Earnings Per Share (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Numerator:
Net income	$ 53.4	$ 61.2	$ 110.9	$ 130.5
Denominator:
Denominator for basic earnings per share — weighted average shares	39.0	40.6	39.2	41.1
Effect of dilutive securities:
Equity compensation plans	0.5	0.5	0.5	0.4
Denominator for diluted earnings per share — adjusted weighted average and assumed conversion	39.5	41.1	39.7	41.5
Basic earnings per share (in dollars per share)	$ 1.37	$ 1.51	$ 2.83	$ 3.18
Diluted earnings per share (in dollars per share)	$ 1.35	$ 1.49	$ 2.79	$ 3.15