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Asset Retirement Obligations (Details) - USD ($)
12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Reconciliation of changes in the asset retirement obligation    
Beginning Balance $ 15,300,000 $ 14,600,000
Liabilities settled (100,000) (1,200,000)
Accretion expense 1,100,000 1,000,000.0
Revisions in estimated cash flows(2) (4,900,000) 900,000
Ending Balance 11,400,000 15,300,000
Current portion of asset retirement obligation liabilities $ 0 $ 900,000