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Commitments and Contingencies (Sonatide Joint Venture) - Additional Information (Detail)
 2 Months Ended 3 Months Ended 7 Months Ended 9 Months Ended
Sep. 30, 2017
USD ($)
Vessel
Sep. 30, 2018
USD ($)
Vessel
Kz / $
Jul. 31, 2017
USD ($)
Vessel
Sep. 30, 2018
USD ($)
Vessel
Kz / $
Jul. 15, 2018
Dec. 31, 2017
USD ($)
Kz / $
Sep. 30, 2014
Vessel
Commitments and Contingencies Disclosure [Line Items]              
Due from affiliates   $ 174,349,000   $ 174,349,000   $ 230,315,000  
Cash and cash equivalents   461,088,000   461,088,000   432,035,000  
Due to affiliates   48,064,000   48,064,000   99,448,000  
Commissions payable [1]   1,932,000   1,932,000   1,898,000  
Total revenues $ 74,300,000 99,192,000   296,286,000      
Ownership Interest In Joint Venture         74.00%    
Investments in, at equity, and advances to unconsolidated companies   $ 1,129,000   1,129,000   $ 29,216,000  
Dividend received in excess of investment       $ 5,000,000      
Devaluation Of Angolan kwanza Versus US Dollar              
Commitments and Contingencies Disclosure [Line Items]              
Foreign currency exchange rate | Kz / $   294   294   168  
Percentage of devaluation of currency       75.00%      
Vessel Revenues              
Commitments and Contingencies Disclosure [Line Items]              
Total revenues $ 70,571,000 $ 97,011,000   $ 288,679,000      
ANGOLA              
Commitments and Contingencies Disclosure [Line Items]              
Number of vessels operating | Vessel   36   36     81
Decrease in number of vessels operating | Vessel   45   45      
Decrease in number of active vessels | Vessel   55   55      
Number of active vessels | Vessel   21   21     76
Sonatide joint venture              
Commitments and Contingencies Disclosure [Line Items]              
Due from affiliates   $ 134,000,000   $ 134,000,000   $ 230,000,000  
Unpaid vessel revenue   23,000,000   23,000,000      
Due to affiliates   31,000,000   31,000,000   99,000,000  
Commissions payable   $ 26,200,000   26,200,000   36,400,000  
Due from affiliates and due to affiliates       $ 71,000,000      
Number of vessels operating | Vessel   4   4      
Number of vessels stacked | Vessel   2   2      
Ownership Interest In Joint Venture   49.00%   49.00%      
Investments in, at equity, and advances to unconsolidated companies   $ 0   $ 0   $ 27,000,000  
Dividend received       12,000,000      
Sonatide joint venture | ANGOLA              
Commitments and Contingencies Disclosure [Line Items]              
Proceeds from related party       $ 69,000,000      
Percentage of Angolan operation revenue 20.00%   20.00% 15.00%      
Number of vessels operating | Vessel 44 38 52 38      
Number of vessels stacked | Vessel 16 16 22 16      
Number of vessels transferred out of Angola | Vessel       2      
Sonatide joint venture | ANGOLA | Vessel Revenues              
Commitments and Contingencies Disclosure [Line Items]              
Total revenues $ 14,000,000   $ 60,000,000 $ 44,000,000      
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       61,000,000      
Sonatide joint venture | Sonatide's converting kwanzas into dollars and subsequent payment to company              
Commitments and Contingencies Disclosure [Line Items]              
Proceeds from related party       8,000,000      
Sonatide joint venture | Angolan kwanza-denominated              
Commitments and Contingencies Disclosure [Line Items]              
Bank deposits maintained   $ 67,000,000   67,000,000      
Cash and cash equivalents   $ 36,000,000   $ 36,000,000      
[1] Excludes $26.2 million and $36.4 million of commissions due to Sonatide at September 30, 2018 and December 31, 2017,