Segment Information (Summary Of Segment Information) (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Segment Reporting Information [Line Items]
Revenues	$ 528,783	$ 534,448	$ 534,644	$ 495,654	$ 553,186	$ 551,961	$ 542,120	$ 492,022	$ 2,093,529	$ 2,139,289	$ 2,029,183
Operating income	$ 57,261	$ 52,800	$ 52,210	$ 38,185	$ 52,627	$ 41,690	$ 42,330	$ 23,441	200,456	160,088	139,726
Truckload Transportation Services
Segment Reporting Information [Line Items]
Revenues									1,644,874	1,702,137	1,657,854
Operating income									189,850	152,992	119,597
Value Added Services
Segment Reporting Information [Line Items]
Revenues									393,174	390,645	361,384
Operating income									16,898	7,535	14,664
Other
Segment Reporting Information [Line Items]
Revenues									54,512	46,588	11,342
Operating income									(7,513)	(3,991)	3,947
Corporate
Segment Reporting Information [Line Items]
Revenues									2,297	2,803	3,081
Operating income									1,221	3,552	1,518
Subtotal
Segment Reporting Information [Line Items]
Revenues									2,094,857	2,142,173	2,033,661
Inter-segment eliminations
Segment Reporting Information [Line Items]
Revenues									$ (1,328)	$ (2,884)	$ (4,478)