Net Income Per Share (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015
Numerator (basic and diluted):
Net income	$ 21.1	$ 18.8
Less: Dividends paid to participating securities	(0.1)	(0.2)
Less: Undistributed earnings available to participating securities	0.0	0.0
Undistributed and distributed earnings available to common stockholders	$ 21.0	$ 18.6
Denominator:
Average number of common shares outstanding (in shares)	30,294,900	30,178,300
Effect of dilutive stock-based compensation (in shares)	127,800	114,900
Average number of common and potential common shares outstanding (in shares)	30,422,700	30,293,200
Anti-dilutive stock options (in shares)	0	0