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Goodwill and Intangible Assets - Changes To The Net Book Value Of Goodwill And Intangible Assets (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Changes in intangible assets and goodwill [abstract]      
Intangible assets and goodwill, beginning balance $ 48.7 $ 49.9  
Additions 4.9 4.5  
Acquisitions through business combinations (note 3)   3.8  
Amortization (8.9) (9.4) $ (9.2)
Foreign exchange and other 0.1 (0.1)  
Intangible assets and goodwill, ending balance 44.8 48.7 49.9
Goodwill      
Changes in intangible assets and goodwill [abstract]      
Intangible assets and goodwill, beginning balance 23.2 19.5  
Acquisitions through business combinations (note 3)   3.7  
Foreign exchange and other 0.0 0.0  
Intangible assets and goodwill, ending balance 23.2 23.2 19.5
Other intangible assets      
Changes in intangible assets and goodwill [abstract]      
Intangible assets and goodwill, beginning balance 15.9 21.9  
Additions 0.0 0.0  
Acquisitions through business combinations (note 3)   0.0  
Amortization (5.5) (6.0)  
Foreign exchange and other 0.0 0.0  
Intangible assets and goodwill, ending balance 10.4 15.9 21.9
Computer software assets      
Changes in intangible assets and goodwill [abstract]      
Intangible assets and goodwill, beginning balance 9.6 8.5  
Additions 4.9 4.5  
Acquisitions through business combinations (note 3)   0.1  
Amortization (3.4) (3.4)  
Foreign exchange and other 0.1 (0.1)  
Intangible assets and goodwill, ending balance $ 11.2 $ 9.6 $ 8.5