Earnings Per Share (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Numerator:
Net income	$ 57.5	$ 69.3
Denominator:
Denominator for basic earnings per share — weighted average shares	39.4	41.4
Effect of dilutive securities:
Equity compensation plans	0.5	0.4
Denominator for diluted earnings per share — adjusted weighted average and assumed conversion	39.9	41.8
Basic earnings per share (in dollars per share)	$ 1.46	$ 1.67
Diluted earnings per share (in dollars per share)	$ 1.44	$ 1.66