Segment Reporting (Revenue by Geographical Areas) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Revenues from External Customers [Line Items]
Revenues	$ 667,048	$ 629,401	$ 1,946,889	$ 1,819,529
Revenue as Percentage of Consolidated Revenue	100.00%	100.00%	100.00%	100.00%
Domestic [Member]
Revenues from External Customers [Line Items]
Revenues	$ 632,584	$ 599,991	$ 1,849,567	$ 1,732,594
Revenue as Percentage of Consolidated Revenue	94.80%	95.30%	95.00%	95.20%
Foreign [Member]
Revenues from External Customers [Line Items]
Revenues	$ 34,464	$ 29,410	$ 97,322	$ 86,935
Revenue as Percentage of Consolidated Revenue	5.20%	4.70%	5.00%	4.80%