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Inventory (Narrative) (Details) - USD ($) $ in Millions | 12 Months Ended
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Dec. 31, 2020 | Dec. 31, 2019
Disclosure of inventories [Abstract] |  |
Mining and processing costs, inventories | $ 319.9 | $ 345.4
Amortization | 152.7 | 165.0
Amortization costs, Inventories |  | $ 165.0
Indirect Production Costs | 5.4 |
Reclassified Amortization Costs | $ 1.1 |

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