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Fair value (Details) - Fair value measurements on a recurring basis - USD ($)
$ in Thousands
Jun. 30, 2018
Dec. 31, 2017
Dec. 31, 2016
Assets:      
Total $ 53,628 $ 67,203 $ 114,388
Level 1      
Assets:      
Total 24,601 15,703 43,930
Level 2      
Assets:      
Total 29,027 51,500 70,458
Cash and money market funds      
Assets:      
Total 24,601 15,703 43,930
Cash and money market funds | Level 1      
Assets:      
Total 24,601 15,703 43,930
Corporate debt securities (including commercial paper)      
Assets:      
Total 21,046 35,531 48,466
Corporate debt securities (including commercial paper) | Level 2      
Assets:      
Total 21,046 35,531 $ 48,466
U.S. government securities      
Assets:      
Total 7,981 15,969  
U.S. government securities | Level 2      
Assets:      
Total $ 7,981 $ 15,969