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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
Aug. 03, 2020
$ / oz
Apr. 30, 2007
$ / oz
Jun. 30, 2020
$ / oz
Dec. 31, 2020
$ / T
$ / oz
Dec. 31, 2019
$ / T
$ / oz
Disclosure of detailed information about property, plant and equipment [line items]          
Silver price ($/oz) 11.43 3.90 2.00    
GREECE | Individual assets or cash-generating units          
Disclosure of detailed information about property, plant and equipment [line items]          
Gold price ($/oz)         1,400
Silver price ($/oz)         18
Lead price ($/t) | $ / T         2,100
Zinc price ($/t) | $ / T         2,400
Discount rate         6.00%
GREECE | Bottom of range | Individual assets or cash-generating units          
Disclosure of detailed information about property, plant and equipment [line items]          
Gold price ($/oz)       1,850  
Silver price ($/oz)       25  
Lead price ($/t) | $ / T       2,000  
Zinc price ($/t) | $ / T       2,575  
Discount rate       6.00%  
GREECE | Top of range | Individual assets or cash-generating units          
Disclosure of detailed information about property, plant and equipment [line items]          
Gold price ($/oz)       1,550  
Silver price ($/oz)       21  
Lead price ($/t) | $ / T       1,975  
Zinc price ($/t) | $ / T       2,400  
Discount rate       6.50%