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Commitments and Contingencies (Sonatide Joint Venture) - Additional Information (Detail)
$ in Thousands
 4 Months Ended 5 Months Ended 12 Months Ended
Jul. 31, 2017
USD ($)
Vessel
Jul. 31, 2017
USD ($)
Vessel
Dec. 31, 2017
USD ($)
Vessel
Mar. 31, 2017
USD ($)
Vessel
Commitments and Contingencies Disclosure [Line Items]        
Due from affiliate     $ 230,315  
Due to affiliate     99,448  
Commissions payable [1]     1,898  
Vessel revenues     $ 171,884  
Number of vessels operating | Vessel [2]     227  
Investments in, at equity, and advances to unconsolidated companies     $ 29,216  
Sonatide joint venture        
Commitments and Contingencies Disclosure [Line Items]        
Due from affiliate     230,000 $ 263,000
Unpaid vessel revenue     44,000  
Due to affiliate     99,000 133,000
Commissions payable     36,000  
Due from affiliate and due to affiliate     $ 33,000 21,000
Number of vessels operating | Vessel     7  
Number of vessels stacked | Vessel     4  
Ownership Interest In Joint Venture     49.00%  
Investments in, at equity, and advances to unconsolidated companies     $ 27,000 45,000
Investment balance in difference between carrying value and net assets of the joint venture $ 28,000 $ 28,000    
Amortization period of basis difference equity method investments 10 years      
Sonatide joint venture | ANGOLA        
Commitments and Contingencies Disclosure [Line Items]        
Proceeds from related party   22,000 21,000  
Vessel revenues   $ 34,000 $ 34,000 $ 127,000
Percentage of Angolan operation revenue   23.00% 20.00% 22.00%
Number of vessels operating | Vessel 50 50 43 58
Number of vessels stacked | Vessel 21 21 16 20
Number of vessels transferred out of Angola | Vessel   3 3 22
Sonatide joint venture | U.S. dollars initially received by Sonatide on behalf of the company or dollars collected from other customers        
Commitments and Contingencies Disclosure [Line Items]        
Proceeds from related party   $ 19,000 $ 20,000  
Sonatide joint venture | Sonatide's converting kwanzas into dollars and subsequent payment to company        
Commitments and Contingencies Disclosure [Line Items]        
Proceeds from related party   $ 3,000 1,000  
Sonatide joint venture | Angolan kwanza-denominated        
Commitments and Contingencies Disclosure [Line Items]        
Bank deposits maintained     $ 81,000  
[1] Excludes $36.4 million and $34.7 million of commissions due to Sonatide at December 31, 2017 and March 31, 2017, respectively. These amounts are included in amounts due to affiliates.
[2] Vessel count excludes vessels operated under sale leaseback agreements.