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Share-based compensation - Schedule of Recognized Compensation Costs (Detail) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]      
Total compensation costs $ 2,310 $ 5,428 $ 5,294
Sales and marketing expenses [Member]      
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]      
Total compensation costs 185 381 404
General and administrative expenses [Member]      
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]      
Total compensation costs 1,209 2,453 2,245
Research and development expenses [Member]      
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]      
Total compensation costs $ 916 $ 2,594 $ 2,645