Goodwill and Acquired Intangible Assets - Narrative (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Goodwill and Intangible Assets Disclosure [Abstract]
Amortization of acquired intangible assets	$ 33,311	$ 30,904	$ 26,642
Future amortization expense, year 1	36,600
Future amortization expense, year 2	33,900
Future amortization expense, year 3	28,000
Future amortization expense, year 4	22,400
Future amortization expense, year 5	$ 17,100