Fair value measurements (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Recurring
Fair value measurements
Cash and cash equivalents	$ 2,711	$ 3,250
Lease receivable	402	314
Short-term investments	233	238
Assets	3,346	3,802
Recurring | Level 1
Fair value measurements
Cash and cash equivalents	2,711	3,250
Lease receivable	402	314
Short-term investments	233	238
Assets	3,346	3,802
Non-recurring
Fair value Assumptions
Fair value measurements	$ 0	$ 0