Goodwill and Acquired Intangible Assets - Changes In Carrying Amount Of Goodwill (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Goodwill [Roll Forward]
Beginning of year	$ 147,578	$ 64,649	$ 25,211
Changes from business combinations	57,175	82,929	39,438
Total	$ 204,753	$ 147,578	$ 64,649