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Decommissioning Liabilities (Narrative) (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Asset Retirement Obligation Disclosure [Line Items]    
Estimated reclamation discount rate 2.30% 2.30%
Closure cost obligations discount rate 7.60% 7.40%
Bottom of range [member]    
Asset Retirement Obligation Disclosure [Line Items]    
Inflation rate 2.00% 2.00%
Top of range [member]    
Asset Retirement Obligation Disclosure [Line Items]    
Inflation rate 4.00% 4.00%
Provision for decommissioning, restoration and rehabilitation costs [member]    
Asset Retirement Obligation Disclosure [Line Items]    
Asset retirement obligation, undiscounted value $ 53.2 $ 51.3