Earnings Per Share (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Numerator:
Net income	$ 41.5	$ 76.3
Denominator:
Denominator for basic earnings per share — weighted average shares	36.5	37.9
Effect of dilutive securities:
Equity compensation plans	0.6	0.6
Denominator for diluted earnings per share — adjusted weighted average and assumed conversion	37.1	38.5
Basic earnings per share (in dollars per share)	$ 1.14	$ 2.02
Diluted earnings per share (in dollars per share)	$ 1.12	$ 1.98