Annual report pursuant to Section 13 and 15(d)

Significant Accounting Policies (Narrative) (Details)

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Significant Accounting Policies (Narrative) (Details) - USD ($)
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Significant Accounting Policies [Line Items]      
Allocated Share-based Compensation Expense $ 22.7 $ 16.2 $ (2.5)
Depreciation expenses including capital leases 205,600,000 189,600,000 164,800,000
Self-insurance reserve 127,600,000 125,800,000  
Long-term portions of self insurance reserve 77,700,000 78,400,000  
Advertising expense 27,300,000 20,500,000 22,900,000
Debt and Capital Lease Obligations 95,000,000 104,975,000  
Long-term Debt, Fair Value 97,100,000 108,300,000  
Employee Service Share-based Compensation, Tax Benefit from Compensation Expense (9.0) $ (6.3) $ (1.0)
Group Health Claims [Member]      
Significant Accounting Policies [Line Items]      
Insurance maximum, per occurrence 1,000,000    
Workers Compensation Claims [Member]      
Significant Accounting Policies [Line Items]      
Insurance maximum, per occurrence 1,000,000    
Cargo Loss And Damage Claims [Member]      
Significant Accounting Policies [Line Items]      
Insurance maximum, per occurrence 100,000    
Bodily Injury And Property Damage [Member]      
Significant Accounting Policies [Line Items]      
Insurance maximum, per occurrence 2,750,000    
Corridor Deductible 2,500,000    
Insurance Layer to Meet Corridor Deductible 5,000,000    
2016 Share Repurchase Program [Member]      
Significant Accounting Policies [Line Items]      
Stock Repurchase Program, Authorized Amount 250,000,000    
Stock Repurchase Program, Remaining Authorized Repurchase Amount 192,000,000    
2014 Share Repurchase Program [Member]      
Significant Accounting Policies [Line Items]      
Stock Repurchase Program, Authorized Amount $ 200,000,000