Subsequent Events (Details) - Subsequent Event [Member] $ in Thousands, bbl in Millions	Feb. 14, 2018 USD ($) bbl Towboat TankBarge	Feb. 12, 2018 USD ($)
Senior Notes Due March 01, 2028 [Member]
Subsequent Event [Line Items]
Long term debt, face amount		$ 500,000
Debt instrument, interest rate, stated percentage		4.20%
Credit facility, expiration date		Mar. 01, 2028
Periodic payment, interest		$ 10,500
Periodic payment, interest, exception of first payment		$ 11,550
Higman [Member]
Subsequent Event [Line Items]
Cash paid for acquisition	$ 419,692
Tank barge barrel capacity | bbl	4,800,000
Higman [Member] | Inland Tank Barges [Member]
Subsequent Event [Line Items]
Number of maritime vessels acquired | TankBarge	159
Number of maritime vessels under construction | TankBarge	2
Higman [Member] | Inland Towboats [Member]
Subsequent Event [Line Items]
Number of maritime vessels acquired | Towboat	75