Goodwill and Acquired Intangible Assets, Net (Details) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2018	Dec. 31, 2017
Goodwill and Finite-lived Intangible Assets Rollforward [Roll Forward]
Finite-Lived Intangible Assets, Net	$ 128,586	$ 150,543
Finite-Lived Intangible Assets, Amortization Expense	(17,179)
Finite-lived intangible assets, other changes	(4,778)
Goodwill	715,745	717,386
Goodwill, Other Increase (Decrease)	(22,383)
Intangible Assets, Net (Including Goodwill)	844,331	867,929
Total intangible assets amortization expense	(17,179)
Total intangible assets, other changes	(27,161)
Finite-Lived Intangible Assets, Amortization Expense, Remainder of Fiscal Year	5,400
Finite-Lived Intangible Assets, Amortization Expense, Next Twelve Months	21,200
Finite-Lived Intangible Assets, Amortization Expense, Year Two	20,400
Finite-Lived Intangible Assets, Amortization Expense, Year Three	19,500
Finite-Lived Intangible Assets, Amortization Expense, Year Four	18,500
Finite-Lived Intangible Assets, Amortization Expense, Year Five	13,700
Finite-lived Intangible Assets Acquired	0
Goodwill, Acquired During Period	20,742
Total intangible assets acquired during period	$ 20,742
Goodwill, Impairment Loss		$ 31,770