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, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Earnings Per Share [Abstract]
Net income	$ 100.0	$ 110.7	$ 112.9	$ 180.0
Exclude: (Income) loss from discontinued operations	0.6	0.3	0.2	0.4
Income from continuing operations	$ 100.6	$ 111.0	$ 113.1	$ 180.4
Weighted-average shares outstanding – basic	38.2	39.1	38.3	39.4
Add: Potential effect of dilutive securities attributable to stock-based payments	0.2	0.4	0.2	0.4
Weighted-average shares outstanding – diluted	38.4	39.5	38.6	39.8
Earnings per share – Basic:
Income from continuing operations (in dollars per share)	$ 2.63	$ 2.84	$ 2.95	$ 4.58
Loss from discontinued operations (in dollars per share)	(0.01)	(0.01)	0	(0.01)
Net income (in dollars per share)	2.62	2.83	2.95	4.57
Earnings per share – Diluted:
Income from continuing operations (in dollars per share)	2.62	2.81	2.93	4.53
Loss from discontinued operations (in dollars per share)	(0.02)	(0.01)	0	(0.01)
Net income (in dollars per share)	$ 2.60	$ 2.80	$ 2.93	$ 4.52