Acquisitions, 2020 (Details)						3 Months Ended	12 Months Ended
	Mar. 31, 2020 USD ($) bbl Vessel	Jan. 03, 2020 USD ($)	Dec. 28, 2018 USD ($) TankBarge	Dec. 14, 2018 USD ($) bbl TankBarge	Nov. 30, 2018 USD ($) Towboat	Dec. 31, 2020 USD ($) Vessel	Dec. 31, 2020 USD ($) Vessel	Dec. 31, 2019 USD ($) TankBarge	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
Acquisitions [Abstract]
Purchase price							$ 148,185,000	$ 248,164,000	$ 301,861,000
Cash paid for acquisition							81,635,000
Fair values of assets acquired and liabilities assumed, net [Abstract]
Goodwill						$ 657,800,000	$ 657,800,000	$ 953,826,000	$ 953,826,000	$ 935,135,000
Inland Pressure Barges [Member]
Acquisitions [Abstract]
Number of maritime vessels acquired | Vessel						6	6
Purchase price							$ 39,350,000
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
Inland Towboats [Member]
Acquisitions [Abstract]
Number of maritime vessels acquired | Towboat					1
Purchase price					$ 3,050,000
Savage Inland Marine, LLC [Member]
Acquisitions [Abstract]
Cash paid for acquisition	$ 278,999,000
Fair values of assets acquired and liabilities assumed, net [Abstract]
Accounts receivable	0
Inventories	0
Prepaid expenses	1,067,000
Property and equipment	210,065,000
Operating lease right-of-use assets	27,085,000
Goodwill	81,635,000
Other intangibles	2,300,000
Total assets	322,152,000
Accounts payable and accrued liabilities	68,000
Operating lease liabilities, including current portion	43,085,000
Total liabilities	43,153,000
Net assets acquired	278,999,000
Acquisition related costs						$ 376,000
Savage Inland Marine, LLC [Member] | Customer Relationships [Member]
Fair values of assets acquired and liabilities assumed, net [Abstract]
Other intangibles	$ 2,300,000
Intangible assets amortization period							10 years
Savage Inland Marine, LLC [Member] | Inland Tank Barges [Member]
Acquisitions [Abstract]
Number of maritime vessels acquired | Vessel	92
Tank barge barrel capacity (in barrels) | bbl	2,500,000
Savage Inland Marine, LLC [Member] | Inland Towboats [Member]
Acquisitions [Abstract]
Number of maritime vessels acquired | Vessel	45
Convoy [Member]
Acquisitions [Abstract]
Cash paid for acquisition		$ 37,180,000
Fair values of assets acquired and liabilities assumed, net [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
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]
Fair values of assets acquired and liabilities assumed, net [Abstract]
Other intangibles		9,000,000
Intangible assets amortization period							10 years
Convoy [Member] | Distributorships [Member]
Fair values of assets acquired and liabilities assumed, net [Abstract]
Other intangibles		8,000,000
Intangible assets amortization period							12 years
Convoy [Member] | Non-compete Agreements [Member]
Fair values of assets acquired and liabilities assumed, net [Abstract]
Other intangibles		$ 170,000
Intangible assets amortization period							3 years