Goodwill and Intangible Assets - Goodwill (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Goodwill [Roll Forward]
Goodwill balance at January 1	$ 238,147	$ 211,927
Effect of foreign exchange	(1,304)	2,641
Additions as the result of acquisitions	98,590	23,579
Goodwill balance at December 31	$ 335,433	$ 238,147