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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