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, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Earnings Per Share [Abstract]
Net income	$ 115.5	$ 110.7	$ 159.0	$ 135.6
Add: Loss from discontinued operations	0.9	0.5	0.9	0.5
Income from continuing operations	$ 116.4	$ 111.2	$ 159.9	$ 136.1
Weighted-average shares outstanding – basic	42.3	43.6	42.6	43.8
Add: Potential effect of dilutive securities attributable to stock-based payments	0.6	0.5	0.6	0.6
Weighted-average shares outstanding – diluted	42.9	44.1	43.2	44.4
Earnings per share – Basic:
Income from continuing operations (in dollars per share)	$ 2.75	$ 2.55	$ 3.76	$ 3.11
Loss from discontinued operations (in dollars per share)	(0.02)	(0.01)	(0.02)	(0.01)
Net income (in dollars per share)	2.73	2.54	3.74	3.10
Earnings per share – Diluted:
Income from continuing operations (in dollars per share)	2.71	2.52	3.70	3.07
Loss from discontinued operations (in dollars per share)	(0.02)	(0.01)	(0.02)	(0.01)
Net income (in dollars per share)	$ 2.69	$ 2.51	$ 3.68	$ 3.06