Net Income Per Share (Details) (USD $) In Millions, except Share data, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013
Numerator (basic and diluted):
Net income	$ 23.0	$ 29.1	$ 71.2	$ 85.8
Less: Dividends paid to participating securities	(0.1)	(0.1)	(0.3)	(0.2)
Less: Undistributed earnings available to participating securities	(0.1)	(0.2)	(0.4)	(0.6)
Undistributed and distributed earnings available to common stockholders	$ 22.8	$ 28.8	$ 70.5	$ 85.0
Denominator:
Average number of common shares outstanding (in shares)	30,098,300	31,092,600	30,284,900	31,042,200
Effect of dilutive stock-based compensation (in shares)	127,200	187,700	117,800	171,100
Average number of common and potential common shares outstanding (in shares)	30,225,500	31,280,300	30,402,700	31,213,300