Changes in Operations Freightliner (Details) £ in Thousands		3 Months Ended									9 Months Ended	12 Months Ended
	May. 31, 2014	Dec. 31, 2015 USD ($) rail_car employee locomotive	Sep. 30, 2015 USD ($)	Jun. 30, 2015 USD ($)	Mar. 31, 2015 GBP (£) $ / £	Mar. 31, 2015 USD ($) $ / £	Dec. 31, 2014 USD ($) rail_car locomotive $ / £	Sep. 30, 2014 USD ($) $ / £	Jun. 30, 2014 USD ($) $ / £	Mar. 31, 2014 USD ($) $ / £	Dec. 31, 2015 USD ($) rail_car employee locomotive	Dec. 31, 2015 USD ($) rail_car employee locomotive	Dec. 31, 2014 USD ($) rail_car locomotive $ / £	Dec. 31, 2013 USD ($)	Mar. 25, 2015 GBP (£) rail_car employee locomotive $ / £	Mar. 25, 2015 USD ($) rail_car employee locomotive $ / £
Business Acquisition, Name of Acquired Entity	Rapid City, Pierre & Eastern Railroad, Inc. (RCP&E)
Number of employees | employee		7,500									7,500	7,500
Leased locomotives | locomotive		333					162				333	333	162
Leased railcars | rail_car		21,819					18,583				21,819	21,819	18,583
Foreign Currency Exchange Rate, Translation | $ / £					1.51	1.51	1.58	1.67	1.68	1.66			1.58		1.49	1.49
Revenues		$ 514,853,000	$ 546,299,000	$ 542,219,000		$ 397,030,000	$ 415,627,000	$ 432,543,000	$ 414,563,000	$ 376,279,000
Income from operations		$ 94,631,000	$ 117,559,000	$ 99,451,000		$ 72,620,000	$ 113,471,000	$ 123,116,000	$ 110,109,000	$ 74,875,000		$ 384,261,000	$ 421,571,000	$ 380,188,000
Loss on settlement of foreign currency forward purchase contracts												(18,686,000)	$ 0	$ 0
Freightliner [Member]
Business Acquisition, Percentage of Voting Interests Acquired															100.00%	100.00%
Business Acquisition, Effective Date of Acquisition					Mar. 25, 2015	Mar. 25, 2015
Business Acquisition, Name of Acquired Entity					Freightliner Group Limited (Freightliner)	Freightliner Group Limited (Freightliner)
Percentage of economic interest retained by acquiree's management					6.00%	6.00%
Number of employees | employee															2,500	2,500
Leased locomotives | locomotive															250	250
Electric locomotives | locomotive															45	45
Leased railcars | rail_car															5,500	5,500
Cash consideration					£ 492,083	$ 733,006,000
Deferred consideration					24,200	36,048,000
Total consideration					£ 516,283	$ 769,054,000
Business Combination, Contingent Consideration, Liability, Noncurrent															£ 24,200	$ 36,000,000
Change in estimated fair value of deferred consideration												0
Change in contingent consideration liability												0
Revenues											$ 531,300,000
Income from operations											$ 33,400,000
Business Combination, Acquisition Related Costs												(12,600,000)
Business Combination, Integration Related Costs												(2,600,000)
Loss on settlement of foreign currency forward purchase contracts												$ (18,700,000)