Business Acquisitions (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2017 USD ($) equipment	Dec. 31, 2016 USD ($) equipment	Jul. 03, 2017 USD ($)	Dec. 02, 2016	Dec. 31, 2015 USD ($)
Business Acquisition [Line Items]
Restricted cash	$ 13	$ 0			$ 0
Trade and other receivables	15,823	937			0
Other current assets	2,054	150			0
Investments, at Equity, and Advances to 50% or Less Owned Companies	10,000	(3,437)			0
Property and Equipment	15,190	15,765			0
Goodwill	0	0			3,157
Intangible Assets	10,957	7,098			(3,157)
Other Assets	(17,863)	0			0
Accounts payable	0	39			0
Other current liabilities	(17,214)	(13)			0
Long-Term Debt	(28,725)	0			0
Deferred Income Taxes	(3,939)
Deferred Income Taxes, Noncurrent		0			0
Other Liabilities	(42)	0			0
Purchase price	(5,868)	20,539			$ 0
Cash Acquired from Acquisition	$ 16,400	900
Central Contracting & Marine, Inc. [Member]
Business Acquisition [Line Items]
Payments to Acquire Businesses, Gross		18,100
SeaJon II [Member]
Business Acquisition [Line Items]
Business Acquisition, Percentage of Voting Interests Acquired				50.00%
Payments to Acquire Businesses, Gross		$ 3,400
Inland River Harbor Boats [Member] | Central Contracting & Marine, Inc. [Member]
Business Acquisition [Line Items]
Number Of Equipments Acquired | equipment	5	5
U.S-flag Offshore Tug [Member] | SeaJon II [Member]
Business Acquisition [Line Items]
Number Of Equipments Acquired | equipment		1
ISH [Member]
Business Acquisition [Line Items]
Payments to Acquire Businesses, Gross	$ 10,500
Debt Conversion, Converted Instrument, Amount	$ 18,100
Long-Term Debt			$ (28,700)