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Asset Retirement Obligations - Asset retirement obligation activity (Details) - USD ($)
$ in Thousands
3 Months Ended
Mar. 31, 2022
Mar. 31, 2021
Asset retirement obligations    
Beginning balance $ 12,833  
Liabilities incurred 1,001 $ 628
Accretion expense 459 $ 300
Liabilities settled (219)  
Ending balance $ 14,074