Quarterly report pursuant to Section 13 or 15(d)

Segment Information (Segment Activity) (Details)

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Segment Information (Segment Activity) (Details) - USD ($)
$ in Thousands
 3 Months Ended 6 Months Ended
Jun. 30, 2018
Jun. 30, 2017
Jun. 30, 2018
Jun. 30, 2017
Segment Reporting Information [Line Items]        
Revenues-oil and natural gas sales $ 24,426 $ 20,425 $ 52,071 $ 41,691
Depreciation, depletion and amortization 1,035 1,970 2,159 3,839
Bad debt expense and other 145 183 89 281
Operating income (loss) 5,723 5,587 18,761 13,735
Other, net (1,224) 338 (1,155) 222
Interest expense, net (30) (378) (384) (781)
Income tax expense 3,582 3,096 7,624 6,290
Additions to properties and equipment - accrual (542) (625) (969) (814)
Operating Segments [Member] | Gabon Segment [Member]        
Segment Reporting Information [Line Items]        
Revenues-oil and natural gas sales 24,425 20,415 52,068 41,661
Depreciation, depletion and amortization 971 1,902 2,030 3,711
Bad debt expense and other 145 183 89 281
Operating income (loss) 10,147 8,090 25,844 19,050
Other, net (199) 237 (130) 299
Interest expense, net (43) (378) (397) (781)
Income tax expense 3,582 3,096 7,624 6,290
Additions to properties and equipment - accrual (527) (625) (955) (814)
Operating Segments [Member] | Equatorial Guinea Segment [Member]        
Segment Reporting Information [Line Items]        
Operating income (loss) (85) (15) (115) (53)
Other, net (6) 7 (3) 9
Corporate And Other [Member]        
Segment Reporting Information [Line Items]        
Revenues-oil and natural gas sales 1 10 3 30
Depreciation, depletion and amortization 64 68 129 128
Operating income (loss) (4,339) (2,488) (6,968) (5,262)
Other, net (1,019) $ 94 (1,022) $ (86)
Interest expense, net 13   13  
Additions to properties and equipment - accrual $ (15)   $ (14)