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Decommissioning Liabilities (Narrative) (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Asset Retirement Obligation Disclosure [Line Items]    
Estimated reclamation discount rate 1.20% 1.80%
Closure cost obligations discount rate 10.50% 10.50%
Provision for decommissioning, restoration and rehabilitation costs $ 76.6  
Bottom of range [member]    
Asset Retirement Obligation Disclosure [Line Items]    
Inflation rate 1.50% 1.50%
Top of range [member]    
Asset Retirement Obligation Disclosure [Line Items]    
Inflation rate 9.50% 9.50%
Provision for decommissioning, restoration and rehabilitation costs [member]    
Asset Retirement Obligation Disclosure [Line Items]    
Asset retirement obligation, undiscounted value   $ 66.1
Reclamation, Care and Maintenance Expenses $ 3.5 $ 1.9
Reclamation Expenses, Noncurrent $ 57.1