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Financing Agreement - Debt Instruments (Details) - USD ($)
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Debt Instrument [Line Items]      
Long-term debt $ 883,000,000 $ 1,112,142,000 $ 425,707,000
Interest rate at December 31 2.91% 2.25% 2.33%
Debt issuance costs $ (2,261,000) $ (3,156,000)  
Total debt, capital lease obligations, and notes payable 913,012,000 1,141,910,000  
Less: current maturities 47,746,000 3,847,000  
Total long-term debt, capital lease obligations, and notes payable $ 865,266,000 1,138,063,000  
Revolving loan facility      
Debt Instrument [Line Items]      
Average interest rate 2.56%    
Long-term debt $ 3,000,000 172,142,000  
Senior Notes | Senior Unsecured Notes, 3.81 Percent, Due May 2018      
Debt Instrument [Line Items]      
Long-term debt $ 25,000,000 25,000,000  
Interest rate, stated percentage 3.81%    
Senior Notes | Senior Unsecured Notes, 4.60 Percent, Due May 2021      
Debt Instrument [Line Items]      
Long-term debt $ 75,000,000 75,000,000  
Interest rate, stated percentage 4.60%    
Senior Notes | Senior Unsecured Notes 3.13 Percent Due December 2020      
Debt Instrument [Line Items]      
Long-term debt $ 100,000,000 100,000,000  
Interest rate, stated percentage 3.13%    
Capital lease obligations      
Debt Instrument [Line Items]      
Interest rate at December 31 5.20%    
Capital lease obligations $ 19,889,000 19,306,000  
Notes payable and other      
Debt Instrument [Line Items]      
Interest rate, stated percentage 3.40%    
Notes payable and other | Notes Payable 3.50 Percent Due June 2027      
Debt Instrument [Line Items]      
Long-term debt $ 12,384,000 13,618,000  
Long-term Debt      
Debt Instrument [Line Items]      
Interest rate at period end 2.66%    
Long-term line of credit $ 680,000,000 $ 740,000,000