Acquisitions, 2017 (Details) $ / shares in Units, $ in Thousands								1 Months Ended	12 Months Ended
	Dec. 28, 2018 USD ($) TankBarge	Dec. 14, 2018 USD ($) TankBarge bbl	Nov. 30, 2018 USD ($) Towboat	Mar. 15, 2018 USD ($) TankBarge	Oct. 20, 2017 USD ($)	Sep. 13, 2017 USD ($) Branch State $ / shares shares	Jul. 10, 2017 USD ($) TankBarge Towboat bbl hp	Jul. 31, 2017 USD ($) TankBarge	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Feb. 14, 2018 USD ($)	Jun. 25, 2017 USD ($)
Acquisitions [Abstract]
Purchase price									$ 301,861	$ 177,222	$ 231,066
Property acquired												$ 497,951
Revolving Credit Facility [Member]
Acquisitions [Abstract]
Maximum borrowing capacity									$ 850,000
Maturity date									Jun. 26, 2022
Inland Pressure Tank Barges [Member]
Acquisitions [Abstract]
Purchase price				$ 10,400
Number of maritime vessels acquired | TankBarge				2
Average age of property				5 years
Inland Towboats [Member]
Acquisitions [Abstract]
Purchase price			$ 3,050
Number of maritime vessels acquired | Towboat			1
Inland Tank Barges [Member]
Acquisitions [Abstract]
Purchase price	$ 3,120	$ 28,500
Number of maritime vessels acquired | TankBarge	3	27
Tank barge barrel capacity (in barrels) | bbl		306,000
Average age of property		8 years
Sneed Shipbuilding, Inc [Member]
Acquisitions [Abstract]
Cash paid for acquisition					$ 14,852
Stewart & Stevenson LLC [Member]
Acquisitions [Abstract]
Cash paid for acquisition						$ 377,935
Purchase price before post-closing adjustment and transaction fees						758,213
Assumption of debt						13,724
Stock consideration through issuance of Company common stock						$ 366,554
Issuance of shares (in shares) | shares						5,696,259
Price per share (in dollars per share) | $ / shares						$ 64.35
Term debt assumed						$ 12,135
Short-term secured loans						$ 1,589
Number of branches | Branch						40
Number of states having branches | State						12
Stewart & Stevenson LLC [Member] | Revolving Credit Facility [Member]
Acquisitions [Abstract]
Maximum borrowing capacity									$ 850,000				$ 550,000
Maturity date									Jun. 26, 2022
Undisclosed Competitor [Member]
Acquisitions [Abstract]
Purchase price							$ 68,000
Property acquired							67,970
Intangible assets acquired							$ 30
Weighted average amortization period of intangibles							2 years
Undisclosed Competitor [Member] | Inland Pressure Tank Barges [Member]
Acquisitions [Abstract]
Number of maritime vessels acquired | TankBarge							9
Undisclosed Competitor [Member] | Barrel Tank Barges [Member]
Acquisitions [Abstract]
Number of maritime vessels acquired | TankBarge							4
Tank barge barrel capacity (in barrels) | bbl							30,000
Undisclosed Competitor [Member] | Inland Towboats [Member]
Acquisitions [Abstract]
Number of maritime vessels acquired | Towboat							3
Horsepower of inland towboats | hp							1,320
Undisclosed Competitor [Member] | Inland Tank Barges [Member]
Acquisitions [Abstract]
Number of maritime vessels acquired | TankBarge							13
Average age of property							5 years
Barge Fleeting and Marine Fueling Operation Business [Member]
Acquisitions [Abstract]
Cash paid for acquisition								$ 3,900
Barge Fleeting and Marine Fueling Operation Business [Member] | Inland Tank Barges [Member]
Acquisitions [Abstract]
Purchase price								$ 1,450
Number of maritime vessels acquired | TankBarge								4