Income Taxes (Details 3) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Deferred income tax assets:
Self-insurance accruals	$ 52,177	$ 37,296
Net operating loss carryforwards	258,808	275,124
Alternative minimum taxes	9,679	9,679
Accrued compensation and benefits	61,095	67,323
Federal benefit on state tax positions	17,925	18,847
Pension benefits	204,069	179,159
Miscellaneous other accruals	39,708	38,588
Deferred tax assets gross	643,461	626,016
Valuation allowance	(38,182)	(41,324)
Deferred tax assets net	605,279	584,692
Deferred income tax liabilities:
Property and equipment bases difference	(1,734,508)	(1,649,494)
Other items	(18,716)	(25,265)
Deferred tax liabilities	(1,753,224)	(1,674,759)
Net deferred income tax liability	$ (1,147,945)	$ (1,090,067)