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Asset Retirement Obligations (Narrative) (Details) - USD ($)
$ in Thousands
3 Months Ended 9 Months Ended
Sep. 30, 2020
Sep. 30, 2020
Dec. 31, 2019
Segment Reporting Information [Line Items]      
Liabilities incurred during the period $ 9,800 $ 9,809  
ARO capitalized as property, plant and equipment   9,800  
Bellaire      
Segment Reporting Information [Line Items]      
Fair value of trust assets 10,300 10,300 $ 10,100
Centennial      
Segment Reporting Information [Line Items]      
Reduction in asset retirement obligation 4,800    
Cash paid 1,900    
Settlement receivable from third parties 1,400 1,400  
Accounts payable, cash to be paid to settle 1,900 1,900  
Other long-term liabilities, contingent and noncontingent guarantee liabilities 2,400 2,400  
Centennial | Surety Bond      
Segment Reporting Information [Line Items]      
Surety bonds outstanding to be transferred or replaced 10,700 10,700  
Centennial | Third Party | Surety Bond | Transfer Or Replacement Period One      
Segment Reporting Information [Line Items]      
Surety bonds outstanding to be transferred or replaced 4,900 4,900  
Centennial | Third Party | Surety Bond | Transfer Or Replacement Period Two      
Segment Reporting Information [Line Items]      
Surety bonds outstanding to be transferred or replaced $ 5,800 $ 5,800