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Share-based compensation - Summary of Restricted Shares Activities (Detail) - $ / shares
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
2010 share incentive plan [Member] | Restricted shares [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Number of restricted shares, Unvested, beginning balance 943,220 432,217  
Number of restricted shares, Granted 2,050,000 1,170,000  
Number of restricted shares, Vested (115,125) (274,960)  
Number of restricted shares, Forfeited (1,605,945) (384,037)  
Number of restricted shares, Unvested, Ending balance 1,272,150 943,220 432,217
Number of restricted shares, Vested and expected to vest, Ending balance 1,081,327 801,737  
Weighted-Average Grant Date Fair Value, Granted $ 0.69 $ 1.18  
2013 Plan [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Number of restricted shares, Granted 8,664,980 8,664,980  
2013 Plan [Member] | Restricted shares [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Number of restricted shares, Unvested, beginning balance 1,714,535 3,796,398 7,200,778
Number of restricted shares, Vested (996,835) (1,520,760) (2,627,815)
Number of restricted shares, Forfeited (129,940) (561,103) (776,565)
Number of restricted shares, Unvested, Ending balance 587,760 1,714,535 3,796,398
Number of restricted shares, Vested and expected to vest, Ending balance 499,596 1,457,355 3,226,939
2014 Plan [Member] | Restricted shares [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Number of restricted shares, Unvested, beginning balance 10,276,300 5,761,400  
Number of restricted shares, Granted 0 6,749,000  
Number of restricted shares, Vested (2,447,950) (1,262,200)  
Number of restricted shares, Forfeited (2,022,000) (971,900)  
Number of restricted shares, Unvested, Ending balance 5,806,350 10,276,300 5,761,400
Number of restricted shares, Vested and expected to vest, Ending balance 4,935,398 8,734,855  
Weighted-Average Grant Date Fair Value, Granted $ 0 $ 1.12