Changes in Allowance for Doubtful Accounts (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Allowance For Doubtful Accounts Receivable Rollforward
Balance at beginning of year	$ 554	$ 565	$ 500
Provision charged to selling, general and administrative expenses	66	283	135
Allowance resulting from acquisitions		52	15
Write-offs, net of recoveries of amounts previously reserved	(295)	(358)	(82)
Exchange rate changes	(4)	12	(3)
Balance at end of year	$ 321	$ 554	$ 565