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Segment Information (Segment Activity) (Details) - USD ($) $ in Thousands | 3 Months Ended | 12 Months Ended
--- | --- | ---
Dec. 31, 2017 | Sep. 30, 2017 | Jun. 30, 2017 | Mar. 31, 2017 | Dec. 31, 2016 | Sep. 30, 2016 | Jun. 30, 2016 | Mar. 31, 2016 | Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |  |  |
Revenues-oil and natural gas sales | $ 17,156 | $ 18,178 | $ 20,425 | $ 21,266 | $ 15,326 | $ 14,635 | $ 18,847 | $ 10,976 | $ 77,025 | $ 59,784 | $ 80,445
Depreciation, depletion and amortization |  |  |  |  |  |  |  |  | 6,457 | 6,926 | 32,998
Impairment of proved properties |  |  |  |  |  |  |  |  |  | 88 | 81,322
Bad debt expense and other |  |  |  |  |  |  |  |  | 452 | 1,222 | 2,968
Other operating expense |  |  |  |  |  |  |  |  |  | 8,853 |
Operating income (loss) | 2,495 | $ 3,721 | $ 5,587 | $ 8,148 | 819 | $ 3,690 | $ 4,615 | $ (13,515) | 19,951 | (4,391) | (103,106)
Other, net |  |  |  |  |  |  |  |  | 2,113 | (2,015) | (1,536)
Interest expense, net |  |  |  |  |  |  |  |  | (1,414) | (2,613) | (1,325)
Income tax expense |  |  |  |  |  |  |  |  | 10,378 | 9,248 | 14,587
Additions to properties and equipment - accrual |  |  |  |  |  |  |  |  | 1,702 | (4,061) | 66,419
Total assets | 79,633 |  |  |  | 81,032 |  |  |  | 79,633 | 81,032 |
Continuing Operations [Member] |  |  |  |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |  |  |
Total assets | 23,221 |  |  |  | 28,019 |  |  |  | 23,221 | 28,019 |
Operating Segments [Member] | Gabon Segment [Member] |  |  |  |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |  |  |
Revenues-oil and natural gas sales |  |  |  |  |  |  |  |  | 76,978 | 59,460 | 79,947
Depreciation, depletion and amortization |  |  |  |  |  |  |  |  | 6,196 | 6,531 | 32,125
Impairment of proved properties |  |  |  |  |  |  |  |  |  |  | 78,080
Bad debt expense and other |  |  |  |  |  |  |  |  | 452 | 1,222 | 2,968
Other operating expense |  |  |  |  |  |  |  |  |  | 8,853 |
Operating income (loss) |  |  |  |  |  |  |  |  | 28,488 | 3,901 | (87,243)
Other, net |  |  |  |  |  |  |  |  | 3,142 | (22) | (1,034)
Interest expense, net |  |  |  |  |  |  |  |  | (1,414) | (2,614) | (1,144)
Income tax expense |  |  |  |  |  |  |  |  | 11,638 | 9,248 | 13,238
Additions to properties and equipment - accrual |  |  |  |  |  |  |  |  | 1,576 | (4,242) | 66,269
Operating Segments [Member] | Equatorial Guinea Segment [Member] |  |  |  |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |  |  |
Operating income (loss) |  |  |  |  |  |  |  |  | (122) | (384) | (1,342)
Other, net |  |  |  |  |  |  |  |  | 15 | (8) | (33)
Operating Segments [Member] | U. S. Segment [Member] |  |  |  |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |  |  |
Revenues-oil and natural gas sales |  |  |  |  |  |  |  |  | 47 | 324 | 498
Depreciation, depletion and amortization |  |  |  |  |  |  |  |  | 1 | 151 | 633
Impairment of proved properties |  |  |  |  |  |  |  |  |  | 88 | 3,242
Operating income (loss) |  |  |  |  |  |  |  |  | 352 | (72) | (4,366)
Operating Segments [Member] | Continuing Operations [Member] | Gabon Segment [Member] |  |  |  |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |  |  |
Total assets | 12,638 |  |  |  | 17,291 |  |  |  | 12,638 | 17,291 |
Operating Segments [Member] | Continuing Operations [Member] | Equatorial Guinea Segment [Member] |  |  |  |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |  |  |
Total assets | 10,000 |  |  |  | 10,000 |  |  |  | 10,000 | 10,000 |
Corporate And Other [Member] |  |  |  |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |  |  |
Depreciation, depletion and amortization |  |  |  |  |  |  |  |  | 260 | 244 | 240
Operating income (loss) |  |  |  |  |  |  |  |  | (8,767) | (7,836) | (10,155)
Other, net |  |  |  |  |  |  |  |  | (1,044) | (1,985) | (469)
Interest expense, net |  |  |  |  |  |  |  |  |  | 1 | (181)
Income tax expense |  |  |  |  |  |  |  |  | (1,260) |  | 1,349
Additions to properties and equipment - accrual |  |  |  |  |  |  |  |  | 126 | 181 | $ 150
Corporate And Other [Member] | Continuing Operations [Member] |  |  |  |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |  |  |
Total assets | $ 583 |  |  |  | $ 728 |  |  |  | $ 583 | $ 728 |

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