Summary of significant accounting policies, Allowance for doubtful accounts (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Allowance for doubtful accounts [Roll Forward]
Balance, beginning of period	$ 221	$ 205
Additions charged to costs and expenses	1	39
Deductions	(2)	(23)
Balance, end of period	$ 220	$ 221