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Commitments and contingencies (Tables)
 12 Months Ended
Dec. 31, 2017
Statement [Line Items]  
Disclosure of maturity analysis of operating lease payments [Table Text Block]
      2017     2016  
  Within one year $   5,682   $ 5,591  
  After one year but not more than five years   12,291     12,606  
  More than five years   1,781     442  
               
    $   19,754   $ 18,639  
  Payments recognized in operating expenses:            
      2017     2016  
  Minimum lease payments $ 4,972   $ 4,575  
               
    $ 4,972   $ 4,575