Fair value measurements (Details) - Recurring - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Fair value measurements
Assets measured at fair value	$ 179,402	$ 89,870
Cash and cash equivalents
Fair value measurements
Assets measured at fair value	179,085	89,553
Restricted cash
Fair value measurements
Assets measured at fair value	317	317
Level 1
Fair value measurements
Assets measured at fair value	179,402	89,870
Level 1 | Cash and cash equivalents
Fair value measurements
Assets measured at fair value	179,085	89,553
Level 1 | Restricted cash
Fair value measurements
Assets measured at fair value	$ 317	$ 317