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Lease Obligations - Rent Expense and Future Minimum Rentals (Details) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Lease Obligations      
Net rental expense $ 144,000 $ 152,000 $ 169,000
Obligations for minimum rentals under non-cancelable operating leases      
2018 86,300    
2019 70,900    
2020 55,800    
2021 35,500    
2022 22,700    
Thereafter $ 55,500