Net Income per Common Share (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Basic net income per common share:
Net income	$ 80.1	$ 69.7	$ 210.5	$ 195.9
Weighted average common shares outstanding — Basic (in shares)	87.0	90.1	88.6	90.7
Basic net income per common share	$ 0.92	$ 0.77	$ 2.37	$ 2.16
Diluted net income per common share:
Net income	$ 80.1	$ 69.7	$ 210.5	$ 195.9
Weighted average common shares outstanding — Basic (in shares)	87.0	90.1	88.6	90.7
Plus incremental shares from assumed conversions:
Restricted stock units (in shares)	0.5	0.4	0.4	0.4
Stock options (in shares)	0.6	0.9	0.7	1.0
Weighted average common shares outstanding — Dilutive (in shares)	88.1	91.4	89.7	92.1
Diluted net income per common share	$ 0.91	$ 0.76	$ 2.35	$ 2.13