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Financial Instruments and Risk Management (Market Price Risk) (Details) $ in Millions | 1 Months Ended | 12 Months Ended
--- | --- | ---
Aug. 31, 2018 USD ($) | Dec. 31, 2018 USD ($) | Dec. 31, 2017 USD ($) $ / oz
Disclosure of detailed information about hedged items [line items] |  |  |
Financial assets, at fair value |  |  | $ 0.4
Gains (losses) on change in fair value of derivatives | $ 0.1 |  |
Unrealized gains |  | $ 0.4 | 0.3
Market risk [member] |  |  |
Disclosure of detailed information about hedged items [line items] |  |  |
Potential percentage change in value |  | 1000.00% |
Impact on other comprehensive income |  | $ 0.8 | 3.5
Gold Options [Member] | Market risk [member] |  |  |
Disclosure of detailed information about hedged items [line items] |  |  |
Potential percentage change in value |  | 10.00% |
Impact on pre-tax earnings |  | $ 63.0 | $ 54.3
Bottom of range [member] | Gold Options [Member] | Market risk [member] |  |  |
Disclosure of detailed information about hedged items [line items] |  |  |
Average realized price per ounce | $ / oz |  |  | 1,146
Top of range [member] | Gold Options [Member] | Market risk [member] |  |  |
Disclosure of detailed information about hedged items [line items] |  |  |
Average realized price per ounce | $ / oz |  |  | 1,349

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