Cash, cash equivalents and restricted cash (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017	Dec. 31, 2016
Cash and Cash Equivalents [Abstract]
Cash and cash equivalents	$ 19,688	$ 14,680
Restricted cash	10	10
Total cash, cash equivalents and restricted cash	$ 19,698	$ 14,690	$ 20,278	$ 25,352