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OPERATING SEGMENT REPORTING - Geographic information (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019	Dec. 31, 2019	Dec. 31, 2018
Operating Segment Reporting
Revenue from gold and silver sales	$ 18,291	$ 36,383	$ 49,691	$ 51,966
Long-lived Assets	445,412		445,412		$ 539,150
Investment in MSC	106,180		106,180		110,183
Lease assets	1,100		1,100
MSC
Operating Segment Reporting
Revenue from gold and silver sales	47,081	65,997	84,471	114,818
Investment in MSC	106,180		106,180		110,183	$ 127,814
United States
Operating Segment Reporting
Revenue from gold and silver sales	10,770	11,522	25,088	12,364
Long-lived Assets	50,906		50,906		135,854
Canada
Operating Segment Reporting
Revenue from gold and silver sales	4,166	16,049	16,905	24,992
Long-lived Assets	76,697		76,697		77,147
MEXICO
Operating Segment Reporting
Revenue from gold and silver sales	3,355	$ 8,812	7,698	$ 14,610
Long-lived Assets	20,139		20,139		23,551
ARGENTINA
Operating Segment Reporting
Long-lived Assets	297,670		297,670		302,598
ARGENTINA \| MSC
Operating Segment Reporting
Investment in MSC	$ 106,200		$ 106,200		$ 110,200