MINERAL PROPERTY INTERESTS - Mineral Property Interests (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Mineral Property Interests
Amortization of mineral property interests and asset retirement obligations			$ 3,624	$ 1,065
El Gallo 1 mine
Mineral Property Interests
Amortization of mineral property interests and asset retirement obligations	$ 600	$ 600	1,400	1,100
Black Fox
Mineral Property Interests
Amortization of mineral property interests and asset retirement obligations	$ 1,200		$ 2,000	$ 0