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Financial Instruments and Risk Management (Fair Value for Assets and Liabilities) (Details) - USD ($) $ in Millions | Dec. 31, 2017 | Dec. 31, 2016
--- | --- | ---
Disclosure of fair value measurement of assets [line items] |  |
Financial assets | $ 3,313.8 | $ 2,492.2
Financial liabilities | (632.6) | (732.8)
Level 1 [member] | Recurring fair value measurement [member] |  |
Disclosure of fair value measurement of assets [line items] |  |
Fair value, net asset (liability) | 35.7 | 11.7
Level 2 [member] | Recurring fair value measurement [member] |  |
Disclosure of fair value measurement of assets [line items] |  |
Fair value, net asset (liability) | 5.5 | 9.6
Fair value through profit or loss | Share Purchase Warrants [Member] | Level 1 [member] | Recurring fair value measurement [member] |  |
Disclosure of fair value measurement of assets [line items] |  |
Financial liabilities | (0.1) | (2.4)
Fair value through profit or loss | Share Purchase Warrants [Member] | Level 2 [member] | Recurring fair value measurement [member] |  |
Disclosure of fair value measurement of assets [line items] |  |
Financial liabilities | 0.0 | 0.0
Fair value through profit or loss | Currency Derivative Instruments [Member] | Level 1 [member] | Recurring fair value measurement [member] |  |
Disclosure of fair value measurement of assets [line items] |  |
Financial liabilities | 0.0 | 0.0
Fair value through profit or loss | Currency Derivative Instruments [Member] | Level 2 [member] | Recurring fair value measurement [member] |  |
Disclosure of fair value measurement of assets [line items] |  |
Financial liabilities | 0.0 | (3.3)
Fair value through profit or loss | Prepayment Option Embedded Derivative [Member] | Level 1 [member] | Recurring fair value measurement [member] |  |
Disclosure of fair value measurement of assets [line items] |  |
Financial assets | 0.0 | 0.0
Fair value through profit or loss | Prepayment Option Embedded Derivative [Member] | Level 2 [member] | Recurring fair value measurement [member] |  |
Disclosure of fair value measurement of assets [line items] |  |
Financial assets | 0.0 | 9.6
Fair value through profit or loss | Gold Options [Member] | Level 1 [member] | Recurring fair value measurement [member] |  |
Disclosure of fair value measurement of assets [line items] |  |
Financial assets | 0.0 | 0.0
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 assets | 0.5 | 3.3
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 | 35.8 | 14.1
Fair value through OCI | Trading securities [member] | Level 2 [member] | Recurring fair value measurement [member] |  |
Disclosure of fair value measurement of assets [line items] |  |
Financial assets | 0.0 | 0.0
Fair value through OCI | Currency Hedging Derivative Instruments [Member] | Level 1 [member] | Recurring fair value measurement [member] |  |
Disclosure of fair value measurement of assets [line items] |  |
Financial assets | 0.0 | 0.0
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 | $ 5.0 | $ 0.0

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