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Fair Value Measurements - Schedule of Recurring Measurements of the Fair Value of Assets and Liabilities (Details) - USD ($) $ in Thousands | Dec. 31, 2017 | Dec. 31, 2016
--- | --- | ---
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Outstanding warrant liability | $ 1,200 | $ 1,030
Recurring Measurements [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Marketable equity securities | 876 | 946
Outstanding warrant liability | 1,200 | 1,030
Crude oil price risk derivatives | 161 |
Total | 1,361 | 1,030
Sutter Gold Mining Company [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Marketable equity securities | 8 | 16
Anfield Resources, Inc. [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Marketable equity securities | 868 | 930
Level 1 [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Marketable equity securities | 868 | 930
Outstanding warrant liability |  |
Crude oil price risk derivatives |  |
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 |  |
Level 1 [Member] | Anfield Resources, Inc. [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Marketable equity securities | 868 | 930
Level 2 [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Marketable equity securities | 8 | 16
Outstanding warrant liability |  |
Crude oil price risk derivatives | 161 |
Total | 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 | 8 | 16
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 |  |
Outstanding warrant liability | 1,200 | 1,030
Crude oil price risk derivatives |  |
Total | 1,200 | 1,030
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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