Acquisitions, 2019 (Details)					12 Months Ended
	Mar. 14, 2019 USD ($) bbl Towboat TankBarge	Dec. 28, 2018 USD ($) TankBarge	Dec. 14, 2018 USD ($) bbl TankBarge	Nov. 30, 2018 USD ($) Towboat	Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($) TankBarge	Dec. 31, 2018 USD ($)
Acquisitions [Abstract]
Purchase price					$ 148,185,000	$ 248,164,000	$ 301,861,000
Inland Tank Barges [Member]
Acquisitions [Abstract]
Number of maritime vessels acquired | TankBarge		3	27			9
Purchase price		$ 3,120,000	$ 28,500,000			$ 17,991,000
Tank barge barrel capacity (in barrels) | bbl			306,000
Average age of property			8 years		5 years
Inland Towboats [Member]
Acquisitions [Abstract]
Number of maritime vessels acquired | Towboat				1
Purchase price				$ 3,050,000
Cenac Marine Services, LLC [Member]
Fair values of assets acquired and liabilities assumed, net [Abstract]
Prepaid expenses	$ 1,138,000
Property and equipment	247,122,000
Other intangibles	340,000
Total assets	248,600,000
Other long-term liabilities	4,100,000
Net assets acquired	$ 244,500,000
Weighted average amortization period of intangibles					2 years
Weighted average amortization period of unfavorable contracts					1 year 3 months 18 days
Acquisition related costs						$ 442,000
Cenac Marine Services, LLC [Member] | Inland Barrel Tank Barges [Member]
Acquisitions [Abstract]
Number of maritime vessels acquired | TankBarge	63
Tank barge barrel capacity (in barrels) | bbl	30,000
Aggregate tank barge barrel capacity (in barrels) | bbl	1,833,000
Cenac Marine Services, LLC [Member] | Inland Towboats [Member]
Acquisitions [Abstract]
Number of maritime vessels acquired | Towboat	34
Average age of property					7 years
Cenac Marine Services, LLC [Member] | Offshore Tugboats [Member]
Acquisitions [Abstract]
Number of maritime vessels acquired | Towboat	2