Valuation and qualifying accounts (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Allowance for doubtful accounts
Valuation and qualifying accounts
Balance at the beginning of the year	$ 1,056	$ 1,158	$ 1,343
Additions charged to earnings	531	1,475	208
Changes to net	(716)	(1,577)	(393)
Balance at the end of the year	871	1,056	1,158
Valuation of deferred tax assets
Valuation and qualifying accounts
Balance at the beginning of the year	777	640	647
Changes to net	696	137	(7)
Balance at the end of the year	$ 1,473	$ 777	$ 640