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Financial Instruments and Risk Management (Foreign Currency Exchange Rate Risk) (Details) - USD ($) $ in Millions | 12 Months Ended
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Dec. 31, 2017 | Dec. 31, 2016
Disclosure of nature and extent of risks arising from financial instruments [line items] |  |
Total financial instrument exposure to credit risk | $ 204.8 | $ 258.1
Financial liabilities, at fair value |  | 2.4
Unrealized gains | $ 0.3 | (1.6)
Currency risk [member] |  |
Disclosure of nature and extent of risks arising from financial instruments [line items] |  |
Potential percentage change in value | 10.00% |
Canadian dollar | Currency risk [member] |  |
Disclosure of nature and extent of risks arising from financial instruments [line items] |  |
Total financial instrument exposure to credit risk | $ (3.2) | (7.7)
Impact on pre-tax earnings | 7.8 | 2.3
Mexican peso | Currency risk [member] |  |
Disclosure of nature and extent of risks arising from financial instruments [line items] |  |
Total financial instrument exposure to credit risk | 14.1 | 29.4
Impact on pre-tax earnings | 1.5 | 0.7
Cash and Cash Equivalents [Member] |  |
Disclosure of nature and extent of risks arising from financial instruments [line items] |  |
Total financial instrument exposure to credit risk | 200.8 | 252.2
Cash and Cash Equivalents [Member] | Canadian dollar | Currency risk [member] |  |
Disclosure of nature and extent of risks arising from financial instruments [line items] |  |
Total financial instrument exposure to credit risk | 17.2 | 22.8
Cash and Cash Equivalents [Member] | Mexican peso | Currency risk [member] |  |
Disclosure of nature and extent of risks arising from financial instruments [line items] |  |
Total financial instrument exposure to credit risk | 6.8 | 7.3
Equity Securities [Member] | Canadian dollar | Currency risk [member] |  |
Disclosure of nature and extent of risks arising from financial instruments [line items] |  |
Total financial instrument exposure to credit risk | 35.8 | 14.1
Equity Securities [Member] | Mexican peso | Currency risk [member] |  |
Disclosure of nature and extent of risks arising from financial instruments [line items] |  |
Total financial instrument exposure to credit risk | 0.0 | 0.0
Trade receivables [member] |  |
Disclosure of nature and extent of risks arising from financial instruments [line items] |  |
Total financial instrument exposure to credit risk | 4.0 | 5.9
Trade receivables [member] | Canadian dollar | Currency risk [member] |  |
Disclosure of nature and extent of risks arising from financial instruments [line items] |  |
Total financial instrument exposure to credit risk | 7.4 | 3.3
Trade receivables [member] | Mexican peso | Currency risk [member] |  |
Disclosure of nature and extent of risks arising from financial instruments [line items] |  |
Total financial instrument exposure to credit risk | 22.0 | 38.2
Accounts Payable And Accrued Liabilities [Member] | Canadian dollar | Currency risk [member] |  |
Disclosure of nature and extent of risks arising from financial instruments [line items] |  |
Total financial instrument exposure to credit risk | (56.1) | (42.7)
Accounts Payable And Accrued Liabilities [Member] | Mexican peso | Currency risk [member] |  |
Disclosure of nature and extent of risks arising from financial instruments [line items] |  |
Total financial instrument exposure to credit risk | (14.7) | (16.1)
Current portion of financing obligations [Member] | Canadian dollar | Currency risk [member] |  |
Disclosure of nature and extent of risks arising from financial instruments [line items] |  |
Total financial instrument exposure to credit risk | (4.2) | (1.5)
Current portion of financing obligations [Member] | Mexican peso | Currency risk [member] |  |
Disclosure of nature and extent of risks arising from financial instruments [line items] |  |
Total financial instrument exposure to credit risk | 0.0 | 0.0
Financial Obligations [Member] | Canadian dollar | Currency risk [member] |  |
Disclosure of nature and extent of risks arising from financial instruments [line items] |  |
Total financial instrument exposure to credit risk | (3.3) | (3.7)
Financial Obligations [Member] | Mexican peso | Currency risk [member] |  |
Disclosure of nature and extent of risks arising from financial instruments [line items] |  |
Total financial instrument exposure to credit risk | $ 0.0 | $ 0.0

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