Goodwill and Other Intangible Assets, Net Goodwill and Other Intangible Assets, Net - Goodwill Rollforward (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Goodwill [Roll Forward]
Goodwill	$ 1,000,123	$ 941,389	$ 675,484
Acquisitions	87,865	67,480	265,905
Foreign exchange	32,920	(8,746)
Goodwill	1,156,288	$ 1,000,123	$ 941,389
David Michael & Company, Inc. [Member]
Goodwill [Roll Forward]
Goodwill	$ 35,380