Net Income Per Common Share - Summary of Earnings Per Share Basic and Diluted (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Basic net income per common share:
Net income	$ 306.6	$ 276.6	$ 284.0
Weighted average common shares outstanding - Basic (in shares)	84.9	87.9	90.6
Basic net income per common share	$ 3.61	$ 3.15	$ 3.13
Diluted net income per common share:
Weighted average common shares outstanding - Basic (in shares)	84.9	87.9	90.6
Plus incremental shares from assumed conversions:
Weighted average common shares outstanding -Dilutive (in shares)	85.8	89.0	91.9
Dilutive net income per common share	$ 3.57	$ 3.11	$ 3.09
Anti-dilutive shares outstanding, excluded from computation (in shares)	0.1	0.2	0.2
Restricted Stock Units
Plus incremental shares from assumed conversions:
Incremental shares from assumed conversions	0.3	0.4	0.4
Stock Options
Plus incremental shares from assumed conversions:
Incremental shares from assumed conversions	0.6	0.7	0.9