Segment Information (Revenues by geographic region) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Segment Reporting Information [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	$ 761,385	$ 710,545	$ 1,502,208	$ 1,430,734
United States
Segment Reporting Information [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	600,195	548,555	1,186,763	1,148,610
Europe
Segment Reporting Information [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	116,422	108,076	202,686	186,750
Other Geographic Locations
Segment Reporting Information [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	$ 44,768	$ 53,914	$ 112,759	$ 95,374