Segment Information (Summary Of Segment Information) (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Segment Reporting Information [Line Items]
Revenues	$ 509,694	$ 506,504	$ 521,812	$ 498,376	$ 507,937	$ 509,587	$ 515,897	$ 469,429	$ 2,036,386	$ 2,002,850	$ 1,815,020
Operating Income	43,124	41,805	51,113	35,402	49,399	50,066	46,767	27,442	171,444	173,674	134,582
Truckload Transportation Services [Member]
Segment Reporting Information [Line Items]
Revenues									1,699,349	1,694,965	1,550,601
Operating Income									153,142	157,604	122,208
Value Added Services [Member]
Segment Reporting Information [Line Items]
Revenues									320,933	291,109	250,983
Operating Income									15,999	17,036	11,005
Other Segment [Member]
Segment Reporting Information [Line Items]
Revenues									11,782	12,367	9,847
Operating Income									1,212	(383)	(427)
Corporate Segment [Member]
Segment Reporting Information [Line Items]
Revenues									4,322	4,409	3,589
Operating Income									$ 1,091	$ (583)	$ 1,796