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, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Earnings Per Share [Abstract]
Net income	$ 108.0	$ 103.5	$ 283.4	$ 262.5
Add: Loss from discontinued operations	0.0	0.5	1.7	1.4
Income from continuing operations	$ 108.0	$ 104.0	$ 285.1	$ 263.9
Weighted-average shares outstanding – basic	40.3	41.9	40.8	42.3
Add: Potential effect of dilutive securities attributable to stock-based payments	0.4	0.5	0.5	0.6
Weighted-average shares outstanding – diluted	40.7	42.4	41.3	42.9
Earnings per share – Basic:
Income from continuing operations (in dollars per share)	$ 2.68	$ 2.48	$ 6.98	$ 6.23
Loss from discontinued operations (in dollars per share)	0.00	(0.01)	(0.04)	(0.03)
Net income (in dollars per share)	2.68	2.47	6.94	6.20
Earnings per share – Diluted:
Income from continuing operations (in dollars per share)	2.65	2.45	6.90	6.15
Loss from discontinued operations (in dollars per share)	0.00	(0.01)	(0.04)	(0.03)
Net income (in dollars per share)	$ 2.65	$ 2.44	$ 6.86	$ 6.12