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Financial Instruments and Risk Management (Market Price Risk) (Details) $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2017 USD ($) | Dec. 31, 2016 USD ($) $ / 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 | 2.3 | $ 1.6
Unrealized gains | $ 0.3 | (1.6)
Market risk [member] |  |
Disclosure of detailed information about hedged items [line items] |  |
Potential percentage change in value | 1000.00% |
Impact on other comprehensive income | $ 3.5 | 1.4
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 | $ 54.3 | $ 48.2
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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