Earnings Per Share (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Numerator:
Net income	$ 47.0	$ 49.0	$ 162.1	$ 159.9
Denominator:
Denominator for basic earnings per share — weighted average shares	37.7	38.6	37.8	39.0
Effect of dilutive securities:
Equity compensation plans	0.8	0.6	0.7	0.6
Denominator for diluted earnings per share — adjusted weighted average and assumed conversion	38.5	39.2	38.5	39.6
Basic earnings per share (in dollars per share)	$ 1.25	$ 1.27	$ 4.29	$ 4.10
Diluted earnings per share (in dollars per share)	$ 1.22	$ 1.25	$ 4.21	$ 4.04