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, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Earnings Per Share [Abstract]
Net income	$ 110.7	$ 81.2	$ 135.6	$ 95.1
Add: Loss from discontinued operations	0.5	0.4	0.5	0.5
Income from continuing operations	$ 111.2	$ 81.6	$ 136.1	$ 95.6
Weighted-average shares outstanding – basic	43.6	45.0	43.8	44.9
Add: Potential effect of dilutive securities attributable to stock-based payments	0.5	0.6	0.6	0.7
Weighted-average shares outstanding – diluted	44.1	45.6	44.4	45.6
Earnings per share – Basic:
Income from continuing operations (in dollars per share)	$ 2.55	$ 1.81	$ 3.11	$ 2.13
Loss from discontinued operations (in dollars per share)	(0.01)	(0.01)	(0.01)	(0.01)
Net income (in dollars per share)	2.54	1.80	3.10	2.12
Earnings per share – Diluted:
Income from continuing operations (in dollars per share)	2.52	1.79	3.07	2.10
Loss from discontinued operations (in dollars per share)	(0.01)	(0.01)	(0.01)	(0.01)
Net income (in dollars per share)	$ 2.51	$ 1.78	$ 3.06	$ 2.09