Segment and Geographic Information, Segment and Geographic Info (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenue from external customers	$ 1,362,265	$ 1,383,210	$ 1,375,964
Long-lived assets	990,865	950,799	974,827
Reportable Geographical Components [Member] | North America [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenue from external customers	718,708	745,660	744,481
Long-lived assets	546,661	548,674	541,654
Reportable Geographical Components [Member] | Europe [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenue from external customers	324,597	342,818	349,100
Long-lived assets	405,238	361,029	397,244
Reportable Geographical Components [Member] | Asia Pacific [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenue from external customers	208,935	196,884	177,559
Long-lived assets	31,390	30,006	26,589
Reportable Geographical Components [Member] | Other [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenue from external customers	110,025	97,848	104,824
Long-lived assets	7,576	11,090	9,340
Reportable Geographical Components [Member] | United States [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenue from external customers	574,500	595,000	592,900
Long-lived assets	$ 491,800	$ 495,300	$ 486,100