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Commitments and Contingencies (Sonatide Joint Venture) - Additional Information (Detail)
 3 Months Ended 6 Months Ended
Jun. 30, 2018
USD ($)
Vessel
Kz / $
Jun. 30, 2018
USD ($)
Vessel
Kz / $
Jun. 30, 2017
USD ($)
Vessel
Dec. 31, 2017
USD ($)
Kz / $
Jun. 30, 2014
Vessel
Commitments and Contingencies Disclosure [Line Items]          
Due from affiliates $ 197,059,000 $ 197,059,000   $ 230,315,000  
Cash and cash equivalents 459,286,000 459,286,000   432,035,000  
Due to affiliates 62,353,000 62,353,000   99,448,000  
Commissions payable [1] 2,488,000 2,488,000   1,898,000  
Total revenues 105,601,000 197,094,000      
Investments in, at equity, and advances to unconsolidated companies $ 1,335,000 1,335,000   $ 29,216,000  
Dividend received in excess of investment   $ 4,900,000      
Devaluation Of Angolan kwanza Versus US Dollar          
Commitments and Contingencies Disclosure [Line Items]          
Foreign currency exchange rate | Kz / $ 250 250   168  
Percentage of devaluation of currency   49.00%      
Percentage of foreign exchange loss recognized   49.00%      
Estimated exchange loss   $ 20,600,000      
Vessel Revenues          
Commitments and Contingencies Disclosure [Line Items]          
Total revenues $ 104,174,000 $ 191,668,000      
ANGOLA          
Commitments and Contingencies Disclosure [Line Items]          
Number of vessels operating | Vessel 37 37     84
Decrease in number of vessels operating | Vessel 47 47      
Decrease in number of active vessels | Vessel 58 58      
Number of active vessels | Vessel 22 22     80
Sonatide joint venture          
Commitments and Contingencies Disclosure [Line Items]          
Due from affiliates $ 153,000,000 $ 153,000,000   $ 230,000,000  
Unpaid vessel revenue 25,000,000 25,000,000      
Due to affiliates 47,000,000 47,000,000   99,000,000  
Commissions payable $ 30,200,000 30,200,000   36,400,000  
Due from affiliates and due to affiliates   $ 55,000,000      
Number of vessels operating | Vessel 7 7      
Number of vessels stacked | Vessel 5 5      
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   12,300,000      
Sonatide joint venture | ANGOLA          
Commitments and Contingencies Disclosure [Line Items]          
Proceeds from related party   $ 51,000,000      
Percentage of Angolan operation revenue   15.00% 20.00%    
Number of vessels operating | Vessel 38 38 53    
Number of vessels stacked | Vessel 17 17 23    
Number of vessels transferred out of Angola | Vessel   8      
Sonatide joint venture | ANGOLA | Vessel Revenues          
Commitments and Contingencies Disclosure [Line Items]          
Total revenues   $ 29,000,000 $ 53,000,000    
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   47,000,000      
Sonatide joint venture | Sonatide's converting kwanzas into dollars and subsequent payment to company          
Commitments and Contingencies Disclosure [Line Items]          
Proceeds from related party   4,000,000      
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 $ 43,000,000 $ 43,000,000      
[1] Excludes $30.2 million and $36.4 million of commissions due to Sonatide at June 30, 2018 and December 31, 2017,