Segment Reporting (Operating Income (Loss)) (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 before Inter-segment Eliminations	$ 1,363,877	$ 1,404,113	$ 1,446,096	$ 1,366,161	$ 1,318,698	$ 1,377,079	$ 1,348,549	$ 1,245,627	$ 5,580,247	$ 5,289,953	$ 4,952,000
Inter-segment Revenue Eliminations									231,769	208,847	223,683
Revenues from External Customers									5,580,247	5,289,953	4,952,000
Operating Income (Loss)	37,795	55,213	80,143	55,690	49,942	61,125	60,165	36,696	228,841	207,928	78,170
Depreciation and Amortization, net of Accretion									215,202	201,638	189,517
Capital Expenditures									293,135	288,994	186,496
Assets	3,141,464				3,100,016				3,141,464	3,100,016	2,943,732
Freight [Member]
Segment Reporting Information [Line Items]
Revenues before Inter-segment Eliminations									3,392,596	3,247,107	3,075,064
Inter-segment Revenue Eliminations									52,991	50,543	49,027
Revenues from External Customers									3,339,605	3,196,564	3,026,037
Operating Income (Loss)									143,869	119,779	28,908
Depreciation and Amortization, net of Accretion									124,372	109,875	101,391
Capital Expenditures									190,218	146,001	127,415
Assets	1,459,576				1,368,249				1,459,576	1,368,249	1,331,795
Logistics [Member]
Segment Reporting Information [Line Items]
Revenues before Inter-segment Eliminations									1,726,200	1,589,994	1,477,988
Inter-segment Revenue Eliminations									48,921	34,572	18,768
Revenues from External Customers									1,677,279	1,555,422	1,459,220
Operating Income (Loss)									44,616	54,747	26,275
Depreciation and Amortization, net of Accretion									7,532	8,783	10,328
Capital Expenditures									7,186	10,055	5,490
Assets	336,291				315,410				336,291	315,410	291,903
Truckload [Member]
Segment Reporting Information [Line Items]
Revenues before Inter-segment Eliminations									635,556	615,014	569,741
Inter-segment Revenue Eliminations									76,842	82,849	108,631
Revenues from External Customers									558,714	532,165	461,110
Operating Income (Loss)									44,921	34,813	20,844
Depreciation and Amortization, net of Accretion									69,799	70,003	64,109
Capital Expenditures									93,117	131,413	51,129
Assets	807,470				791,864				807,470	791,864	720,540
Other [Member]
Segment Reporting Information [Line Items]
Revenues before Inter-segment Eliminations									57,664	46,685	52,890
Inter-segment Revenue Eliminations									53,015	40,883	47,257
Revenues from External Customers									4,649	5,802	5,633
Operating Income (Loss)									(4,565)	(1,411)	2,143
Depreciation and Amortization, net of Accretion									13,499	12,977	13,689
Capital Expenditures									2,614	1,525	2,462
Assets	538,127				624,493				538,127	624,493	599,494
Inter-Segment Revenue Eliminations [Member]
Segment Reporting Information [Line Items]
Revenues before Inter-segment Eliminations									$ (231,769)	$ (208,847)	$ (223,683)