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 Per Share [Abstract]
Net income	$ 131.7	$ 114.7	$ 244.6	$ 294.7
Exclude: Loss from discontinued operations	0.0	0.0	0.3	0.3
Income from continuing operations	$ 131.7	$ 114.7	$ 244.9	$ 295.0
Weighted-average shares outstanding – basic	38.3	38.6	38.3	39.1
Add: Potential effect of dilutive securities attributable to stock-based payments	0.3	0.4	0.3	0.4
Weighted-average shares outstanding – diluted	38.6	39.0	38.6	39.5
Earnings per share – Basic:
Income from continuing operations (in dollars per share)	$ 3.44	$ 2.97	$ 6.39	$ 7.54
Loss from discontinued operations (in dollars per share)	0	0	(0.01)	(0.01)
Net income (in dollars per share)	3.44	2.97	6.38	7.53
Earnings per share – Diluted:
Income from continuing operations (in dollars per share)	3.42	2.94	6.35	7.46
Loss from discontinued operations (in dollars per share)	0	0	(0.01)	0
Net income (in dollars per share)	$ 3.42	$ 2.94	$ 6.34	$ 7.46