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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%

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