Net Income Per Share (Details) (USD $) In Millions, except Share data, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013
Net Income Per Share [Abstract]
Net income	$ 25.0	$ 26.6	$ 48.2	$ 56.7
Less: Dividends paid to participating securities	(0.1)	0	(0.2)	(0.1)
Less: Undistributed earnings available to participating securities	(0.2)	(0.2)	(0.3)	(0.4)
Undistributed and distributed earnings available to common shareholders	$ 24.7	$ 26.4	$ 47.7	$ 56.2
Common Stock, Number of Shares, Par Value and Other Disclosures [Abstract]
Average number of common shares outstanding	30,096,500	31,097,300	30,379,700	31,021,900
Effect of dilutive stock-based compensation	114,200	162,200	113,200	162,700
Average number of common and potential common shares outstanding	30,210,700	31,259,500	30,492,900	31,184,600