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Income Taxes (Narrative) (Details) $ in Thousands | 12 Months Ended |
--- | --- | ---
Dec. 31, 2018 USD ($) item | Dec. 31, 2017 USD ($) | Dec. 31, 2016 USD ($)
Income Taxes [Line Items] |  |  |
Interest or penalties accrued | $ 0 | $ 0 |
Deferred tax assets | 40,077 | 1,260 |
Alternative minimum tax credit | 674 | 1,349 |
Valuation allowance | $ 90,935 | $ 153,205 | $ 211,800
U.S. corporate income tax rate | 21.00% | 35.00% |
Cost account associated with interest in profit, remaining portion |  | $ 97,600 |
Change in valuation allowance | $ (16,500) | $ 57,600 |
Prior Production Sharing Contract, Through September 17, 2018 [Member] |  |  |
Income Taxes [Line Items] |  |  |
Entitled percent for consortium after initial royalty percentage | 70.00% |  |
Production Sharing Contract, September 17, 2018 Through September 16, 2028 [Member] |  |  |
Income Taxes [Line Items] |  |  |
Entitled percent for consortium after initial royalty percentage | 80.00% |  |
Production Sharing Contract, After September 16, 2028 [Member] |  |  |
Income Taxes [Line Items] |  |  |
Entitled percent for consortium after initial royalty percentage | 70.00% |  |
Expected Refunded Between 2018 And 2021 [Member] |  |  |
Income Taxes [Line Items] |  |  |
Alternative minimum tax credit | $ 1,400 |  |
Minimum |  |  |
Income Taxes [Line Items] |  |  |
Allocation of remaining profit production, government payments, percentage | 50.00% |  |
Planned wells in the next fiscal year | item | 2 |  |
Maximum |  |  |
Income Taxes [Line Items] |  |  |
Allocation of remaining profit production, government payments, percentage | 60.00% |  |
Planned wells in the next fiscal year | item | 3 |  |

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