Acquisitions (Details) bbl in Millions			3 Months Ended	6 Months Ended
	Mar. 31, 2020 USD ($) bbl TankBarge Towboat	Jan. 03, 2020 USD ($)	Jun. 30, 2020 USD ($) TankBarge	Jun. 30, 2020 USD ($) TankBarge	Jun. 30, 2019 USD ($)	Dec. 31, 2019 USD ($)
Acquisitions [Abstract]
Purchase price				$ 92,830,000	$ 127,268,000
Cash paid for acquisition				81,667,000
Assets [Abstract]
Goodwill			$ 657,832,000	$ 657,832,000		$ 953,826,000
Liabilities [Abstract]
Acquisition related costs			$ 281,000
Inland Pressure Barges [Member]
Acquisitions [Abstract]
Number of maritime vessels acquired | TankBarge			4	4
Purchase price				$ 26,625,000
Savage Inland Marine, LLC [Member]
Acquisitions [Abstract]
Cash paid for acquisition	$ 278,999,000
Assets [Abstract]
Accounts receivable	0
Inventories	0
Prepaid expenses	1,067,000
Property and equipment	210,065,000
Operating lease right-of-use assets	27,755,000
Goodwill	81,667,000
Other intangibles	2,200,000
Total assets	322,754,000
Liabilities [Abstract]
Accounts payable and accrued liabilities	0
Operating lease liabilities, including current portion	43,755,000
Total liabilities	43,755,000
Net assets acquired	278,999,000
Savage Inland Marine, LLC [Member] | Customer Relationships [Member]
Assets [Abstract]
Other intangibles	$ 2,200,000
Liabilities [Abstract]
Intangible assets amortization period				10 years
Savage Inland Marine, LLC [Member] | Inland Tank Barges [Member]
Acquisitions [Abstract]
Number of maritime vessels acquired | TankBarge	92
Tank barge barrel capacity (in barrels) | bbl	2.5
Savage Inland Marine, LLC [Member] | Inland Towboats [Member]
Acquisitions [Abstract]
Number of maritime vessels acquired | Towboat	45
Convoy [Member]
Acquisitions [Abstract]
Cash paid for acquisition		$ 37,180,000
Assets [Abstract]
Accounts receivable		5,677,000
Inventories		11,771,000
Prepaid expenses		177,000
Property and equipment		415,000
Operating lease right-of-use assets		3,713,000
Goodwill		10,309,000
Other intangibles		17,170,000
Total assets		49,232,000
Liabilities [Abstract]
Accounts payable and accrued liabilities		8,339,000
Operating lease liabilities, including current portion		3,713,000
Total liabilities		12,052,000
Net assets acquired		37,180,000
Weighted average amortization period of intangibles				11 years
Convoy [Member] | Customer Relationships [Member]
Assets [Abstract]
Other intangibles		9,000,000
Liabilities [Abstract]
Intangible assets amortization period				10 years
Convoy [Member] | Distributorships [Member]
Assets [Abstract]
Other intangibles		8,000,000
Liabilities [Abstract]
Intangible assets amortization period				12 years
Convoy [Member] | Non-compete Agreements [Member]
Assets [Abstract]
Other intangibles		$ 170,000
Liabilities [Abstract]
Intangible assets amortization period				3 years