Operating Segments and Related Information (Details 4) - 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
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenue	$ 494,784	$ 464,712	$ 434,124	$ 406,814	$ 474,738	$ 405,228	$ 402,729	$ 379,472	$ 1,800,434	$ 1,662,167	$ 1,557,067
United States
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenue									956,438	903,582	830,485
Europe [Member]
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenue									375,474	338,805	338,886
Asia [Member]
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenue									227,047	195,913	175,616
Mid_East/Africa [Member]
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenue									127,796	130,890	125,848
Canada/Latin_America [Member]
Segment Reporting, Revenue Reconciling Item [Line Items]
Revenue									$ 113,679	$ 92,977	$ 86,232