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, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Earnings Per Share [Abstract]
Net income	$ 114.7	$ 108.0	$ 294.7	$ 283.4
Add: Loss from discontinued operations	0.0	0.0	0.3	1.7
Income from continuing operations	$ 114.7	$ 108.0	$ 295.0	$ 285.1
Weighted-average shares outstanding – basic	38.6	40.3	39.1	40.8
Add: Potential effect of dilutive securities attributable to stock-based payments	0.4	0.4	0.4	0.5
Weighted-average shares outstanding – diluted	39.0	40.7	39.5	41.3
Earnings per share – Basic:
Income from continuing operations (in dollars per share)	$ 2.97	$ 2.68	$ 7.54	$ 6.98
Loss from discontinued operations (in dollars per share)	0	0	(0.01)	(0.04)
Net income (in dollars per share)	2.97	2.68	7.53	6.94
Earnings per share – Diluted:
Income from continuing operations (in dollars per share)	2.94	2.65	7.46	6.90
Loss from discontinued operations (in dollars per share)	0	0	0	(0.04)
Net income (in dollars per share)	$ 2.94	$ 2.65	$ 7.46	$ 6.86