SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Products, Properties and Production of Minerals (Details)	12 Months Ended
	Dec. 31, 2019
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