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Asset retirement obligations - Summary of Asset Retirement Obligations (Detail) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Disclosure of asset retirement obligations [line items]    
Beginning Balance $ 99,684 $ 89,778
Acquired during the year   9,453
Accretion during the year 2,038 2,006
Revisions to estimate (2,043) 1,544
Settlements (5,536) (3,097)
Ending Balance 94,143 99,684
Less: Current portion (824) (3,489)
Long term portion 93,319 96,195
Estimated undiscounted amount 135,173 139,591
Turkey [member]    
Disclosure of asset retirement obligations [line items]    
Beginning Balance 37,321 36,196
Acquired during the year   0
Accretion during the year 896 913
Revisions to estimate (1,117) 502
Settlements (621) (290)
Ending Balance 36,479 37,321
Less: Current portion 0 0
Long term portion 36,479 37,321
Estimated undiscounted amount 48,454 49,257
Canada [member]    
Disclosure of asset retirement obligations [line items]    
Beginning Balance 9,453 0
Acquired during the year   9,453
Accretion during the year 0 0
Revisions to estimate 2,762 0
Settlements 0 0
Ending Balance 12,215 9,453
Less: Current portion 0 0
Long term portion 12,215 9,453
Estimated undiscounted amount 14,989 12,286
Greece [member]    
Disclosure of asset retirement obligations [line items]    
Beginning Balance 47,461 48,131
Acquired during the year   0
Accretion during the year 1,035 1,025
Revisions to estimate (3,512) 1,112
Settlements (4,915) (2,807)
Ending Balance 40,069 47,461
Less: Current portion (824) (3,489)
Long term portion 39,245 43,972
Estimated undiscounted amount 65,274 71,591
Romania [member]    
Disclosure of asset retirement obligations [line items]    
Beginning Balance 1,405 1,359
Acquired during the year   0
Accretion during the year 36 36
Revisions to estimate (77) 10
Settlements 0 0
Ending Balance 1,364 1,405
Less: Current portion 0 0
Long term portion 1,364 1,405
Estimated undiscounted amount 2,335 2,340
Brazil [member]    
Disclosure of asset retirement obligations [line items]    
Beginning Balance 4,044 4,092
Acquired during the year   0
Accretion during the year 71 32
Revisions to estimate (99) (80)
Settlements 0 0
Ending Balance 4,016 4,044
Less: Current portion 0 0
Long term portion 4,016 4,044
Estimated undiscounted amount $ 4,121 $ 4,117