Earnings Per Share (Computation 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, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Earnings Per Share [Abstract]
Net income	$ 170.0	$ 100.0	$ 254.2	$ 112.9
Exclude: Income from discontinued operations	0.0	0.6	0.0	0.2
Income from continuing operations	$ 170.0	$ 100.6	$ 254.2	$ 113.1
Weighted-average shares outstanding – basic	37.4	38.2	37.7	38.3
Add: Potential effect of dilutive securities attributable to stock-based payments	0.3	0.2	0.3	0.3
Weighted-average shares outstanding – diluted	37.7	38.4	38.0	38.6
Earnings per share – Basic:
Income from continuing operations (in dollars per share)	$ 4.55	$ 2.63	$ 6.74	$ 2.95
Loss from discontinued operations (in dollars per share)	0	(0.01)	0	0
Net income (in dollars per share)	4.55	2.62	6.74	2.95
Earnings per share – Diluted:
Income from continuing operations (in dollars per share)	4.51	2.62	6.70	2.93
Loss from discontinued operations (in dollars per share)	0	(0.02)	0	0
Net income (in dollars per share)	$ 4.51	$ 2.60	$ 6.70	$ 2.93