Net Income Per Share (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Numerator (basic and diluted):
Net income	$ 27.7	$ 41.0	$ 65.6	$ 87.3
Less: Dividends paid to participating securities	0.0	(0.1)	(0.2)	(0.2)
Less: Undistributed earnings available to participating securities	(0.1)	(0.2)	(0.2)	(0.4)
Undistributed and distributed earnings available to common stockholders	$ 27.6	$ 40.7	$ 65.2	$ 86.7
Denominator:
Average number of common shares outstanding (in shares)	30,662,800	30,569,600	30,648,400	30,541,600
Effect of dilutive stock-based compensation (in shares)	169,100	153,200	156,300	141,500
Average number of common and potential common shares outstanding (in shares)	30,831,900	30,722,800	30,804,700	30,683,100