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Segment Reporting (Revenue by Geographical Areas) (Details) - USD ($)
$ in Thousands
 3 Months Ended
Mar. 31, 2018
Mar. 31, 2017
Revenues from External Customers [Line Items]    
Revenues $ 685,173 $ 626,528
Revenue as Percentage of Consolidated Revenue 100.00% 100.00%
Domestic [Member]    
Revenues from External Customers [Line Items]    
Revenues $ 647,267 $ 596,301
Revenue as Percentage of Consolidated Revenue 94.50% 95.20%
Foreign [Member]    
Revenues from External Customers [Line Items]    
Revenues $ 37,906 $ 30,227
Revenue as Percentage of Consolidated Revenue 5.50% 4.80%