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Share-based compensation - 2013 Plan (Detail) - 2013 Plan [Member] - Restricted shares [Member] - USD ($)
12 Months Ended
Dec. 31, 2019
Dec. 31, 2021
Dec. 31, 2020
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