Earnings Per Share (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Numerator:
Net income	$ 49.0	$ 95.7	$ 159.9	$ 226.2
Denominator:
Denominator for basic earnings per share — weighted average shares	38.6	40.1	39.0	40.7
Effect of dilutive securities:
Equity compensation plans	0.6	0.5	0.6	0.5
Denominator for diluted earnings per share — adjusted weighted average and assumed conversion	39.2	40.6	39.6	41.2
Basic earnings per share (in dollars per share)	$ 1.27	$ 2.39	$ 4.10	$ 5.55
Diluted earnings per share (in dollars per share)	$ 1.25	$ 2.36	$ 4.04	$ 5.49