Earnings Per Share (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Earnings
Net income	$ 16.2	$ 22.5	$ 17.7	$ 1.3	$ 12.1	$ 23.7	$ 11.0	$ (1.3)	$ 57.7	$ 45.5	$ 33.5
Shares
Weighted-average common shares outstanding (in shares)	45.7	45.8	45.9	45.9	45.7	45.9	46.0	46.2	45.7	46.0	46.3
Adjustment for assumed dilution:
Weighted-average shares assuming dilution (in shares)	46.0	46.1	46.1	46.0	46.0	46.2	46.1	46.2	46.0	46.1	46.4
Earnings per share
Basic (in dollars per share)	$ 0.35	$ 0.49	$ 0.39	$ 0.03	$ 0.26	$ 0.52	$ 0.24	$ (0.03)	$ 1.26	$ 0.99	$ 0.72
Diluted (in dollars per share)	$ 0.35	$ 0.49	$ 0.38	$ 0.03	$ 0.26	$ 0.51	$ 0.24	$ (0.03)	$ 1.25	$ 0.99	$ 0.72
Restricted stock units
Adjustment for assumed dilution:
Adjustment for assumed dilution (in shares)									0.2	0.1	0.1
Performance shares
Adjustment for assumed dilution:
Adjustment for assumed dilution (in shares)									0.1	0.0	0.0