Segment Reporting (Operating Income (Loss)) (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011
Segment Reporting Information [Line Items]
Revenues before Inter-segment Eliminations	$ 1,404,113	$ 1,377,079	$ 4,216,370	$ 3,971,255
Inter-segment Revenue Eliminations	57,276	54,228	170,935	156,569
Revenues from External Customers	1,404,113	1,377,079	4,216,370	3,971,255
Operating Income (Loss)	55,213	61,125	191,046	157,986
Freight [Member]
Segment Reporting Information [Line Items]
Revenues before Inter-segment Eliminations	858,276	843,300	2,567,847	2,450,870
Inter-segment Revenue Eliminations	11,996	13,205	37,868	36,931
Revenues from External Customers	846,280	830,095	2,529,979	2,413,939
Operating Income (Loss)	34,441	40,721	122,372	100,220
Logistics [Member]
Segment Reporting Information [Line Items]
Revenues before Inter-segment Eliminations	427,790	417,142	1,294,965	1,181,129
Inter-segment Revenue Eliminations	12,625	9,411	34,792	24,216
Revenues from External Customers	415,165	407,731	1,260,173	1,156,913
Operating Income (Loss)	10,990	12,679	35,972	33,420
Truckload [Member]
Segment Reporting Information [Line Items]
Revenues before Inter-segment Eliminations	160,094	158,705	480,337	459,371
Inter-segment Revenue Eliminations	18,502	20,921	57,273	63,291
Revenues from External Customers	141,592	137,784	423,064	396,080
Operating Income (Loss)	11,273	7,867	36,442	25,273
Other [Member]
Segment Reporting Information [Line Items]
Revenues before Inter-segment Eliminations	15,229	12,160	44,156	36,454
Inter-segment Revenue Eliminations	14,153	10,691	41,002	32,131
Revenues from External Customers	1,076	1,469	3,154	4,323
Operating Income (Loss)	(1,491)	(142)	(3,740)	(927)
Inter-Segment Revenue Eliminations [Member]
Segment Reporting Information [Line Items]
Revenues before Inter-segment Eliminations	$ (57,276)	$ (54,228)	$ (170,935)	$ (156,569)