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Accounting policies - Summary of Key assumptions used in the Income and Market Approach (Detail)
 12 Months Ended
Dec. 31, 2020
$ / per_oz
Dec. 31, 2019
$ / per_oz
Dec. 31, 2020
$ / oz
Dec. 31, 2019
$ / oz
Dec. 31, 2018
$ / per_oz
Disclosure of Detailed Information of Key Assumptions Used in the Income and Market Approach Explanatory [line items]          
Resource value per ounce (with infrastructure) | $ / per_oz 76 70      
Discount rates - real 8.40% 8.50%      
Life of mine 7 years 10 years      
US Dollars [member] | Year 4 [member]          
Disclosure of Detailed Information of Key Assumptions Used in the Income and Market Approach Explanatory [line items]          
Gold price 1,500 1,350 1,500 1,350 1,300
US Dollars [member] | Year 1 To 3 [member] | Bottom of range [member]          
Disclosure of Detailed Information of Key Assumptions Used in the Income and Market Approach Explanatory [line items]          
Gold price     1,600 1,400  
US Dollars [member] | Year 1 To 3 [member] | Top of range [member]          
Disclosure of Detailed Information of Key Assumptions Used in the Income and Market Approach Explanatory [line items]          
Gold price     1,800 1,500