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Significant accounting policies - Leases (Detail) - USD ($)
$ in Thousands
Jun. 30, 2019
Jan. 01, 2019
Lessee, Lease, Description [Line Items]    
Weighted-average remaining lease term, finance lease (in years) 6 years 15 days  
Right-of-use asset   $ 8,295
Lease liabilities (note 1(c)) $ 10,388 $ 8,295
Weighted-average discount rate, operating leases 4.34%  
Weighted-average remaining lease term, operating lease (in years) 14 years 3 months 4 days  
Lessee, Operating Lease, Liability, Payment, Due [Abstract]    
Year 1 $ 1,840  
Year 2 1,256  
Year 3 669  
Year 4 510  
Year 5 491  
Thereafter 4,752  
Total 9,518  
Finance Lease, Liability, Payment, Due [Abstract]    
Year 1 537  
Year 2 537  
Year 3 537  
Year 4 537  
Year 5 537  
Thereafter $ 610  
Minimum    
Lessee, Lease, Description [Line Items]    
Lease term, renewal period (in years) 1 year  
Maximum    
Lessee, Lease, Description [Line Items]    
Lease term, renewal period (in years) 5 years