Significant Accounting Policies Significant Accounting Policies (Disaggregated Revenue) (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Disaggregated Revenue [Line Items]
Revenue from operations	$ 745,738	$ 782,611	$ 755,435	$ 707,733	$ 734,572	$ 779,474	$ 762,151	$ 696,245	$ 2,991,517	$ 2,972,442	$ 2,787,897
LTL Service Revenue [Member]
Disaggregated Revenue [Line Items]
Revenue from operations									2,939,572	2,893,683	2,706,654
Other Service Revenue [Member]
Disaggregated Revenue [Line Items]
Revenue from operations									$ 51,945	$ 78,759	$ 81,243