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Net Income Per Share (Details) (USD $)
In Millions, except Share data, unless otherwise specified
 3 Months Ended 9 Months Ended
Sep. 30, 2013
Sep. 30, 2012
Sep. 30, 2013
Sep. 30, 2012
Net Income Per Share [Abstract]        
Net income $ 29.1 $ 27.7 $ 85.8 $ 63.3
Less: Dividends paid to participating securities (0.1) 0 (0.2) (0.1)
Less: Undistributed earnings available to participating securities (0.2) (0.3) (0.6) (0.7)
Undistributed and distributed earnings available to common shareholders $ 28.8 $ 27.4 $ 85.0 $ 62.5
Common Stock, Number of Shares, Par Value and Other Disclosures [Abstract]        
Average number of common shares outstanding 31,092,600 30,659,900 31,042,200 31,097,400
Effect of dilutive stock-based compensation 187,700 392,700 171,100 362,900
Average number of common and potential common shares outstanding 31,280,300 31,052,600 31,213,300 31,460,300