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Commitments and contingencies (Details 1) - USD ($)
$ in Thousands
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Disclosure of Commitments and contingencies [Line Items]      
Minimum lease payments receivable under cancellable operating lease $ 543 $ 935 $ 1,277
Within 1 year      
Disclosure of Commitments and contingencies [Line Items]      
Minimum lease payments receivable under cancellable operating lease 300 289 455
After 1 year but not more than 5 years      
Disclosure of Commitments and contingencies [Line Items]      
Minimum lease payments receivable under cancellable operating lease $ 243 $ 646 $ 822