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Income taxes- Unrecognized Tax Benefits (Details) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Jun. 30, 2018
Dec. 31, 2017
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]        
Unrecognized Tax Benefits, Beginning Balance $ 3,338 $ 3,921    
Additions for tax positions of prior years 114      
Lapse of statutes of limitations (989) (603)    
Foreign currency impact 29 20    
Unrecognized Tax Benefits $ 3,338 $ 3,921 $ 2,500 $ 2,492