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Commitments and Contingencies (Sonatide Joint Venture) - Additional Information (Detail)
$ in Thousands
 3 Months Ended
Mar. 31, 2018
USD ($)
Vessel
Kz / $
Mar. 31, 2017
USD ($)
Vessel
Dec. 31, 2017
USD ($)
Kz / $
Mar. 31, 2014
Vessel
Commitments and Contingencies Disclosure [Line Items]        
Due from affiliate $ 207,919   $ 230,315  
Cash and cash equivalents 442,472   432,035  
Due to affiliate 78,135   99,448  
Commissions payable [1] 2,146   1,898  
Vessel revenues 87,494      
Investments in, at equity, and advances to unconsolidated companies $ 13,503   $ 29,216  
Devaluation Of Angolan kwanza Versus US Dollar        
Commitments and Contingencies Disclosure [Line Items]        
Foreign currency exchange rate | Kz / $ 214   168  
Percentage of devaluation of currency 27.00%      
Percentage of foreign exchange loss recognized 49.00%      
Estimated exchange loss $ 14,800      
ANGOLA        
Commitments and Contingencies Disclosure [Line Items]        
Number of vessels operating | Vessel 38     85
Decrease in number of vessels operating | Vessel 47      
Decrease in number of active vessels | Vessel 58      
Number of active vessels | Vessel 21     79
Sonatide joint venture        
Commitments and Contingencies Disclosure [Line Items]        
Due from affiliate $ 208,000   $ 230,000  
Unpaid vessel revenue 26,000      
Due to affiliate 78,000   99,000  
Commissions payable 37,600   36,400  
Due from affiliate and due to affiliate $ 27,000      
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 $ 12,000   $ 27,000  
Sonatide joint venture | ANGOLA        
Commitments and Contingencies Disclosure [Line Items]        
Proceeds from related party 8,000      
Vessel revenues $ 14,000 $ 26,000    
Percentage of Angolan operation revenue 16.00% 17.00%    
Number of vessels operating | Vessel 40 55    
Number of vessels stacked | Vessel 18 24    
Number of vessels transferred out of Angola | Vessel 12      
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 $ 6,000      
Sonatide joint venture | Sonatide's converting kwanzas into dollars and subsequent payment to company        
Commitments and Contingencies Disclosure [Line Items]        
Proceeds from related party 2,000      
Sonatide joint venture | Angolan kwanza-denominated        
Commitments and Contingencies Disclosure [Line Items]        
Bank deposits maintained 103,000      
Cash and cash equivalents $ 66,000      
[1] Excludes $37.6 million and $36.4 million of commissions due to Sonatide at March 31, 2018 and December 31, 2017,