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, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Earnings Per Share [Abstract]
Net income	$ 103.5	$ 101.7	$ 262.5	$ 237.3
Add: Loss from discontinued operations	0.5	0.0	1.4	0.6
Income from continuing operations	$ 104.0	$ 101.7	$ 263.9	$ 237.9
Weighted-average shares outstanding – basic	41.9	43.2	42.3	43.6
Add: Potential effect of dilutive securities attributable to stock-based payments	0.5	0.5	0.6	0.6
Weighted-average shares outstanding – diluted	42.4	43.7	42.9	44.2
Earnings per share – Basic:
Income from continuing operations (in dollars per share)	$ 2.48	$ 2.35	$ 6.23	$ 5.46
Loss from discontinued operations (in dollars per share)	(0.01)	0.00	(0.03)	(0.01)
Net income (in dollars per share)	2.47	2.35	6.20	5.45
Earnings per share – Diluted:
Income from continuing operations (in dollars per share)	2.45	2.33	6.15	5.39
Loss from discontinued operations (in dollars per share)	(0.01)	0.00	(0.03)	(0.01)
Net income (in dollars per share)	$ 2.44	$ 2.33	$ 6.12	$ 5.38