Earnings Per Share Calculation of Basic and Diluted Earnings per Share (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions		3 Months Ended		6 Months Ended
		Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Calculation of Basic and Diluted Earnings per Share [Abstract]
Net income		$ 48.8	$ 43.1	$ 92.6	$ 78.0
Weighted average number of common shares		52.5	52.8	52.7	52.7
Dilutive effect of stock-based awards		0.5	0.6	0.4	0.6
Number of shares used to compute diluted earnings per share		53.0	53.4	53.1	53.3
Basic		$ 0.93	$ 0.82	$ 1.76	$ 1.48
Diluted		$ 0.92	$ 0.81	$ 1.74	$ 1.46
Antidilutive Securities Excluded from Computation of Earnings Per Share, Amount	[1]	0.5	0.2	0.3

[1] Anti-dilutive stock-based awards that were excluded from the calculation of the number of shares used to compute diluted earnings per share because they were anti-dilutive when applying the treasury-stock method.