Supplemental Information For Statements Of Cash Flows (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Property, Plant and Equipment [Line Items]
Income taxes paid	$ 6,152	$ 11,933	$ 23,791
Income taxes refunded	14,347	3,933	4,550
Interest paid, excluding capitalized interest	32,341	26,662	23,957
Company financed sale of equipment and real property	0	7,950	1,768
Reclassification of Dorian to Marketable Securities	0	0	102,509
Distribution of Dorian shares to shareholders	31,379	0	0
Services and Equipment received to settle notes receivable	5,954	0	0
Noncash or Part Noncash Divestiture, Amount of Consideration Received		2,000
Property, Plant and Equipment [Member]
Property, Plant and Equipment [Line Items]
Services and Equipment received to settle notes receivable	0	2,500	0
U.S.-flag Harbor Tugs [Member]
Property, Plant and Equipment [Line Items]
Noncash or Part Noncash Divestiture, Amount of Consideration Received	$ 0	$ 2,000	$ 0