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Decommissioning Liabilities (Narrative) (Details) - USD ($) $ in Millions | 12 Months Ended
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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 |

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