Earnings Per Share - Computation of Basic and Diluted Earnings Per Share (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Earnings
Net income	$ 23.0	$ 22.5	$ 44.2	$ 41.5
Shares
Weighted-average common shares outstanding (basic shares)	45.1	45.8	45.1	45.8
Adjustment for assumed dilution:
Weighted-average shares assuming dilution (diluted shares)	45.4	46.1	45.3	46.1
Earnings per share
Basic (in dollars per share)	$ 0.51	$ 0.49	$ 0.98	$ 0.91
Diluted (in dollars per share)	$ 0.51	$ 0.49	$ 0.98	$ 0.90
Restricted stock units
Adjustment for assumed dilution:
Incremental common shares attributable to share-based payment arrangements (in shares)	0.2	0.2	0.1	0.2
Performance shares
Adjustment for assumed dilution:
Incremental common shares attributable to share-based payment arrangements (in shares)	0.1	0.1	0.1	0.1