Earnings Per Share (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Numerator:
Net income	$ 76.3	$ 57.5
Denominator:
Denominator for basic earnings per share — weighted average shares	37.9	39.4
Effect of dilutive securities:
Equity compensation plans	0.6	0.5
Denominator for diluted earnings per share — adjusted weighted average and assumed conversion	38.5	39.9
Basic earnings per share (in dollars per share)	$ 2.02	$ 1.46
Diluted earnings per share (in dollars per share)	$ 1.98	$ 1.44