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, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Earnings Per Share [Abstract]
Net income	$ 137.6	$ 115.5	$ 175.5	$ 159.0
Add: Loss from discontinued operations	1.6	0.9	1.6	0.9
Income from continuing operations	$ 139.2	$ 116.4	$ 177.1	$ 159.9
Weighted-average shares outstanding – basic	40.7	42.3	41.1	42.6
Add: Potential effect of dilutive securities attributable to stock-based payments	0.4	0.6	0.5	0.6
Weighted-average shares outstanding – diluted	41.1	42.9	41.6	43.2
Earnings per share – Basic:
Income from continuing operations (in dollars per share)	$ 3.42	$ 2.75	$ 4.31	$ 3.76
Loss from discontinued operations (in dollars per share)	(0.04)	(0.02)	(0.04)	(0.02)
Net income (in dollars per share)	3.38	2.73	4.27	3.74
Earnings per share – Diluted:
Income from continuing operations (in dollars per share)	3.39	2.71	4.26	3.70
Loss from discontinued operations (in dollars per share)	(0.04)	(0.02)	(0.04)	(0.02)
Net income (in dollars per share)	$ 3.35	$ 2.69	$ 4.22	$ 3.68