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Deferred Consideration - Additional Information (Detail)
 3 Months Ended 6 Months Ended
Jun. 30, 2018
USD ($)
oz
Jun. 30, 2018
USD ($)
oz
Apr. 11, 2018
USD ($)
Business Acquisition [Line Items]      
Fixed payments ounces of gold per year | oz 9,500 9,500  
Upfront consideration paid   $ 0  
Deferred consideration $ 162,848,000 162,848,000 $ 172,746,000
Deferred consideration, current 11,378,000 11,378,000  
Deferred consideration, non-current 151,470,000 $ 151,470,000  
Discount rate used to value the deferred consideration   10.00%  
Contractual gold payments 2,715,000 $ 2,715,000  
Gain/loss on revaluation of deferred consideration $ 9,898,000 $ 9,898,000  
April 1, 2018 through March 31, 2058 [Member]      
Business Acquisition [Line Items]      
Fixed payments ounces of gold per year | oz 9,500 9,500  
April 1, 2058 and Thereafter [Member]      
Business Acquisition [Line Items]      
Fixed payments ounces of gold per year | oz 6,333 6,333