Segment Reporting (Operating Income (Loss)) (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011
Segment Reporting Information [Line Items]
Revenues before Inter-segment Eliminations	$ 1,446,096	$ 1,348,549	$ 2,812,257	$ 2,594,176
Inter-segment Revenue Eliminations	58,013	53,040	113,659	102,341
Revenues from External Customers	1,446,096	1,348,549	2,812,257	2,594,176
Operating Income (Loss)	80,143	60,165	135,833	96,861
Freight [Member]
Segment Reporting Information [Line Items]
Revenues before Inter-segment Eliminations	878,524	839,829	1,709,571	1,607,570
Inter-segment Revenue Eliminations	13,466	12,435	25,872	23,726
Revenues from External Customers	865,058	827,394	1,683,699	1,583,844
Operating Income (Loss)	53,429	39,155	87,931	59,499
Logistics [Member]
Segment Reporting Information [Line Items]
Revenues before Inter-segment Eliminations	448,029	394,012	867,175	763,987
Inter-segment Revenue Eliminations	11,822	8,644	22,167	14,805
Revenues from External Customers	436,207	385,368	845,008	749,182
Operating Income (Loss)	12,688	12,095	24,982	20,741
Truckload [Member]
Segment Reporting Information [Line Items]
Revenues before Inter-segment Eliminations	162,920	155,451	320,243	300,666
Inter-segment Revenue Eliminations	19,205	21,524	38,771	42,370
Revenues from External Customers	143,715	133,927	281,472	258,296
Operating Income (Loss)	14,619	10,323	25,169	17,406
Other [Member]
Segment Reporting Information [Line Items]
Revenues before Inter-segment Eliminations	14,636	12,297	28,927	24,294
Inter-segment Revenue Eliminations	13,520	10,437	26,849	21,440
Revenues from External Customers	1,116	1,860	2,078	2,854
Operating Income (Loss)	(593)	(1,408)	(2,249)	(785)
Inter-Segment Revenue Eliminations [Member]
Segment Reporting Information [Line Items]
Revenues before Inter-segment Eliminations	$ (58,013)	$ (53,040)	$ (113,659)	$ (102,341)