Net Income Per Common Share (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Basic net income per common share:
Net income	$ 276.6	$ 284.0	$ 249.8
Weighted average common shares outstanding — Basic (in shares)	87.9	90.6	92.8
Basic net income per common share	$ 3.15	$ 3.13	$ 2.69
Diluted net income per common share:
Weighted average common shares outstanding — Basic (in shares)	87.9	90.6	92.8
Plus incremental shares from assumed conversions:
Restricted stock units (in shares)	0.4	0.4	0.4
Stock options (in shares)	0.7	0.9	1.0
Weighted average common shares outstanding — Dilutive (in shares)	89.0	91.9	94.2
Dilutive net income per common share	$ 3.11	$ 3.09	$ 2.65
Anti-dilutive shares outstanding, excluded from computation (in shares)	0.2	0.2	0.4