Quarterly report pursuant to Section 13 or 15(d)

Asset Retirement Obligations (Details)

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Asset Retirement Obligations (Details) - USD ($)
$ in Thousands
3 Months Ended 9 Months Ended
Sep. 30, 2021
Sep. 30, 2020
Sep. 30, 2021
Sep. 30, 2020
Asset Retirement Obligations        
Asset retirement obligations - beginning of period     $ 54,452  
Obligations incurred     2,359  
Accretion expense $ 828 $ 1,115 2,947 $ 3,330
Settlement of obligations     (50)  
Revisions to prior estimates     (549)  
Asset retirement obligations - end of period $ 59,159   $ 59,159