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Asset Retirement Obligations (Narrative) (Details) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
asset retirement obligations [Line Items]    
Adjustments to the liability from changes in estimates $ (14,889) $ (11,438)
Surety bonds outstanding 31,700  
Letters of credit outstanding 20,000  
Cash collateral for surety bond 600  
Asset retirement obligations    
asset retirement obligations [Line Items]    
Surety bonds outstanding $ 552,400