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Fair Value Measurements - Schedule of Recurring Measurements of Fair Value of Assets and Liabilities (Details) - USD ($)
$ in Thousands
Jun. 30, 2018
Dec. 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Outstanding warrant liability $ 1,010 $ 1,200
Recurring Measurements [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Marketable equity securities 753 876
Commodity price risk derivatives 24 161
Outstanding warrant liability 1,010 1,200
Total 1,034 1,361
Sutter Gold Mining Company [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Marketable equity securities 10 8
Anfield Resources, Inc. [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Marketable equity securities 743 868
Level 1 [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Marketable equity securities 753 878
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 10 8
Level 1 [Member] | Anfield Resources, Inc. [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Marketable equity securities 743 868
Level 2 [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Marketable equity securities
Commodity price risk derivatives 24 161
Outstanding warrant liability
Total 24 161
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 1,010 1,200
Total 1,010 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