Calculation of Basic and Diluted Earnings per Share (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Calculation of Basic and Diluted Earnings per Share [Abstract]
Net income	$ 48.7	$ 43.8
Weighted-average number of common shares outstanding - basic	53.0	52.8
Dilutive effect of share equivalents	0.7	0.5
Number of common shares and share equivalents outstanding - diluted	53.7	53.3
Basic earnings per share	$ 0.92	$ 0.83
Diluted earnings per share	$ 0.91	$ 0.82
Number of anti-dilutive share equivalents	0.0	0.3