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Share Capital (Narrative) (Details) - USD ($)
$ / shares in Units, shares in Millions, $ in Millions
9 Months Ended
Sep. 30, 2017
Sep. 30, 2016
Dec. 31, 2016
Class of Stock [Line Items]      
Treasury stock shares 92.5   95.9
Treasury shares held, as unvested restricted stock awards 9.2   9.3
Common shares market price (per share) $ 35.04    
Treasury shares market value $ 3,200.0    
Purchase of shares $ 58.6 $ 424.9