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Financial Instruments and Risk Management (Fair Value for Assets and Liabilities) (Details) - USD ($) $ in Millions | Dec. 31, 2020 | Dec. 31, 2019
--- | --- | ---
Disclosure of fair value measurement of assets [line items] |  |
Financial assets (liabilities) | $ 3,636.5 | $ 3,396.5
Financial liabilities | (785.0) | (701.2)
Level 1 [member] | Recurring fair value measurement [member] |  |
Disclosure of fair value measurement of assets [line items] |  |
Fair value, net asset (liability) | 43.7 | 22.8
Level 2 [member] | Recurring fair value measurement [member] |  |
Disclosure of fair value measurement of assets [line items] |  |
Fair value, net asset (liability) | 4.4 | 2.6
Fair value through profit or loss | Gold Options [Member] | Level 2 [member] | Recurring fair value measurement [member] |  |
Disclosure of fair value measurement of assets [line items] |  |
Financial liabilities | 0.0 | (0.7)
Fair value through OCI | Currency Derivative Instruments [Member] | Level 2 [member] | Recurring fair value measurement [member] |  |
Disclosure of fair value measurement of assets [line items] |  |
Financial liabilities |  | 3.3
Fair value through OCI | Fuel Options [Member] | Level 2 [member] | Recurring fair value measurement [member] |  |
Disclosure of fair value measurement of assets [line items] |  |
Financial liabilities |  | 0.0
Fair value through OCI | Trading securities [member] | Level 1 [member] | Recurring fair value measurement [member] |  |
Disclosure of fair value measurement of assets [line items] |  |
Financial assets (liabilities) | 43.7 | $ 22.8
Fair value through OCI | Currency Hedging Derivative Instruments [Member] | Level 2 [member] | Recurring fair value measurement [member] |  |
Disclosure of fair value measurement of assets [line items] |  |
Financial assets (liabilities) | $ 4.3 |

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