Goodwill and Acquired Intangible Assets - Changes In Carrying Amount Of Goodwill (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Beginning of year	$ 64,649	$ 25,211	$ 25,309
Changes from business combinations	82,929	39,438	0
Other	0	0	(98)
Total Goodwill	$ 147,578	$ 64,649	$ 25,211