Income Taxes (Schedule Of Deferred Tax Assets And Liabilities) (Details) (USD $)	Dec. 31, 2012	Dec. 31, 2011
Income Tax Disclosure [Abstract]
Claims and insurance reserves	$ 32,499,000	$ 30,003,000
Allowance for doubtful accounts	2,829,000	2,827,000
Accrued vacation	12,399,000	9,339,000
Deferred compensation	17,734,000	14,441,000
Other	9,764,000	5,992,000
Total deferred tax assets	75,225,000	62,602,000
Valuation allowance	(559,000)	0
Net deferred tax assets	74,666,000	62,602,000
Depreciation	(197,875,000)	(169,309,000)
Unrecognized revenue	(8,171,000)	(6,995,000)
Other	(1,752,000)	(1,748,000)
Total deferred tax liabilities	(207,798,000)	(178,052,000)
Net deferred tax liability	$ (133,132,000)	$ (115,450,000)