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Commitments and Contingencies - Additional Information (Detail)
$ in Millions
 1 Months Ended 9 Months Ended 12 Months Ended
Nov. 22, 2017
USD ($)
Dec. 31, 2017
USD ($)
T
Mar. 31, 2016
USD ($)
Jun. 30, 2018
USD ($)
Vessel
T
Jul. 31, 2017
USD ($)
Significant Purchase And Supply Commitment [Line Items]          
Cash compensation on termination of employment, maximum   $ 68.0      
Fast Crew Supply Boat [Member]          
Significant Purchase And Supply Commitment [Line Items]          
Number of vessels under construction   1      
Remaining accumulated cost reclassified from construction in progress to other assets as insurance receivable     $ 5.6    
Remaining insurance receivable   $ 1.8     $ 1.8
Vessel Commitments          
Significant Purchase And Supply Commitment [Line Items]          
Significant commitment, new construction deadweight tons capacity | T   5,400   5,400  
'Number of Vessels, commitments   2   1  
Settlement agreement, date November 22, 2017        
Delivery date of first PSV 2017-11        
Final installment paid $ 4.3        
Delivery date of second PSV 2018-07        
Remaining installment payment for second PSV $ 4.5        
Settlement credit for future dry docking services $ 0.3        
'Vessel under construction, fair value, after fresh start accounting         $ 7.0
Remaining Balance, commitments       $ 2.3