Stockholders' Equity - Basic and Diluted Shares Reconciliation (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Numerator (basic and diluted):
Net income	$ (27.3)	$ 25.8	$ 22.3	$ 13.7	$ 17.0	$ 18.7	$ 26.0	$ 21.1	$ 34.5	$ 82.8	$ 89.7
Less: Dividends paid to participating securities									(0.4)	(0.3)	(0.2)
Less: Undistributed earnings available to participating securities									0.0	(0.2)	(0.3)
Undistributed and distributed earnings available to common stockholders									$ 34.1	$ 82.3	$ 89.2
Denominator:
Average number of common shares outstanding (shares)									30,407,100	30,310,900	30,251,400
Effect of dilutive stock-based compensation (shares)									142,200	152,500	122,900
Average number of common and potential common shares outstanding (shares)									30,549,300	30,463,400	30,374,300