Segment Information (Revenues by geographic region) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Segment Reporting Information [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	$ 696,875	$ 648,023	$ 2,199,083	$ 2,078,757
United States
Segment Reporting Information [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	550,573	504,732	1,737,336	1,653,342
Europe
Segment Reporting Information [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	108,607	91,447	311,294	278,198
Other Geographic Locations
Segment Reporting Information [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	$ 37,695	$ 51,844	$ 150,453	$ 147,217