Calculation 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
Calculation of Basic and Diluted Earnings per Share [Abstract]
Net income	$ 52.3	$ 57.4	$ 144.9	$ 135.4
Weighted-average number of common shares outstanding - basic	52.5	52.8	52.6	52.7
Dilutive effect of share equivalents	0.5	0.6	0.5	0.7
Number of common shares and share equivalents outstanding - diluted	53.0	53.4	53.1	53.4
Basic earnings per share	$ 1.00	$ 1.09	$ 2.75	$ 2.57
Diluted earnings per share	$ 0.99	$ 1.08	$ 2.73	$ 2.54
Number of anti-dilutive share equivalents	0.0	0.0	0.2	0.2