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SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Products, Properties and Production of Minerals (Details)
 12 Months Ended
Dec. 31, 2020
SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES  
Option to sell prior to the completion of refining (as a percent) 90.00%
Minimum  
SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES  
Period over which concentrates are provisionally priced at the end of a period after delivery to the customer 30 days
Maximum  
SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES  
Period over which concentrates are provisionally priced at the end of a period after delivery to the customer 90 days