Income Taxes (Details 3) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Deferred income tax assets:
Self-insurance accruals	$ 81,908	$ 69,291
Net operating loss carryforwards	377,740	322,380
Alternative minimum taxes	10,727	10,727
Accrued compensation and benefits	68,626	60,039
Federal benefit on state tax positions	18,847	18,417
Pension benefits	157,082	87,745
Miscellaneous other accruals	33,090	39,414
Deferred tax assets gross	748,020	608,013
Valuation allowance	(24,742)	(33,793)
Deferred tax assets net	723,278	574,220
Deferred income tax liabilities:
Property and equipment bases difference	(2,149,699)	(1,943,923)
Other items	(16,996)	(22,503)
Deferred tax liabilities	(2,166,695)	(1,966,426)
Net deferred income tax liability	$ (1,443,417)	$ (1,392,206)