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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, 2018 | Dec. 31, 2017
Disclosure of nature and extent of risks arising from financial instruments [line items] |  |
Total financial instrument exposure to credit risk | $ 214.0 | $ 208.0
Financial liabilities, at fair value | 0.1 |
Unrealized gains | $ 0.4 | 0.3
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 | $ 10.4 | 78.0
Impact on pre-tax earnings | 1.4 | 7.8
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 | 21.3 | 14.4
Impact on pre-tax earnings | 2.1 | 1.5
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 | 206.0 | 200.8
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 | 26.8 | 43.1
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 | 10.8 | 7.1
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 | 7.8 | 35.8
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 | 5.7 | 4.0
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 | 11.9 | 7.4
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 | 21.4 | 22.0
Other monetary assets [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 | 33.0 | 47.8
Other monetary assets [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 | 5.9 | 0.0
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 | (69.1) | (56.1)
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 | $ (16.8) | $ (14.7)

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