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 |