Segment Information (Schedule of Revenue and Other Income) (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Segment Reporting [Abstract]
Total Segment Revenue and Freight from External Customers									$ 1,321,412	$ 1,143,514	$ 1,340,502
Other Income not Allocated to Segments (Note 3)	$ 20,771	$ 19,713	$ 10,145	$ 22,650	$ 32,418	$ 13,569	$ 20,627	$ 15,506	73,279	82,120	68,193
Gain on Sale of Assets	$ 4,188	$ (513)	$ 5,582	$ 7,955	$ 1,129	$ 194	$ 3,933	$ (28)	17,212	5,228	13,025
Total Consolidated Revenue and Other Income									$ 1,411,903	$ 1,230,862	$ 1,421,720