MINERAL PROPERTY INTERESTS - Mineral Property Interests (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Mineral Property Interests
Amortization of mineral property interests and asset retirement obligations			$ 5,437	$ 1,513
Black Fox
Mineral Property Interests
Amortization of mineral property interests and asset retirement obligations	$ 1,000	$ 0	3,100	0
El Gallo Project
Mineral Property Interests
Amortization of mineral property interests and asset retirement obligations	$ 400	$ 400	$ 1,800	$ 1,500