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Repurchase of shares - Summary of the Shares Repurchased by the Company (Detail) - Share Buyback Program [Member] - USD ($)
1 Months Ended 12 Months Ended
Aug. 18, 2020
Jul. 31, 2020
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Jun. 30, 2020
Equity, Class of Treasury Stock [Line Items]            
Share repurchase authorized amount           $ 20,000,000
Total number of ADS Purchased 334,245 857,147 1,191,392 0 0  
Average price paid per share $ 3.86 $ 3.72        
Approximate Dollar Value of ADSs that May Yet Be Purchased Under the Plan     $ 4,475,000