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Property, plant and equipment - Summary of Key Assumptions Used for Assessing Recoverable Amount of Company's CGUs Versus Carrying Values (Detail)
 1 Months Ended 3 Months Ended 12 Months Ended
Jul. 01, 2018
$ / oz
Apr. 30, 2007
$ / oz
Sep. 30, 2018
$ / oz
Jun. 30, 2018
$ / oz
Dec. 31, 2018
$ / T
$ / oz
Disclosure of detailed information about property, plant and equipment [line items]          
Silver price ($/oz) 6.77 3.90   2.50  
Turkey [member] | Individual assets or cash-generating units [member]          
Disclosure of detailed information about property, plant and equipment [line items]          
Gold price ($/oz)     1,250    
Silver price ($/oz)     17    
Discount rate     6.50%    
Greece [member] | Individual assets or cash-generating units [member]          
Disclosure of detailed information about property, plant and equipment [line items]          
Discount rate         7.00%
Greece [member] | Bottom of range [member] | Individual assets or cash-generating units [member]          
Disclosure of detailed information about property, plant and equipment [line items]          
Gold price ($/oz)         1,275
Silver price ($/oz)         17
Lead price ($/t) | $ / T         2,200
Zinc price ($/t) | $ / T         2,800
Greece [member] | Top of range [member] | Individual assets or cash-generating units [member]          
Disclosure of detailed information about property, plant and equipment [line items]          
Gold price ($/oz)         1,300
Silver price ($/oz)         18
Lead price ($/t) | $ / T         2,300
Zinc price ($/t) | $ / T         2,900