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Segment Reporting (Revenue by Geographical Areas) (Details) - USD ($)
$ in Thousands
 3 Months Ended
Mar. 31, 2017
Mar. 31, 2016
Revenues from External Customers [Line Items]    
Revenues $ 626,528 $ 582,040
Revenue as Percentage of Consolidated Revenue 100.00% 100.00%
Domestic [Member]    
Revenues from External Customers [Line Items]    
Revenues $ 596,301 $ 554,438
Revenue as Percentage of Consolidated Revenue 95.20% 95.30%
Foreign [Member]    
Revenues from External Customers [Line Items]    
Revenues $ 30,227 $ 27,602
Revenue as Percentage of Consolidated Revenue 4.80% 4.70%