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Fair Value Measurements - Schedule of Recurring Measurements of Fair Value of Assets and Liabilities (Details) - USD ($) $ in Thousands | Sep. 30, 2018 | Dec. 31, 2017
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Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Outstanding warrant liability | $ 722 | $ 1,200
Recurring Measurements [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Marketable equity securities | 956 | 876
Commodity price risk derivatives | 37 | 161
Outstanding warrant liability | 722 | 1,200
Total | 759 | 1,361
Sutter Gold Mining Company [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Marketable equity securities | 5 | 8
Anfield Resources, Inc. [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Marketable equity securities | 951 | 868
Level 1 [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Marketable equity securities | 956 | 878
Commodity price risk derivatives |  |
Outstanding warrant liability |  |
Total |  |
Level 1 [Member] | Sutter Gold Mining Company [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Marketable equity securities | 5 | 8
Level 1 [Member] | Anfield Resources, Inc. [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Marketable equity securities | 951 | 868
Level 2 [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Marketable equity securities |  |
Commodity price risk derivatives | 37 | 161
Outstanding warrant liability |  |
Total | 37 | 161
Level 2 [Member] | Sutter Gold Mining Company [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Marketable equity securities |  |
Level 2 [Member] | Anfield Resources, Inc. [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Marketable equity securities |  |
Level 3 [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Marketable equity securities |  |
Commodity price risk derivatives |  |
Outstanding warrant liability | 722 | 1,200
Total | 722 | 1,200
Level 3 [Member] | Sutter Gold Mining Company [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Marketable equity securities |  |
Level 3 [Member] | Anfield Resources, Inc. [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Marketable equity securities |  |

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