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, 2018	Mar. 31, 2017
Earnings Per Share [Abstract]
Net income	$ 37.9	$ 43.5
Add: Loss from discontinued operations	0.0	0.0
Income from continuing operations	$ 37.9	$ 43.5
Weighted-average shares outstanding – basic	41.5	42.8
Add: Potential effect of dilutive securities attributable to stock-based payments	0.6	0.7
Weighted-average shares outstanding – diluted	42.1	43.5
Earnings per share – Basic:
Income from continuing operations (in dollars per share)	$ 0.91	$ 1.02
Loss from discontinued operations (in dollars per share)	0.00	0.00
Net income (in dollars per share)	0.91	1.02
Earnings per share – Diluted:
Income from continuing operations (in dollars per share)	0.90	1.00
Loss from discontinued operations (in dollars per share)	0.00	0.00
Net income (in dollars per share)	$ 0.90	$ 1.00