Summary of significant accounting policies, Allowance for doubtful accounts (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Allowance for doubtful accounts [Roll Forward]
Balance, beginning of period	$ 205	$ 100	$ 50
Additions charged to costs and expenses	39	105	50
Deductions	(23)	0	0
Balance, end of period	$ 221	$ 205	$ 100