Member] - $ / shares
12 Months Ended
Dec. 31, 2019
Dec. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Number of RSUs unvested at beginning of period 7,551,480 7,822,280
Number of RSUs, granted 9,406,495  
Number of RSUs, vested (1,534,570)  
Number of RSUs, Canceled/forfeited (3,399,610) (270,800)
Number of RSUs unvested at end of period 12,023,795  
Weighted-average grant date fair value at beginning of period $ 1.4416 $ 1.4375
Weighted-average grant date fair value, granted 0.7353  
Weighted-average grant date fair value, vested 1.4645  
Weighted-average grant date fair value, Canceled/forfeited 1.0740 $ 1.3233
Weighted-average grant date fair value at end of period $ 0.9880