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