Net Income per Common Share (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Basic net income per common share:
Net income	$ 72.2	$ 61.6
Weighted average common shares outstanding - Basic (in shares)	85.0	90.0
Basic net income per common share	$ 0.85	$ 0.68
Diluted net income per common share:
Net income	$ 72.2	$ 61.6
Weighted average common shares outstanding - Basic (in shares)	85.0	90.0
Plus incremental shares from assumed conversions:
Restricted stock units (in shares)	0.4	0.5
Stock options (in shares)	0.6	0.7
Weighted average common shares outstanding -Dilutive (in shares)	86.0	91.2
Diluted net income per common share	$ 0.84	$ 0.68