Income Taxes (Details 3) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Deferred income tax assets:
Self-insurance accruals	$ 75,198	$ 107,252
Net operating loss carryforwards	171,053	396,313
Alternative minimum taxes	22,552	13,901
Accrued compensation and benefits	65,532	81,454
Federal benefit on state tax positions	11,950	19,247
Pension benefits	82,547	162,141
Miscellaneous other accruals	19,608	28,313
Deferred tax assets gross	448,440	808,621
Valuation allowance	(18,667)	(16,387)
Deferred tax assets net	429,773	792,234
Deferred income tax liabilities:
Property and equipment basis difference	(1,614,963)	(2,451,151)
Other	(16,857)	(20,735)
Deferred Tax Liabilities, Gross	(1,631,820)	(2,471,886)
Net deferred income tax liability	(1,202,047)	(1,679,652)
Deferred Tax Assets Direct Financing Leases And Other Assets	$ 7,000	$ 9,000