MINERAL PROPERTY INTERESTS AND PLANT AND EQUIPMENT - Leased Assets (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Property, Plant and Equipment [Line Items]
Leased asset, cost	$ 482,153	$ 459,006
Accumulated amortization	(57,316)	(35,127)
Net book value	424,837	$ 423,879
Operating Leases
Property, Plant and Equipment [Line Items]
Leased asset, cost	1,167
Accumulated amortization	(441)
Net book value	726
Finance Leases
Property, Plant and Equipment [Line Items]
Leased asset, cost	8,048
Accumulated amortization	(864)
Net book value	$ 7,184