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Inventory (Narrative) (Details) - USD ($) $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2019 | Dec. 31, 2018
Inventory [Line Items] |  |
Long-term inventory | $ 25.7 | $ 30.0
Mining and processing costs, inventories | 345.4 | 393.1
Amortization costs, Inventories | $ 165.0 | 166.6
Inventory write-down |  | 64.0
Impairment loss |  | 64.0
Impairment loss net of tax |  | 49.9
Mining [Member] |  |
Inventory [Line Items] |  |
Inventory write-down |  | 62.5
Amortization [Member] |  |
Inventory [Line Items] |  |
Inventory write-down |  | $ 1.5

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