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Share-based compensation - 2010 Plan (Detail) - USD ($)
1 Months Ended 12 Months Ended
Dec. 31, 2010
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
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   8,315,463      
Total fair value, share options vested   $ 0 $ 0 $ 132,000  
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 [Member]          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Granted   800,000 6,750,520 2,050,000  
Number of restricted share vested   1,296,540 267,630 115,125  
Unrecognized compensation costs on restricted shares   $ 8,981,000 $ 12,347,000    
2010 Plan | Restricted shares [Member] | Officers and Employees [Member]          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Percent of options vesting monthly   20.00%      
2010 Plan | Restricted shares [Member] | Directors [Member]          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Restricted shares expected to vest   10,770,520 9,970,520    
2010 Plan | Restricted shares [Member] | 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 [Member] | Tranche One [Member]          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Restricted shares expected to vest   1,331,500      
2010 Plan | Restricted shares [Member] | Tranche Two [Member]          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Restricted shares expected to vest   1,321,500      
2010 Plan | Restricted shares [Member] | Tranche Three [Member]          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Restricted shares expected to vest   1,311,500      
2010 Plan | Restricted shares [Member] | Tranche Four [Member]          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Restricted shares expected to vest   1,070,000      
2010 Plan | Restricted shares [Member] | Tranche Five [Member]          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Restricted shares expected to vest   150,000      
2010 Plan | Maximum          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Number of shares available for grant 26,822,828