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Income Taxes (Narrative) (Details) $ in Thousands | 3 Months Ended | 9 Months Ended | 12 Months Ended |
--- | --- | --- | --- | ---
Sep. 30, 2018 USD ($) | Sep. 30, 2018 USD ($) item | Dec. 31, 2017 USD ($) | Dec. 31, 2016 USD ($)
Income Taxes [Line Items] |  |  |  |
Deferred tax assets | $ 1,300 | $ 1,300 | $ 154,500 |
Alternative minimum tax credit | 1,400 | $ 1,400 | 1,300 |
Valuation allowance |  |  | $ 153,200 |
U.S. corporate income tax rate |  | 21.00% | 35.00% |
Entitled Percent For Consortium After Initial Royalty Percentage |  | 70.00% |  |
Future Entitled Percent For Consortium After Initial Royalty Percentage |  | 80.00% |  |
Cost account associated with interest in profit, remaining portion |  |  | $ 97,600 |
Deferred tax asset, excess over the book basis | 46,500 | $ 46,500 |  |
Deferred tax liability, excess over the book basis | 9,800 | 9,800 |  |
Interest or penalties accrued |  |  | $ 0 | $ 0
Deferred income tax benefit | 19,677 | 19,677 |  |
Restatement Adjustment [Member] |  |  |  |
Income Taxes [Line Items] |  |  |  |
Valuation allowance | $ (29,900) | (29,900) |  |
Deferred income tax benefit |  | $ 19,900 |  |
Maximum [Member] |  |  |  |
Income Taxes [Line Items] |  |  |  |
Allocation of remaining profit production, government payments, percentage |  | 60.00% |  |
Planned wells in the next fiscal year | item |  | 3 |  |
Minimum [Member] |  |  |  |
Income Taxes [Line Items] |  |  |  |
Allocation of remaining profit production, government payments, percentage |  | 50.00% |  |
Planned wells in the next fiscal year | item |  | 2 |  |

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