Income Taxes (Schedule Of Deferred Tax Assets And Liabilities) (Details) - USD ($)	Dec. 31, 2015	Dec. 31, 2014
Income Tax Disclosure [Abstract]
Claims and insurance reserves	$ 41,576,000	$ 36,690,000
Allowance for doubtful accounts	1,730,000	2,161,000
Accrued vacation	22,174,000	19,291,000
Deferred compensation	33,382,000	28,648,000
Other	12,008,000	12,924,000
Total deferred tax assets	110,870,000	99,714,000
Valuation allowance	0	(559,000)
Net deferred tax assets	110,870,000	99,155,000
Depreciation	(334,379,000)	(279,324,000)
Unrecognized revenue	(10,062,000)	(9,506,000)
Other	(1,483,000)	(1,737,000)
Deferred Tax Liabilities, Gross	345,924,000	290,567,000
Total deferred tax liabilities	$ (235,054,000)	$ (191,412,000)