Net Income Per Share (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Numerator (basic and diluted):
Net income	$ 20.5	$ 13.7
Less: Dividends paid to participating securities	(0.1)	(0.1)
Less: Undistributed earnings available to participating securities	0.0	0.0
Undistributed and distributed earnings available to common stockholders	$ 20.4	$ 13.6
Denominator:
Average number of common shares outstanding (in shares)	30,488,600	30,364,700
Effect of dilutive stock-based compensation (in shares)	131,100	122,600
Average number of common and potential common shares outstanding (in shares)	30,619,700	30,487,300