Earnings Per Share (Computation of Basic and Diluted Earnings Per Share) (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Earnings Per Share [Abstract]
Net income	$ 69.3	$ 37.9
Add: Loss from discontinued operations	0.1	0.0
Income from continuing operations	$ 69.4	$ 37.9
Weighted-average shares outstanding – basic	39.7	41.5
Add: Potential effect of dilutive securities attributable to stock-based payments	0.4	0.6
Weighted-average shares outstanding – diluted	40.1	42.1
Earnings per share – Basic:
Income from continuing operations (in dollars per share)	$ 1.75	$ 0.91
Loss from discontinued operations (in dollars per share)	0	0
Net income (in dollars per share)	1.75	0.91
Earnings per share – Diluted:
Income from continuing operations (in dollars per share)	1.73	0.90
Loss from discontinued operations (in dollars per share)	0	0
Net income (in dollars per share)	$ 1.73	$ 0.90