Segment Reporting (Operating Income (Loss)) (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Segment Reporting Information [Line Items]
Revenues from external customers									$ 5,806,069	$ 5,473,356	$ 5,580,247
Revenues from internal customers									253,455	248,232	231,769
Operating Income	41,313	91,375	102,700	33,062	33,380	67,675	76,299	31,599	268,450	208,953	228,841
Depreciation and Amortization, net of Accretion									242,507	229,236	215,202
Capital Expenditures									289,776	281,943	293,135
Assets	3,335,618				3,279,931				3,335,618	3,279,931	3,152,415
Freight
Segment Reporting Information [Line Items]
Revenues from external customers									3,586,333	3,424,002	3,339,605
Revenues from internal customers									45,732	42,098	52,991
Operating Income									210,324	146,047	143,869
Depreciation and Amortization, net of Accretion									150,528	135,311	124,372
Capital Expenditures									176,933	180,576	190,218
Assets	1,560,324				1,529,681				1,560,324	1,529,681	1,459,576
Logistics
Segment Reporting Information [Line Items]
Revenues from external customers									1,638,967	1,474,507	1,677,279
Revenues from internal customers									78,744	65,892	48,921
Operating Income									27,193	23,467	44,616
Depreciation and Amortization, net of Accretion									11,225	7,571	7,532
Capital Expenditures									15,577	24,587	7,186
Assets	367,081				318,266				367,081	318,266	302,295
Truckload
Segment Reporting Information [Line Items]
Revenues from external customers									572,990	567,255	558,714
Revenues from internal customers									58,521	69,555	76,842
Operating Income									41,245	38,691	44,921
Depreciation and Amortization, net of Accretion									68,382	74,449	69,799
Capital Expenditures									91,731	74,637	93,117
Assets	792,088				799,775				792,088	799,775	807,470
Corporate and Eliminations
Segment Reporting Information [Line Items]
Revenues from external customers									7,779	7,592	4,649
Revenues from internal customers									70,458	70,687	53,015
Operating Income									(10,312)	748	(4,565)
Depreciation and Amortization, net of Accretion									12,372	11,905	13,499
Capital Expenditures									5,535	2,143	2,614
Assets	$ 616,125				$ 632,209				$ 616,125	$ 632,209	$ 583,074