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, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Earnings Per Share [Abstract]
Net income	$ 110.7	$ 137.6	$ 180.0	$ 175.5
Add: Loss from discontinued operations	0.3	1.6	0.4	1.6
Income from continuing operations	$ 111.0	$ 139.2	$ 180.4	$ 177.1
Weighted-average shares outstanding – basic	39.1	40.7	39.4	41.1
Add: Potential effect of dilutive securities attributable to stock-based payments	0.4	0.4	0.4	0.5
Weighted-average shares outstanding – diluted	39.5	41.1	39.8	41.6
Earnings per share – Basic:
Income from continuing operations (in dollars per share)	$ 2.84	$ 3.42	$ 4.58	$ 4.31
Loss from discontinued operations (in dollars per share)	(0.01)	(0.04)	(0.01)	(0.04)
Net income (in dollars per share)	2.83	3.38	4.57	4.27
Earnings per share – Diluted:
Income from continuing operations (in dollars per share)	2.81	3.39	4.53	4.26
Loss from discontinued operations (in dollars per share)	(0.01)	(0.04)	(0.01)	(0.04)
Net income (in dollars per share)	$ 2.80	$ 3.35	$ 4.52	$ 4.22