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Asset Retirement Obligations Asset Retirement Obligations (Details) - USD ($)
$ in Thousands
3 Months Ended 9 Months Ended 12 Months Ended
Sep. 30, 2021
Sep. 30, 2020
Sep. 30, 2021
Sep. 30, 2020
Dec. 31, 2020
Dec. 31, 2019
Asset Retirement Obligation Disclosure [Abstract]            
Asset Retirement Obligation $ 5,198   $ 5,198   $ 4,974 $ 4,447
Accretion expense 77 $ 90 226 $ 263 354  
Liabilities incurred for new wells and facilities construction     347   281  
Reductions due to plugged wells and facilities     (192)   (103)  
Revisions in estimates     (157)   (5)  
Asset retirement obligation - current portion $ 492   $ 492   $ 441