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, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Earnings Per Share [Abstract]
Net income	$ 101.7	$ 80.3	$ 237.3	$ 175.3
Add: Loss from discontinued operations	0.0	0.0	0.6	0.5
Income from continuing operations	$ 101.7	$ 80.3	$ 237.9	$ 175.8
Weighted-average shares outstanding – basic	43.2	45.0	43.6	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	43.7	45.6	44.2	45.6
Earnings per share – Basic:
Income from continuing operations (in dollars per share)	$ 2.35	$ 1.78	$ 5.46	$ 3.92
Loss from discontinued operations (in dollars per share)	0.00	0.00	(0.01)	(0.01)
Net income (in dollars per share)	2.35	1.78	5.45	3.91
Earnings per share – Diluted:
Income from continuing operations (in dollars per share)	2.33	1.76	5.39	3.86
Loss from discontinued operations (in dollars per share)	0.00	0.00	(0.01)	(0.01)
Net income (in dollars per share)	$ 2.33	$ 1.76	$ 5.38	$ 3.85