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Repurchase of shares - Summary of the Shares Repurchased by the Company (Detail) - USD ($)
$ / shares in Units, $ in Thousands
1 Months Ended 12 Months Ended
Feb. 10, 2017
Jan. 13, 2017
Dec. 31, 2017
Mar. 31, 2017
Dec. 31, 2017
Dec. 31, 2019
Equity, Class of Treasury Stock [Line Items]            
Payments for Repurchase of Preferred Stock and Preference Stock         $ 358  
Approximate Dollar Value of ADSs that May Yet Be Purchased Under the Plan           $ 5,300
Share Repurchase Program 2017 [Member]            
Equity, Class of Treasury Stock [Line Items]            
Total Number of ADS Purchased 5,553 994 93,070 86,523    
Average Price Paid Per Share $ 3.75 $ 4.34   $ 3.86