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Schedule of Vessel Commitments (Parenthetical) (Detail)
$ in Millions
 1 Months Ended 12 Months Ended
Apr. 30, 2017
USD ($)
Mar. 31, 2017
Vessel
Option Agreement    
Significant Purchase and Supply Commitment [Line Items]    
Number of vessels under construction | Vessel   2
Novation Agreement | Subsequent Event    
Significant Purchase and Supply Commitment [Line Items]    
Net consideration received from third party $ 5.3  
Contractual obligation future payments $ 27.2