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Accounting Policies - Asset Retirement Obligations (Details) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]      
Beginning balance $ 5,588 $ 5,191  
Accretion expense 427 397 $ 359
Ending balance $ 6,015 $ 5,588 $ 5,191