Cash and cash equivalents (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017	Jun. 30, 2017	Dec. 31, 2016
Cash, Cash Equivalents, and Short-term Investments [Abstract]
Cash and cash equivalents	$ 61,233	$ 136,986	$ 81,960	$ 82,387