SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Ore Stockpile Inventory (Details)	12 Months Ended	64 Months Ended	76 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2018
El Gallo 1 mine
Stockpiles, Material on Leach Pads, In-process Inventory, Precious Metals Inventory and Materials and Supplies
Cumulative metallurgical recovery rate for gold production (as a percent)		57.00%	61.00%
Minimum
Stockpiles, Material on Leach Pads, In-process Inventory, Precious Metals Inventory and Materials and Supplies
Recovery percentage of leach pads of the recoverable ounces in the first year of leaching	50.00%
Maximum
Stockpiles, Material on Leach Pads, In-process Inventory, Precious Metals Inventory and Materials and Supplies
Recovery percentage of leach pads of the recoverable ounces in the first year of leaching	95.00%