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Fair value (Details) - Fair value measurements on a recurring basis - USD ($)
$ in Thousands
Jun. 30, 2018
Dec. 31, 2017
Assets:    
Total $ 53,628 $ 67,203
Level 1    
Assets:    
Total 24,601 15,703
Level 2    
Assets:    
Total 29,027 51,500
Cash and money market funds    
Assets:    
Total 24,601 15,703
Cash and money market funds | Level 1    
Assets:    
Total 24,601 15,703
Corporate debt securities (including commercial paper)    
Assets:    
Total 21,046 35,531
Corporate debt securities (including commercial paper) | Level 2    
Assets:    
Total 21,046 35,531
U.S. government securities    
Assets:    
Total 7,981 15,969
U.S. government securities | Level 2    
Assets:    
Total $ 7,981 $ 15,969