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Financial Instruments and Risk Management (Foreign Currency Exchange Rate Risk) (Details) - USD ($)
$ in Millions
 12 Months Ended
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