Earnings Per Share (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Earnings
Net income	$ 19.0	$ 23.0	$ 16.9	$ 4.3	$ 16.2	$ 22.5	$ 17.7	$ 1.3	$ 63.2	$ 57.7	$ 45.5
Shares
Weighted-average common shares outstanding (in shares)	44,900,000	45,100,000	45,100,000	45,300,000	45,700,000	45,800,000	45,900,000	45,900,000	45,100,000	45,700,000	46,000,000.0
Adjustment for assumed dilution:
Weighted-average shares assuming dilution (in shares)	45,400,000	45,400,000	45,300,000	45,400,000	46,000,000.0	46,100,000	46,100,000	46,000,000.0	45,400,000	46,000,000.0	46,100,000
Earnings per share
Basic (in dollars per share)	$ 0.42	$ 0.51	$ 0.38	$ 0.09	$ 0.35	$ 0.49	$ 0.39	$ 0.03	$ 1.40	$ 1.26	$ 0.99
Diluted (in dollars per share)	$ 0.42	$ 0.51	$ 0.38	$ 0.09	$ 0.35	$ 0.49	$ 0.38	$ 0.03	$ 1.39	$ 1.25	$ 0.99
Antidilutive securities excluded from computation of diluted earnings per share (in shares)									266,101	228,596,000,000	340,819,000,000
Restricted stock units
Adjustment for assumed dilution:
Adjustment for assumed dilution (in shares)									200,000	200,000	100,000
Performance shares
Adjustment for assumed dilution:
Adjustment for assumed dilution (in shares)									100,000	100,000	0