Summary of significant accounting policies, Revenue recognition - Contract balances (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Summary of significant accounting policies [Abstract]
Revenue recognized	$ 384
Contract liabilities [Abstract]
Customer pre-payments	232	$ 50
Deferred revenue, current	700	384
Deferred revenue, non-current	219	265
Total contract liabilities	$ 1,151	$ 699