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Summary of Significant Accounting Policies - Asset Retirement Obligations (Details) - USD ($)
$ in Thousands
1 Months Ended 12 Months Ended
Sep. 30, 2020
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Asset Retirement Obligations        
Asset retirement obligation, period increase (decrease) $ 800      
Asset retirement obligations   $ 2,284 $ 2,825  
Estimated asset retirement obligations   3,300 3,300  
Accretion reserve   $ (389)    
Accretion expense     215 $ 202
Specialty rental costs        
Asset Retirement Obligations        
Accretion expense     $ 200 $ 200