Annual report pursuant to Section 13 and 15(d)

Operating Leases (Details)

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Operating Leases (Details)
$ in Thousands
 12 Months Ended
Dec. 31, 2017
USD ($)
Years
Dec. 31, 2016
USD ($)
Dec. 31, 2015
USD ($)
Operating Leased Assets [Line Items]      
Operating lease renewal option, minimum (in years) | Years 1    
Operating lease renewal option, maximum (in years) | Years 10    
Operating lease original term, for tractors, trucks and trailers, minimum (in years) 3 years    
Operating lease original term, for tractors, trucks and trailers, maximum (in years) 5 years    
Sublease rental income $ 1,923 $ 1,517 $ 1,611
Expected future sublease rental income 1,206    
Operating Leases, Future Minimum Payments Due [Abstract]      
2018 42,051    
2019 34,693    
2020 22,393    
2021 11,282    
2022 4,948    
Thereafter 2,281    
Total $ 117,648