Earnings Per Share (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Numerator:
Net income	$ 38.8	$ 53.4	$ 115.1	$ 110.9
Denominator:
Denominator for basic earnings per share — weighted average shares	37.7	39.0	37.8	39.2
Effect of dilutive securities:
Equity compensation plans	0.7	0.5	0.7	0.5
Denominator for diluted earnings per share — adjusted weighted average and assumed conversion	38.4	39.5	38.5	39.7
Basic earnings per share (in dollars per share)	$ 1.03	$ 1.37	$ 3.05	$ 2.83
Diluted earnings per share (in dollars per share)	$ 1.01	$ 1.35	$ 2.99	$ 2.79