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Accounting Policies - Asset Retirement Obligations (Details) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2021
Dec. 31, 2020
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]    
Beginning balance $ 6,015 $ 5,588
Accretion expense 461 427
Ending balance $ 6,476 $ 6,015