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Financial Instruments and Risk Management (Foreign Currency Exchange Rate Risk) (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Disclosure of nature and extent of risks arising from financial instruments [line items]    
Total financial instrument exposure to credit risk $ 228.1 $ 187.5
Unrealized gains $ (0.7) 0.6
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 $ 32.6 11.7
Impact on pre-tax earnings 3.3 1.2
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 3.9 15.6
Impact on pre-tax earnings 0.4 1.7
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 220.5 182.8
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 33.5 22.5
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 6.5 9.0
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 43.7 22.8
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 7.6 4.7
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 6.9 10.3
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 18.7 20.9
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 20.2 33.0
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 4.1 3.7
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 (71.7) (76.9)
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 $ (25.4) $ (18.0)