Contingencies and Commitments (Details)	12 Months Ended
	Dec. 31, 2017 PotentiallyResponsibleParty Spill BargeSlip Lawsuit Plaintiff	Mar. 22, 2014 Vessel TankBarge
Collision with M/S Summer Wind [Member]
Loss Contingencies [Line Items]
Number of vessels involved in collision | Vessel		2
Number of vessels damaged in collision resulting in fuel oil discharge | TankBarge		1
Number of actions filed against the Company | Lawsuit	2
Number of crewmembers alleging damages | Plaintiff	2
Portland Harbor Superfund Site [Member]
Site Contingency [Line Items]
Number of other companies also named as Potentially Responsible Parties ("PRPs") | PotentiallyResponsibleParty	250
Number of spills | Spill	4
SBA Shipyard Site [Member]
Site Contingency [Line Items]
Number of barge slips | BargeSlip	3