Significant Accounting Policies Significant Accounting Policies (Disaggregated Revenue) (Details) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Disaggregated Revenue [Line Items]
Revenue from operations	$ 872,987,000	$ 782,611,000	$ 2,466,995,000	$ 2,245,779,000
LTL Service Revenue [Member]
Disaggregated Revenue [Line Items]
Revenue from operations	859,832,000	769,854,000	2,426,419,000	2,206,642,000
Other Service Revenue [Member]
Disaggregated Revenue [Line Items]
Revenue from operations	$ 13,155,000	$ 12,757,000	$ 40,576,000	$ 39,137,000