Segment Information (Schedule of Revenue and Other Income) (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Segment Reporting [Abstract]
Total Segment Revenue and Freight from External Customers									$ 1,375,559	$ 1,472,790	$ 1,321,412
Other Income not Allocated to Segments (Note 4)	$ 16,675	$ 11,188	$ 12,194	$ 13,292	$ 11,426	$ 10,978	$ 10,369	$ 25,887	53,349	58,660	73,279
Gain on Sale of Assets	9	714	933	339	292	(85)	104	254	1,995	565	17,212
Total Revenue and Other Income	$ 342,635	$ 333,346	$ 384,309	$ 370,613	$ 382,236	$ 324,248	$ 415,273	$ 410,258	$ 1,430,903	$ 1,532,015	$ 1,411,903