Changes in Operations United States RCP&E (Details) $ in Thousands		3 Months Ended								12 Months Ended
	May. 31, 2014 USD ($)	Dec. 31, 2015 USD ($) employee	Sep. 30, 2015 USD ($)	Jun. 30, 2015 USD ($)	Mar. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Sep. 30, 2014 USD ($)	Jun. 30, 2014 USD ($)	Mar. 31, 2014 USD ($)	Dec. 31, 2015 USD ($) employee carload	Dec. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)	May. 30, 2014 USD ($) employee mile
Business Acquisition, Name of Acquired Entity	Rapid City, Pierre & Eastern Railroad, Inc. (RCP&E)
Revenues		$ 514,853	$ 546,299	$ 542,219	$ 397,030	$ 415,627	$ 432,543	$ 414,563	$ 376,279
Income/(loss) from operations		$ 94,631	$ 117,559	$ 99,451	$ 72,620	$ 113,471	$ 123,116	$ 110,109	$ 74,875	$ 384,261	$ 421,571	$ 380,188
Number of employees acquired | employee		7,500								7,500
RCP&E [Member]
Cash consideration	$ 218,600
Revenues										$ 69,900
Income/(loss) from operations										$ 17,200
Track Miles | mile													670
Annual carloads transported | carload										63,000
Number of employees acquired | employee													180
Materials and supplies													$ 3,621
Prepaid expenses and other													116
Property and equipment													217,032
Deferred income tax assets													325
Total assets													221,094
Current portion of long-term debt													1,121
Accounts payable and accrued expenses													108
Long-term debt, less current portion													1,260
Net assets													$ 218,605