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, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Earnings Per Share [Abstract]
Net income	$ 126.3	$ 131.7	$ 380.4	$ 244.6
Exclude: Income from discontinued operations	0.0	0.0	0.1	0.3
Income from continuing operations	$ 126.3	$ 131.7	$ 380.5	$ 244.9
Weighted-average shares outstanding – basic	36.8	38.3	37.4	38.3
Add: Potential effect of dilutive securities attributable to stock-based payments	0.2	0.3	0.3	0.3
Weighted-average shares outstanding – diluted	37.0	38.6	37.7	38.6
Earnings per share – Basic:
Income from continuing operations (in dollars per share)	$ 3.43	$ 3.44	$ 10.17	$ 6.39
Loss from discontinued operations (in dollars per share)	0	0	0	(0.01)
Net income (in dollars per share)	3.43	3.44	10.17	6.38
Earnings per share – Diluted:
Income from continuing operations (in dollars per share)	3.41	3.42	10.10	6.35
Loss from discontinued operations (in dollars per share)	0	0	0	(0.01)
Net income (in dollars per share)	$ 3.41	$ 3.42	$ 10.10	$ 6.34