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Financial Instruments and Risk Management (Gold Option Contracts) (Details) $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2017 USD ($) $ / oz oz | Dec. 31, 2016 USD ($)
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)
Gold Options [Member] | Commodity price risk [member] |  |
Disclosure of detailed information about hedged items [line items] |  |
Financial assets, at fair value | 0.5 | 3.3
Gains (losses) on change in fair value of derivatives | (0.6) | (1.2)
Unrealized gains | $ 2.6 | $ 3.3
Gold Options [Member] | 2017 [member] | Commodity price risk [member] |  |
Disclosure of detailed information about hedged items [line items] |  |
Ounces subject to contract | oz | 50,100 |
Purchase Put Options [Member] | Gold Options [Member] | 2017 [member] | Commodity price risk [member] |  |
Disclosure of detailed information about hedged items [line items] |  |
Average realized price per ounce | $ / oz | 1,270 |
Sold Call Option [Member] | Gold Options [Member] | 2017 [member] | Commodity price risk [member] |  |
Disclosure of detailed information about hedged items [line items] |  |
Average realized price per ounce | $ / oz | 1,444 |

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