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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