Changes in Operations U.K/Europe Freightliner (Details) £ in Thousands, $ in Thousands	3 Months Ended										12 Months Ended
	Dec. 31, 2017 USD ($)	Sep. 30, 2017 USD ($)	Jun. 30, 2017 USD ($)	Mar. 31, 2017 USD ($)	Dec. 31, 2016 USD ($) locomotive rail_car $ / AUD	Sep. 30, 2016 USD ($)	Jun. 30, 2016 USD ($)	Mar. 31, 2016 USD ($)	Mar. 31, 2015 GBP (£)	Mar. 31, 2015 USD ($)	Dec. 31, 2017 GBP (£) locomotive rail_car employee shares	Dec. 31, 2017 USD ($) shares	Dec. 31, 2016 USD ($) locomotive rail_car $ / AUD	Dec. 31, 2015 USD ($) $ / AUD	Dec. 31, 2017 USD ($) locomotive rail_car employee	May 03, 2017 $ / AUD	Dec. 31, 2015 locomotive	Dec. 31, 2015 rail_car	Dec. 31, 2015 $ / £	Mar. 25, 2015 GBP (£) locomotive rail_car employee $ / £	Mar. 25, 2015 USD ($) locomotive rail_car employee $ / £
Number of employees | employee											8,000				8,000
Leased locomotives | locomotive					309						198		309		198		333
Leased railcars | rail_car					20,738						21,669		20,738		21,669			21,819
Foreign Currency Exchange Rate, Translation					0.74								0.74	0.75		1.29			1.51	1.49	1.49
Revenues	$ 571,576	$ 576,927	$ 540,433	$ 519,108	$ 516,534	$ 501,002	$ 501,375	$ 482,616
Operating income	$ 108,179	$ 111,489	$ 101,340	$ 77,454	$ 53,571	$ 91,851	$ 87,194	$ 56,996				$ 398,462	$ 289,612	$ 384,261
Loss on settlement of foreign currency forward purchase contracts												0	0	18,686
Freightliner [Member]
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
Deferred consideration									24,200	36,048
Total consideration									£ 516,283	$ 769,054
Business Combination, Contingent Consideration, Liability, Noncurrent																				£ 24,200	$ 36,000
Business Combination, Acquisition Related Costs														12,600
Business Combination, Integration Related Costs														2,600
Loss on settlement of foreign currency forward purchase contracts														18,700
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Gain (Loss) Included in Earnings												$ (2,405)	$ (2,278)	$ 0
Class A Common Stock [Member] | Common Stock [Member]
Stock issued during period, shares | shares											238,201	238,201
Settlement of Freightliner Deferred Consideration [Member]
Payment for Contingent Consideration Liability, Operating Activities											£ 2,100	$ 2,900
Notes Payable											6,300				$ 8,600
Notes Payable, Fair Value Disclosure											£ 5,700				$ 7,700
Settlement of Freightliner Deferred Consideration [Member] | Additional Paid-in Capital [Member]
Conversion of Stock, Amount Issued												$ 17,500
Settlement of Freightliner Deferred Consideration [Member] | Class A Common Stock [Member] | Common Stock [Member]
Stock issued during period, shares | shares											238,201	238,201
Other Expense [Member] | Settlement of Freightliner Deferred Consideration [Member]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Gain (Loss) Included in Earnings												$ 8,900