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Guarantees (Details) - USD ($)
3 Months Ended 6 Months Ended
Jun. 30, 2016
Jun. 30, 2015
Jun. 30, 2016
Jun. 30, 2015
Dec. 31, 2015
Guarantor Obligations [Line Items]          
Loss contingency accrual payments $ 2,500,000 $ 1,100,000 $ 5,500,000 $ 2,100,000  
Loss contingency accrual 0   $ 0   $ 1,300,000
Guarantee for portion of value of leased tractors          
Guarantor Obligations [Line Items]          
Guarantees expiration year     2020    
Guarantor obligation maximum exposure 14,600,000   $ 14,600,000    
Guarantor obligation $ 3,800,000   $ 3,800,000   $ 4,700,000