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Fair value measurements (Details) - USD ($)
$ in Thousands
3 Months Ended 6 Months Ended
Jun. 30, 2018
Jun. 30, 2017
Jun. 30, 2018
Jun. 30, 2017
Unobservable Input Reconciliation [Roll Forward]        
Balance, beginning of period     $ 12,274  
Expense included in earnings     1,674  
Settlements     (1,257)  
Balance, end of period $ 12,691   12,691  
RSDL [Member]        
Unobservable Input Reconciliation [Roll Forward]        
Change in fair value of contingent obligations $ 700 $ 200 $ 1,600 $ 500