Segment Information (Schedule of Revenue and Other Income) (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Segment Reporting [Abstract]
Total Segment Revenue and Freight from External Customers									$ 1,472,790	$ 1,321,412	$ 1,143,514
Other Income not Allocated to Segments (Note 4)	$ 11,426	$ 10,978	$ 10,369	$ 25,887	$ 20,771	$ 19,713	$ 10,145	$ 22,650	58,660	73,279	82,120
Gain on Sale of Assets	292	(85)	104	254	4,188	(513)	5,582	7,955	565	17,212	5,228
Total Revenue and Other Income	$ 382,236	$ 324,248	$ 415,273	$ 410,258	$ 352,318	$ 335,313	$ 352,051	$ 372,221	$ 1,532,015	$ 1,411,903	$ 1,230,862