MINERAL PROPERTY INTERESTS AND PLANT AND EQUIPMENT - Leased Assets (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Property, Plant and Equipment [Line Items]
Leased asset, cost	$ 484,126	$ 459,006
Accumulated amortization	(48,006)	(35,127)
Net book value	436,120	$ 423,879
Operating Leases
Property, Plant and Equipment [Line Items]
Leased asset, cost	1,167
Accumulated amortization	(413)
Net book value	754
Finance Leases
Property, Plant and Equipment [Line Items]
Leased asset, cost	8,048
Accumulated amortization	(515)
Net book value	$ 7,533

Amount of accumulated depreciation, depletion and amortization for physical assets used in the normal conduct of business to produce goods and services.

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Amount before accumulated depreciation, depletion and amortization of physical assets used in the normal conduct of business and not intended for resale. Examples include, but are not limited to, land, buildings, machinery and equipment, office equipment, and furniture and fixtures.

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