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Deferred Consideration - Summary of valuation of deferred consideration (Detail) - USD ($)
$ in Thousands
 9 Months Ended 12 Months Ended
Sep. 30, 2021
Dec. 31, 2020
Disclosure Of Valuation Of Deferred Consideration [Line Item]    
Discount rate 9.00% 9.00%
Perpetual growth rate 1.40% 0.90%
Minimum [Member]    
Disclosure Of Valuation Of Deferred Consideration [Line Item]    
Forward-looking gold price $ 1,756 $ 1,903
Maximum [Member]    
Disclosure Of Valuation Of Deferred Consideration [Line Item]    
Forward-looking gold price 2,906 2,662
Weighted Average [Member]    
Disclosure Of Valuation Of Deferred Consideration [Line Item]    
Forward-looking gold price $ 2,093 $ 2,117