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Deferred Consideration - Additional Information (Detail)
 12 Months Ended
Apr. 11, 2018
USD ($)
Dec. 31, 2018
USD ($)
oz
Business Acquisition [Line Items]    
Upfront consideration paid   $ 0
Deferred consideration $ 172,746,000 161,540,000
Deferred consideration, current   11,765,000
Deferred consideration, non-current   149,775,000
Discount rate used to value the deferred consideration 10.00%  
Contractual gold payments   8,512,000
Gain/loss on revaluation of deferred consideration   $ 12,220,000
April 1, 2018 through March 31, 2058 [Member]    
Business Acquisition [Line Items]    
Fixed payments ounces of gold per year | oz   9,500
April 1, 2058 and Thereafter [Member]    
Business Acquisition [Line Items]    
Fixed payments ounces of gold per year | oz   6,333