Income Taxes (Schedule Of Deferred Tax Assets And Liabilities) (Details) (USD $)	Dec. 31, 2014	Dec. 31, 2013
Income Tax Disclosure [Abstract]
Claims and insurance reserves	$ 36,690,000	$ 35,200,000
Allowance for doubtful accounts	2,161,000	1,595,000
Accrued vacation	19,291,000	14,873,000
Deferred compensation	28,648,000	22,067,000
Other	12,924,000us-gaap_DeferredTaxAssetsOther	10,648,000us-gaap_DeferredTaxAssetsOther
Total deferred tax assets	99,714,000us-gaap_DeferredTaxAssetsGross	84,383,000us-gaap_DeferredTaxAssetsGross
Valuation allowance	(559,000)	(460,000)
Net deferred tax assets	99,155,000us-gaap_DeferredTaxAssetsNet	83,923,000us-gaap_DeferredTaxAssetsNet
Depreciation	(279,324,000)	(239,971,000)
Unrecognized revenue	(9,506,000)	(8,169,000)
Other	(1,737,000)us-gaap_DeferredTaxLiabilitiesOther	(1,651,000)us-gaap_DeferredTaxLiabilitiesOther
Total deferred tax liabilities	(290,567,000)us-gaap_DeferredTaxLiabilities	(249,791,000)us-gaap_DeferredTaxLiabilities
Net deferred tax liability	$ (191,412,000)us-gaap_DeferredTaxAssetsLiabilitiesNet	$ (165,868,000)us-gaap_DeferredTaxAssetsLiabilitiesNet

Amount before allocation of valuation allowances of deferred tax asset attributable to deductible temporary differences and carryforwards.

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Amount before allocation of valuation allowances of deferred tax asset attributable to deductible temporary differences from compensation and benefits costs.

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Amount before allocation of valuation allowances of deferred tax asset attributable to deductible temporary differences from compensated absences (includes, but not limited to, sick and personal days).

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Amount before allocation of valuation allowances of deferred tax asset attributable to deductible temporary differences from the allowance for doubtful accounts.

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Amount before allocation of valuation allowances of deferred tax asset attributable to deductible temporary differences from estimated losses under self insurance.

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Amount of deferred tax assets for which it is more likely than not that a tax benefit will not be realized.

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Amount of deferred tax liability attributable to taxable temporary differences from property, plant, and equipment.

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