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Share-based compensation - 2013 Plan (Detail) - 2013 Plan - Restricted shares - USD ($)
12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Number of shares available for grant 9,073,732    
Unrecognized compensation costs on restricted shares $ 0 $ 0  
Number of restricted share vested   27,475 525,140
Nonemployee [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Number of restricted share vested 60,000 60,000  
Officers and Employees [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Number of shares available for grant 8,664,980 8,664,980