MINERAL PROPERTY INTERESTS - Mineral Property Interests (Details) - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017	Aug. 25, 2017
Mineral Property Interests
Mineral property interests	$ 296,034		$ 293,437
Payments to Acquire Mining Assets	4,084
Amortization of mineral property interests and asset retirement obligations	1,800	$ 533
El Gallo 1 mine
Mineral Property Interests
Amortization of mineral property interests and asset retirement obligations	800	$ (500)
Black Fox
Mineral Property Interests
Mineral property interests				$ 8,954
Amortization of mineral property interests and asset retirement obligations	$ 2,700		$ 0