Net Income per Common 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
Basic net income per common share:
Net income	$ 69.7	$ 68.2	$ 195.9	$ 190.3
Weighted average common shares outstanding — Basic (in shares)	90.1	92.7	90.7	93.1
Basic net income per common share	$ 0.77	$ 0.74	$ 2.16	$ 2.04
Diluted net income per common share:
Net income	$ 69.7	$ 68.2	$ 195.9	$ 190.3
Weighted average common shares outstanding — Basic (in shares)	90.1	92.7	90.7	93.1
Plus incremental shares from assumed conversions:
Restricted stock units (in shares)	0.4	0.4	0.4	0.5
Stock options (in shares)	0.9	1.0	1.0	1.0
Weighted average common shares outstanding — Dilutive (in shares)	91.4	94.1	92.1	94.6
Diluted net income per common share	$ 0.76	$ 0.72	$ 2.13	$ 2.01