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INCOME TAXES - Schedule of Deferred Tax Assets and Liabilities (Details) - USD ($) $ in Thousands | Dec. 31, 2017 | Dec. 31, 2016
--- | --- | ---
Current deferred tax assets |  |
Deferred revenue | $ 0 | $ 0
Inventories | 2,148 | 2,900
Short-term investments | 1,216 | 413
Total current deferred tax assets | 3,364 | 3,313
Non-current deferred tax assets |  |
Operating loss carry forwards | 74,644 | 91,441
Capital loss carry forwards | 15,286 | 21,322
Deferred revenue and other | 3,695 | 3,885
Mineral properties and deferred costs | 28,080 | 40,581
Asset retirement obligations | 4,844 | 6,398
Intangibles and other | (663) | (2,524)
Property, plant and equipment | 845 |
Deferred Tax Liabilities, Property, Plant and Equipment |  | (751)
Total non-current deferred tax assets | 126,731 | 160,352
Subtotal deferred tax asset | 130,095 | 163,665
Less: valuation allowance | (130,095) | (163,665)
Net deferred tax asset | $ 0 | $ 0

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