Quarterly report pursuant to Section 13 or 15(d)

Significant Accounting Policies Significant Accounting Policies (Disaggregated Revenue) (Details)

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Significant Accounting Policies Significant Accounting Policies (Disaggregated Revenue) (Details) - USD ($)
$ in Thousands
3 Months Ended 6 Months Ended
Jun. 30, 2018
Jun. 30, 2017
Jun. 30, 2018
Jun. 30, 2017
Disaggregation of Revenue [Line Items]        
Revenue from operations $ 1,033,498 $ 839,912 $ 1,958,518 $ 1,594,008
LTL Service Revenue [Member]        
Disaggregation of Revenue [Line Items]        
Revenue from operations 1,018,491 826,401 1,929,545 1,566,587
Other Service Revenue [Member]        
Disaggregation of Revenue [Line Items]        
Revenue from operations $ 15,007 $ 13,511 $ 28,973 $ 27,421