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Share-based compensation - 2010 Plan (Detail) - USD ($)
1 Months Ended 12 Months Ended
Dec. 31, 2010
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Nov. 30, 2014
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Shares issued to depositary bank to reserve for future exercise of share options or vesting of restricted shares         10,000,000
Minimum | Share options          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Stock award vesting period 7 years        
Maximum | Share options          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Stock award vesting period 10 years        
2010 Plan          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Number of shares available for grant   11,284,463      
2010 Plan | Directors [Member]          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Stock award vesting period   32 months      
2010 Plan | Share options          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Unrecognized compensation costs on share options   $ 0 $ 0    
2010 Plan | Restricted shares          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Granted     800,000 6,750,520  
Number of restricted share vested   965,500 1,296,540 267,630  
Unrecognized compensation costs on restricted shares   $ 2,000,000 $ 8,981,000    
2010 Plan | Restricted shares | Officers and Employees [Member]          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Percent of options vesting monthly   15.00%      
2010 Plan | Restricted shares | Directors [Member]          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Restricted shares expected to vest   10,770,520 10,770,520    
2010 Plan | Restricted shares | Nonemployee [Member]          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Number of restricted share vested   60,000 60,000    
2010 Plan | Restricted shares | Tranche One [Member]          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Restricted shares expected to vest   410,000      
2010 Plan | Restricted shares | Tranche Two [Member]          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Restricted shares expected to vest   390,000      
2010 Plan | Restricted shares | Tranche Three [Member]          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Restricted shares expected to vest   390,000      
2010 Plan | Restricted shares | Tranche Four [Member]          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Restricted shares expected to vest   70,000      
2010 Plan | Maximum          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Number of shares available for grant 26,822,828