SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Products, Properties and Production of Minerals (Details) - Gold and Silver	12 Months Ended
	Dec. 31, 2017
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
El Gallo 1 mine
SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES
Option to sell prior to the completion of refining (as a percent)	90.00%
Period of completion of refining	15 days