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Asset Retirement Obligations - Changes in Asset Retirement Obligation Liabilities (Details)
$ in Thousands
9 Months Ended
Sep. 30, 2021
USD ($)
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]  
Balance at December 31, 2020 $ 7,624
Accretion 462
Amounts paid (184)
Reclassifications 0
Balance at September 30, 2021 7,902
Short-Term Portion  
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]  
Balance at December 31, 2020 335
Accretion (94)
Amounts paid (184)
Reclassifications 395
Balance at September 30, 2021 452
Long-Term Portion  
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]  
Balance at December 31, 2020 7,289
Accretion 556
Amounts paid 0
Reclassifications (395)
Balance at September 30, 2021 $ 7,450