Valuation and qualifying accounts (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Allowance for doubtful accounts
Valuation and qualifying accounts
Balance at the beginning of the year	$ 871	$ 1,056	$ 1,158
Additions charged to earnings	1,361	531	1,475
Changes to net	(745)	(716)	(1,577)
Balance at the end of the year	1,487	871	1,056
Valuation of deferred tax assets
Valuation and qualifying accounts
Balance at the beginning of the year	1,473	777	640
Changes to net	139	696	137
Balance at the end of the year	$ 1,612	$ 1,473	$ 777