Quarterly report pursuant to Section 13 or 15(d)

Commitments And Contingencies (Narrative) (Details)

v3.8.0.1
Commitments And Contingencies (Narrative) (Details)
$ in Thousands
12 Months Ended
Dec. 31, 2016
Dec. 31, 2013
claim
Dec. 31, 2012
claim
Dec. 31, 2010
claim
Jun. 30, 2018
USD ($)
equipment
Mar. 31, 2018
USD ($)
Loss Contingencies [Line Items]            
Unrecorded Unconditional Purchase Obligation, Due in Remainder of Fiscal Year           $ 3,058
Unrecorded Unconditional Purchase Obligation, Due in Second Years           757
Unrecorded Unconditional Purchase Obligation           3,815
Multi-district Litigation [Member]            
Loss Contingencies [Line Items]            
Claims Filed | claim       1    
Loss Contingency, Number of Plaintiffs 11          
Abney Litigation [Member]            
Loss Contingencies [Line Items]            
Claims Filed | claim     668      
Abood Litigation [Member]            
Loss Contingencies [Line Items]            
Claims Filed | claim   174        
Ocean Services [Member]            
Loss Contingencies [Line Items]            
Unrecorded Unconditional Purchase Obligation, Due in Remainder of Fiscal Year           148
Unrecorded Unconditional Purchase Obligation, Due in Second Years           0
Unrecorded Unconditional Purchase Obligation           148
Inland Services [Member]            
Loss Contingencies [Line Items]            
Unrecorded Unconditional Purchase Obligation, Due in Remainder of Fiscal Year           2,910
Unrecorded Unconditional Purchase Obligation, Due in Second Years           757
Unrecorded Unconditional Purchase Obligation           $ 3,667
Subsequent Event [Member]            
Loss Contingencies [Line Items]            
Unrecorded Unconditional Purchase Obligation         $ 13,300  
Subsequent Event [Member] | U.S.-flag Harbor Tugs [Member]            
Loss Contingencies [Line Items]            
Unrecorded Unconditional Purchase Obligation, Maximum Quantity | equipment         1