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Commitments and contingencies (Details) - USD ($)
$ in Thousands
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Disclosure of Commitments and contingencies [Line Items]      
Minimum lease payments payable under cancellable operating lease $ 18,155 $ 19,984 $ 22,914
Within 1 year      
Disclosure of Commitments and contingencies [Line Items]      
Minimum lease payments payable under cancellable operating lease 2,006 1,984 2,055
After 1 year but not more than 5 years      
Disclosure of Commitments and contingencies [Line Items]      
Minimum lease payments payable under cancellable operating lease 7,335 7,362 6,731
More than 5 years      
Disclosure of Commitments and contingencies [Line Items]      
Minimum lease payments payable under cancellable operating lease $ 8,814 $ 10,638 $ 14,128