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Deferred revenue and income - Schedule of Deferred Revenue and Income (Parenthetical) (Detail) - USD ($)
 1 Months Ended 12 Months Ended
Jun. 30, 2017
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Deferred Revenue Arrangement [Line Items]        
Reimbursement received from depository   $ 127,000    
Deferred income withholding tax   $ 42,000    
Deferred income amortized depositary service period   5 years    
Deferred revenue and income, non-current [1]   $ 3,242,000 $ 4,082,000  
Increase Decrease In Deferred Government Grants $ 1,224,000 (3,493,000) (3,473,000) $ (1,969,000)
Deferred Income Amortization Period For Government Grant 5 years      
Membership subscription revenues [Member]        
Deferred Revenue Arrangement [Line Items]        
Deferred revenue and income, non-current   425,000 820,000  
Government grants [Member]        
Deferred Revenue Arrangement [Line Items]        
Deferred revenue and income, non-current   2,509,000 2,719,000  
Reimbursement from the depository [Member]        
Deferred Revenue Arrangement [Line Items]        
Deferred revenue and income, non-current   $ 308,000 $ 543,000  
[1] As of December 31, 2017, the non-current portion included membership subscription revenue of USD 425,000 (2016: USD 820,000), government grants of USD 2,509,000 (2016: USD 2,719,000), and reimbursement from the depositary of USD 308,000 (2016: USD 543,000).