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Exploration Expense (Tables)
12 Months Ended
Dec. 31, 2020
ExplorationExpensesDisclosureAbstract [Abstract]  
Summary of exploration expenses
      
UNITED STATES DOLLAR
 
      
2020
  2019  2018 
6.
  
EXPLORATION EXPENSE
    
  Australia  
 
(16.9
  (30.0  (37.7
  Ghana  
 
—  
 
  —     (0.4
  Peru  
 
(1.4
  (4.4  (1.1
  Chile  
 
(30.1
  (49.1  (61.5
  Other  
 
(1.3
  (0.9  (3.5
    
 
 
  
 
 
  
 
 
 
  
Total exploration expense
  
 
(49.7
  (84.4  (104.2