Fair value measurements (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Recurring
Fair value measurements
Cash and cash equivalents	$ 4,996	$ 3,250
Lease receivable	481	314
Short-term investments	289	238
Assets	5,766	3,802
Recurring | Level 1
Fair value measurements
Cash and cash equivalents	4,996	3,250
Lease receivable	481	314
Short-term investments	289	238
Assets	5,766	3,802
Non-recurring
Fair value Assumptions
Fair value measurements	$ 0	$ 0