Segment Information Revenues By Region (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Revenues from External Customers [Line Items]
Total revenues	$ 550,776	$ 486,747	$ 503,082	$ 477,592	$ 495,713	$ 506,859	$ 513,024	$ 497,590	$ 2,018,197	$ 2,013,186	$ 1,883,533
United States and Canada [Member]
Revenues from External Customers [Line Items]
Total revenues									1,137,508	1,126,787	997,293
EMEA [Member]
Revenues from External Customers [Line Items]
Total revenues									672,942	698,106	701,301
Other Countries [Member]
Revenues from External Customers [Line Items]
Total revenues									$ 207,747	$ 188,293	$ 184,939