Significant Accounting Policies Significant Accounting Policies (Disaggregated Revenue) (Details) - USD ($)	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Disaggregated Revenue [Line Items]
Revenue from operations	$ 925,020,000	$ 754,096,000
LTL Service Revenue [Member]
Disaggregated Revenue [Line Items]
Revenue from operations	911,054,000	740,186,000
Other Service Revenue [Member]
Disaggregated Revenue [Line Items]
Revenue from operations	$ 13,966,000	$ 13,910,000