Annual report pursuant to Section 13 and 15(d)

Oil And Natural Gas Properties And Equipment (Narrative) (Details)

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Oil And Natural Gas Properties And Equipment (Narrative) (Details) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2015
Dec. 31, 2016
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Undeveloped Acreage $ 10,000   $ 10,000
U.S. Fields [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Carrying value of fields   $ 1,200  
Impairment of oil and gas properties   3,200  
Block P Offshore Equatorial Guinea [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Undeveloped Acreage $ 10,000    
Working interest ownership, percentage 31.00%    
Period of development 25 years    
Etame Field [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Carrying value of fields   12,700  
Impairment of oil and gas properties   78,100  
Roosevelt County, Montana [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Impairment of Leasehold   $ 1,200