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Asset Retirement Obligations - Summary of Asset Retirement Obligations (Detail) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Disclosure of asset retirement obligations [line items]    
Beginning Balance $ 89,778  
Acquired during the year 9,453  
Accretion during the year 2,006  
Revisions to estimate of obligation 1,544  
Settlements (3,097)  
Ending Balance 99,684  
Less: Current portion (3,489)  
Long term portion 96,195 $ 89,778
Estimated undiscounted amount 139,591  
Greece [member]    
Disclosure of asset retirement obligations [line items]    
Beginning Balance 48,131  
Accretion during the year 1,025  
Revisions to estimate of obligation 1,112  
Settlements (2,807)  
Ending Balance 47,461  
Less: Current portion (3,489)  
Long term portion 43,972  
Estimated undiscounted amount 71,591  
Brazil [member]    
Disclosure of asset retirement obligations [line items]    
Beginning Balance 4,092  
Accretion during the year 32  
Revisions to estimate of obligation (80)  
Ending Balance 4,044  
Long term portion 4,044  
Estimated undiscounted amount 4,117  
Turkey [member]    
Disclosure of asset retirement obligations [line items]    
Beginning Balance 36,196  
Accretion during the year 913  
Revisions to estimate of obligation 502  
Settlements (290)  
Ending Balance 37,321  
Long term portion 37,321  
Estimated undiscounted amount 49,257  
Romania [member]    
Disclosure of asset retirement obligations [line items]    
Beginning Balance 1,359  
Accretion during the year 36  
Revisions to estimate of obligation 10  
Ending Balance 1,405  
Long term portion 1,405  
Estimated undiscounted amount 2,340  
Canada [member]    
Disclosure of asset retirement obligations [line items]    
Acquired during the year 9,453  
Ending Balance 9,453  
Long term portion 9,453  
Estimated undiscounted amount $ 12,286