Annual report pursuant to Section 13 and 15(d)

Revenue, Contract Assets and Liabilities and Other Contract Matters - Additional Information (Details)

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Revenue, Contract Assets and Liabilities and Other Contract Matters - Additional Information (Details)
$ in Millions
9 Months Ended 12 Months Ended
Sep. 30, 2020
USD ($)
Sep. 30, 2019
USD ($)
Dec. 31, 2020
USD ($)
vessel
ship
Vechicle
Dec. 31, 2019
USD ($)
vessel
Vechicle
Project
Dec. 31, 2018
USD ($)
Mar. 31, 2021
USD ($)
vessel
Mar. 31, 2018
vessel
Long Term Contracts Or Programs Disclosure [Line Items]              
Reduction of estimated contract price for liquidated damages, amount     $ 0.6 $ 12.9      
Number of multi-purpose service vessels | vessel     2 2     2
Contract asset under dispute, noncurrent     $ 12.5 $ 12.5      
Reversal of gross profit recognized   $ 2.5          
Number of towing, salvage and rescue vessels | ship     5        
Hurricane Laura              
Long Term Contracts Or Programs Disclosure [Line Items]              
Date of landfall     2020-08        
Impact of Hurricane Laura, description     In August 2020, Hurricane Laura made landfall as a high-end Category 4 hurricane in Lake Charles, Louisiana, where its high winds and flooding caused significant damage throughout the region. At our Lake Charles Yard, Hurricane Laura primarily damaged drydocks, warehouses, bulkheads and our ninth harbor tug project which was nearing completion and subsequently completed in the fourth quarter 2020.        
Total charges related to deductibles     $ 1.3        
Deductible charges associated with, description     As a result, during 2020, we recorded charges of $1.3 million related to deductibles associated with our builder’s risk, equipment, property and marine liability insurance coverages, and our preliminary estimates of cost associated with uninsurable damage, primarily for bulkheads.        
Shipyard              
Long Term Contracts Or Programs Disclosure [Line Items]              
Change in estimated margins     $ 16.6 12.3 $ 2.4    
F&S              
Long Term Contracts Or Programs Disclosure [Line Items]              
Change in estimated margins     2.7 4.9 6.7    
Contracts For Construction              
Long Term Contracts Or Programs Disclosure [Line Items]              
Reduction of estimated contract price for liquidated damages, amount       11.2      
Towing, Salvage and Rescue Ship              
Long Term Contracts Or Programs Disclosure [Line Items]              
Change in estimated margins     $ 7.3        
Number of vehicle ferry projects | Vechicle     5        
Reserve for loss     $ 3.2        
Number of towing, salvage and rescue vessels | ship     5        
Number of additional vessels constructed upon exercise of option | vessel     3        
Towing, Salvage and Rescue Ship | Scenario Forecast              
Long Term Contracts Or Programs Disclosure [Line Items]              
Number of additional vessels construction not exercised under contract | vessel           3  
Contractual obligation           $ 13.1  
Towing, Salvage and Rescue Ship | Minimum              
Long Term Contracts Or Programs Disclosure [Line Items]              
Projects, percent complete (percentage)     10.00%        
Towing, Salvage and Rescue Ship | Maximum              
Long Term Contracts Or Programs Disclosure [Line Items]              
Projects, percent complete (percentage)     60.00%        
Harbor Tug              
Long Term Contracts Or Programs Disclosure [Line Items]              
Change in estimated margins     $ 1.0 4.9 6.7    
Number of vehicle ferry projects | Vechicle     2        
Reserve for loss       1.6      
Forty-Vehicle Ferry              
Long Term Contracts Or Programs Disclosure [Line Items]              
Change in estimated margins     $ 7.2 $ 5.1      
Number of vehicle ferry projects | Vechicle     2 2      
Reserve for loss     $ 4.8 $ 3.0      
Incremental forecast costs $ 4.1            
Forty-Vehicle Ferry | Other (Income) Expense, Net              
Long Term Contracts Or Programs Disclosure [Line Items]              
Accrued deductible on insurance coverage $ 0.1            
Forty-Vehicle Ferry | Minimum              
Long Term Contracts Or Programs Disclosure [Line Items]              
Incremental forecast costs     1.0        
Forty-Vehicle Ferry | Maximum              
Long Term Contracts Or Programs Disclosure [Line Items]              
Incremental forecast costs     4.0        
First Forty-Vehicle Ferry              
Long Term Contracts Or Programs Disclosure [Line Items]              
Change in estimated margins     6.2        
Second Forty-Vehicle Ferry Project              
Long Term Contracts Or Programs Disclosure [Line Items]              
Change in estimated margins     $ 1.0        
Projects, percent complete (percentage)     80.00%        
Seventy-Vehicle Ferry              
Long Term Contracts Or Programs Disclosure [Line Items]              
Change in estimated margins     $ 1.1        
Projects, percent complete (percentage)     55.00%        
Reserve for loss     $ 0.5        
Jacket and Deck              
Long Term Contracts Or Programs Disclosure [Line Items]              
Change in estimated margins     1.2 2.0      
Reserve for loss       1.1      
Paddlewheel Riverboat and Subsea Components Projects              
Long Term Contracts Or Programs Disclosure [Line Items]              
Change in estimated margins     $ 1.5        
Ice-Breaker Tug              
Long Term Contracts Or Programs Disclosure [Line Items]              
Change in estimated margins       1.5      
Reserve for loss       0.1      
Research Vessel Projects              
Long Term Contracts Or Programs Disclosure [Line Items]              
Change in estimated margins       $ 0.8      
Number of Research Vessels Projects | Project       3      
Paddle Wheel River Boat              
Long Term Contracts Or Programs Disclosure [Line Items]              
Change in estimated margins       $ 1.3      
Reserve for loss       0.2      
Subsea Components Project              
Long Term Contracts Or Programs Disclosure [Line Items]              
Change in estimated margins       1.6      
Reserve for loss       $ 0.2      
Petrochemical Modules              
Long Term Contracts Or Programs Disclosure [Line Items]              
Change in estimated margins         $ 2.4