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Commitments and contingencies (Details) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Disclosure of Commitments and contingencies [Line Items]    
Minimum operating lease payments recognised as expense $ 8,873 $ 10,417
Not later than one year [member]    
Disclosure of Commitments and contingencies [Line Items]    
Minimum operating lease payments recognised as expense 1,543 1,543
Later than one year and not later than five years [member]    
Disclosure of Commitments and contingencies [Line Items]    
Minimum operating lease payments recognised as expense 6,173 6,173
Later than five years [member]    
Disclosure of Commitments and contingencies [Line Items]    
Minimum operating lease payments recognised as expense $ 1,157 $ 2,701