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 ($)
3 Months Ended 6 Months Ended
Jun. 30, 2017
Jun. 30, 2016
Jun. 30, 2017
Jun. 30, 2016
Disaggregated Revenue [Line Items]        
Revenue from operations $ 839,912,000 $ 755,435,000 $ 1,594,008,000 $ 1,463,168,000
LTL Service Revenue [Member]        
Disaggregated Revenue [Line Items]        
Revenue from operations 826,401,000 742,255,000 1,566,587,000 1,436,787,000
Other Service Revenue [Member]        
Disaggregated Revenue [Line Items]        
Revenue from operations $ 13,511,000 $ 13,180,000 $ 27,421,000 $ 26,381,000