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Fair Value Measurements - Schedule of Reconciliation of Changes in Assets and Liabilities Measured at Fair Value On a Recurring Basis (Details) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Net unrealized gains on oil price risk derivatives $ 160  
Recurring Measurements [Member]    
Fair value 1,976 $ 1,302
Acquisition of investment 777 750
Issuance of warrants   1,240
Other comprehensive loss (8) 3
Net unrealized gains on oil price risk derivatives (26)
Net unrealized gain on warrant valuation 170 (210)
Net unrealized loss on Anfield Shares (839) (58)
Offset of derivative assets and liabilities   (194)
Cash settlements paid (135) (857)
Fair value 1,915 1,976
Level 2 [Member] | Recurring Measurements [Member] | Crude Oil Derivative , Assets [Member]    
Fair value 1,634
Acquisition of investment
Issuance of warrants  
Other comprehensive loss
Net unrealized gains on oil price risk derivatives
Net unrealized gain on warrant valuation
Net unrealized loss on Anfield Shares
Offset of derivative assets and liabilities   (194)
Cash settlements paid (1,440)
Fair value
Level 2 [Member] | Recurring Measurements [Member] | Crude Oil Derivative , Liabilities [Member]    
Fair value
Acquisition of investment
Issuance of warrants  
Other comprehensive loss
Net unrealized gains on oil price risk derivatives (26)
Net unrealized gain on warrant valuation
Net unrealized loss on Anfield Shares
Offset of derivative assets and liabilities  
Cash settlements paid (135)
Fair value (161)
Level 2 [Member] | Recurring Measurements [Member] | Sutter Gold Mining Company [Member] | Equity Securities [Member]    
Fair value 16 13
Acquisition of investment
Issuance of warrants  
Other comprehensive loss (8) 3
Net unrealized gains on oil price risk derivatives
Net unrealized gain on warrant valuation
Net unrealized loss on Anfield Shares
Offset of derivative assets and liabilities  
Cash settlements paid
Fair value 8 16
Level 1 [Member] | Recurring Measurements [Member] | Anfield [Member] | Equity Securities [Member]    
Fair value 930 238
Acquisition of investment 777 750
Issuance of warrants  
Other comprehensive loss
Net unrealized gains on oil price risk derivatives
Net unrealized gain on warrant valuation
Net unrealized loss on Anfield Shares (839) (58)
Offset of derivative assets and liabilities  
Cash settlements paid
Fair value 868 930
Level 3 [Member] | Recurring Measurements [Member] | Executive Retirement Programme Liabilities [Member]    
Fair value (583)
Acquisition of investment
Issuance of warrants  
Other comprehensive loss
Net unrealized gains on oil price risk derivatives
Net unrealized gain on warrant valuation
Net unrealized loss on Anfield Shares
Offset of derivative assets and liabilities  
Cash settlements paid 583
Fair value
Level 3 [Member] | Recurring Measurements [Member] | Warrant Liability [Member]    
Fair value 1,030
Acquisition of investment
Issuance of warrants   1,240
Other comprehensive loss
Net unrealized gains on oil price risk derivatives
Net unrealized gain on warrant valuation 170 (210)
Net unrealized loss on Anfield Shares
Offset of derivative assets and liabilities  
Cash settlements paid
Fair value $ 1,200 $ 1,030