Quarterly report pursuant to Section 13 or 15(d)

Equipment Acquisitions, Dispositions and Depreciation and Impairment Policies (Narrative) (Details)

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Equipment Acquisitions, Dispositions and Depreciation and Impairment Policies (Narrative) (Details)
$ in Thousands
3 Months Ended
Mar. 31, 2018
USD ($)
equipment
Mar. 31, 2017
USD ($)
Property, Plant and Equipment [Line Items]    
Payments to Acquire Property, Plant, and Equipment | $ $ 9,542 $ 66,810
Sales Price Of Equipment | $ 15,900  
Gain (Loss) on Disposition of Property Plant Equipment | $ $ 6,500  
U.S.-flag Harbor Tugs [Member]    
Property, Plant and Equipment [Line Items]    
Number of Equipment Acquired 1  
Number Of Equipments Sold (1)  
Short-sea Container/RORO Vessels [Member]    
Property, Plant and Equipment [Line Items]    
Number of Equipment Acquired 2  
U.S.-flag Petroleum and Chemical Carrier [Member]    
Property, Plant and Equipment [Line Items]    
Number Of Equipments Sold (1)  
Inland River Dry Cargo Barges [Member]    
Property, Plant and Equipment [Line Items]    
Number Of Equipments Sold (32)  
Inland River Specialty Barges [Member]    
Property, Plant and Equipment [Line Items]    
Number Of Equipments Sold (2)  
Gain (Loss) on Asset Dispositions and Impairments, Net [Member]    
Property, Plant and Equipment [Line Items]    
Amortization Of Deferred Gains Included In Gains On Asset Dispositions And Impairments, Net | $ $ 518 $ 605