Segment Reporting (Operating Income (Loss)) (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Segment Reporting Information [Line Items]
Revenues from external customers									$ 5,473,356	$ 5,580,247	$ 5,289,953
Revenues from internal customers									248,232	231,769	208,847
Operating Income (Loss)	33,380	67,675	76,299	31,599	37,795	55,213	80,143	55,690	208,953	228,841	207,928
Depreciation and Amortization, net of Accretion									229,236	215,202	201,638
Capital Expenditures									281,943	293,135	288,994
Assets	3,279,931				3,152,415				3,279,931	3,152,415	3,100,016
Freight
Segment Reporting Information [Line Items]
Revenues from external customers									3,424,002	3,339,605	3,196,564
Revenues from internal customers									42,098	52,991	50,543
Operating Income (Loss)									146,047	143,869	119,779
Depreciation and Amortization, net of Accretion									135,311	124,372	109,875
Capital Expenditures									180,576	190,218	146,001
Assets	1,529,681				1,459,576				1,529,681	1,459,576	1,368,249
Logistics
Segment Reporting Information [Line Items]
Revenues from external customers									1,474,507	1,677,279	1,555,422
Revenues from internal customers									65,892	48,921	34,572
Operating Income (Loss)									23,467	44,616	54,747
Depreciation and Amortization, net of Accretion									7,571	7,532	8,783
Capital Expenditures									24,587	7,186	10,055
Assets	318,266				302,295				318,266	302,295	313,253
Truckload
Segment Reporting Information [Line Items]
Revenues from external customers									567,255	558,714	532,165
Revenues from internal customers									69,555	76,842	82,849
Operating Income (Loss)									38,691	44,921	34,813
Depreciation and Amortization, net of Accretion									74,449	69,799	70,003
Capital Expenditures									74,637	93,117	131,413
Assets	799,775				807,470				799,775	807,470	791,864
Corporate and Eliminations
Segment Reporting Information [Line Items]
Revenues from external customers									7,592	4,649	5,802
Revenues from internal customers									70,687	53,015	40,883
Operating Income (Loss)									748	(4,565)	(1,411)
Depreciation and Amortization, net of Accretion									11,905	13,499	12,977
Capital Expenditures									2,143	2,614	1,525
Assets	$ 632,209				$ 583,074				$ 632,209	$ 583,074	$ 626,650