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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, 2019
$ / oz
Apr. 30, 2007
$ / oz
Jun. 30, 2019
$ / oz
Dec. 31, 2019
$ / T
$ / oz
Dec. 31, 2018
$ / T
$ / oz
Disclosure of detailed information about property, plant and equipment [line items]          
Silver price ($/oz) 9.27 3.90 2.50    
TURKEY          
Disclosure of detailed information about property, plant and equipment [line items]          
Discount rate (as a percent)         6.50%
TURKEY | Individual assets or cash-generating units          
Disclosure of detailed information about property, plant and equipment [line items]          
Gold price ($/oz)       1,400 1,250
Silver price ($/oz)       18 17
Discount rate (as a percent)       5.00%  
Average factor to convert contained mineral resource ounces outside of reserves to ounces used in value beyond proven and probable calculations       69.00% 32.00%
Fair value per contained ounce of resources and exploration potential beyond proven and probable reserves       110 50
GREECE | Individual assets or cash-generating units          
Disclosure of detailed information about property, plant and equipment [line items]          
Gold price ($/oz)       1,400  
Silver price ($/oz)       18  
Lead price ($/t) | $ / T       2,100  
Zinc price ($/t) | $ / T       2,400  
Discount rate (as a percent)       6.00% 7.00%
Average factor to convert contained mineral resource ounces outside of reserves to ounces used in value beyond proven and probable calculations       27.00% 27.00%
Fair value per contained ounce of resources and exploration potential beyond proven and probable reserves       130 100
GREECE | Bottom of range | Individual assets or cash-generating units          
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 | Top of range | Individual assets or cash-generating units          
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