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