Segment Information Segments (Details) $ in Thousands	3 Months Ended
	Mar. 31, 2016 USD ($) region reportable_segment	Mar. 31, 2015 USD ($)
Segment Reporting Information [Line Items]
Number of operating regions | region	11
Number of reportable segments | reportable_segment	3
Freight revenues	$ 326,414	$ 298,209
Freight-related revenues	133,458	78,125
All other revenues	22,744	20,696
Operating revenues	482,616	397,030
Income/(loss) from operations	56,996	72,620
Depreciation and amortization	49,330	42,217
Interest expense, net	17,900	13,482
Provision for/(benefit from) income taxes	12,808	16,862
Loss on settlement of foreign currency forward purchase contracts	0	18,686
Expenditures for additions to property & equipment, net of grants from outside parties	39,764	49,898
North American Operations [Member]
Segment Reporting Information [Line Items]
Freight revenues	221,825	243,030
Freight-related revenues	61,525	58,061
All other revenues	16,428	16,533
Operating revenues	299,778	317,624
Income/(loss) from operations	69,978	57,081
Depreciation and amortization	36,189	35,305
Interest expense, net	10,465	10,773
Provision for/(benefit from) income taxes	16,009	14,157
Loss on settlement of foreign currency forward purchase contracts		16,374
Expenditures for additions to property & equipment, net of grants from outside parties	29,177	45,270
Australian Operations [Member]
Segment Reporting Information [Line Items]
Freight revenues	24,777	46,358
Freight-related revenues	25,490	10,864
All other revenues	1,531	2,720
Operating revenues	51,798	59,942
Income/(loss) from operations	(11,751)	14,236
Depreciation and amortization	6,656	6,226
Interest expense, net	2,395	2,333
Provision for/(benefit from) income taxes	(2,525)	2,928
Loss on settlement of foreign currency forward purchase contracts		2,312
Expenditures for additions to property & equipment, net of grants from outside parties	5,187	4,615
U.K./European Operations [Member]
Segment Reporting Information [Line Items]
Freight revenues	79,812	8,821
Freight-related revenues	46,443	9,200
All other revenues	4,785	1,443
Operating revenues	131,040	19,464
Income/(loss) from operations	(1,231)	1,303
Depreciation and amortization	6,485	686
Interest expense, net	5,040	376
Provision for/(benefit from) income taxes	(676)	(223)
Loss on settlement of foreign currency forward purchase contracts		0
Expenditures for additions to property & equipment, net of grants from outside parties	$ 5,400	$ 13