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Deferred Consideration - Summary of valuation of deferred consideration (Detail) - USD ($)
$ in Thousands
 9 Months Ended 12 Months Ended
Sep. 30, 2020
Dec. 31, 2019
Disclosure Of Valuation Of Deferred Consideration [Line Item]    
Discount rate 10.00% 10.00%
Perpetual growth rate 0.90% 1.50%
Minimum [Member]    
Disclosure Of Valuation Of Deferred Consideration [Line Item]    
Forward-looking gold price $ 1,889 $ 1,535
Maximum [Member]    
Disclosure Of Valuation Of Deferred Consideration [Line Item]    
Forward-looking gold price 2,678 2,328
Weighted Average [Member]    
Disclosure Of Valuation Of Deferred Consideration [Line Item]    
Forward-looking gold price $ 2,104 $ 1,757