Business Segments (Long-Lived Assets) (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Segment Reporting Information [Line Items]
Revenues	$ 620,884	$ 629,401	$ 608,088	$ 582,040	$ 577,517	$ 572,123	$ 485,323	$ 430,045	$ 2,440,413	$ 2,065,008	$ 1,724,741
Long-Lived Assets	56,942				53,196				56,942	53,196
Domestic [Member]
Segment Reporting Information [Line Items]
Revenues									2,324,713	1,972,888	1,643,598
Long-Lived Assets	55,801				52,228				55,801	52,228
Foreign [Member]
Segment Reporting Information [Line Items]
Revenues									115,700	92,120	$ 81,143
Long-Lived Assets	$ 1,141				$ 968				$ 1,141	$ 968