Valuation and qualifying accounts (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Allowance for doubtful accounts
Valuation and qualifying accounts
Balance at the beginning of the year	$ 1,343	$ 1,667	$ 1,051
Additions charged to earnings	208	305	577
Changes to net	(393)	(629)	39
Balance at the end of the year	1,158	1,343	1,667
Reserves for inventory
Valuation and qualifying accounts
Balance at the beginning of the year	2,604	2,452	1,599
Additions charged to earnings	2,206	2,251	1,752
Changes to net	(1,677)	(2,099)	(899)
Balance at the end of the year	3,133	2,604	2,452
Valuation of deferred tax assets
Valuation and qualifying accounts
Balance at the beginning of the year	647	1,600	1,395
Changes to net	(7)	(953)	205
Balance at the end of the year	$ 640	$ 647	$ 1,600