Income Taxes (Details) - USD ($)	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Tax Cuts and Jobs Act, Measurement Period Adjustment, Income Tax Expense (Benefit)	$ (1,600,000)
Income tax provision	$ 31,013,000	$ 30,507,000	$ 41,569,000	$ 82,032,000
Effective income tax rate	30.00%	36.40%		38.90%
Unrecognized Tax Benefits, Decrease Resulting from Prior Period Tax Positions		$ 3,300,000
Tax Benefit from United States Short Line Tax Credit, Amount			31,600,000
Unrecognized Tax Benefits, Increase Resulting from Current Period Tax Positions			$ (5,500,000)
Effective income tax excluding short line tax credit and prior period portion of the reserve for uncertain tax positions, percentage			28.40%
Effective Income Tax Rate Reconciliation, at Federal Statutory Income Tax Rate, Percent	21.00%		21.00%		35.00%
Tax credits percentage of qualified maintenance expenditures to reduce federal income tax			50.00%
Tax credit limitation per mile on maintenance expenditures to reduce federal income tax			$ 3,500
March 25, 2015 through December 31, 2017
Unrecognized Tax Benefits, Increase Resulting from Current Period Tax Positions			$ (3,700,000)