Acquisition Additional Acquisition (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Apr. 24, 2017	Dec. 31, 2016	Dec. 31, 2015
Business Acquisition [Line Items]
Total Purchase Price		$ 20,000
Goodwill	$ 359,688	$ 8,007	$ 345,269	$ 342,111
Acquired Finite-lived Intangible Assets, Weighted Average Useful Life	7 years