Earnings Per Share (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Numerator:
Net income	$ 68.0	$ 38.8	$ 109.5	$ 115.1
Denominator:
Denominator for basic earnings per share — weighted average shares	36.0	37.7	36.2	37.8
Effect of dilutive securities:
Equity compensation plans	0.7	0.7	0.7	0.7
Denominator for diluted earnings per share — adjusted weighted average and assumed conversion	36.7	38.4	36.9	38.5
Basic earnings per share (in dollars per share)	$ 1.89	$ 1.03	$ 3.02	$ 3.05
Diluted earnings per share (in dollars per share)	$ 1.86	$ 1.01	$ 2.97	$ 2.99