Net Income per Common Share (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended				6 Months Ended
	Jun. 30, 2019	Mar. 31, 2019	Jun. 30, 2018	Mar. 31, 2018	Jun. 30, 2019	Jun. 30, 2018
Basic net income per common share:
Net income	$ 80.9	$ 72.2	$ 68.8	$ 61.6	$ 153.1	$ 130.4
Weighted average common shares outstanding - Basic (in shares)	85.2		88.8		85.1	89.4
Basic net income per common share	$ 0.95		$ 0.77		$ 1.80	$ 1.45
Diluted net income per common share:
Net income	$ 80.9		$ 68.8		$ 153.1	$ 130.4
Weighted average common shares outstanding - Basic (in shares)	85.2		88.8		85.1	89.4
Plus incremental shares from assumed conversions:
Restricted stock units (in shares)	0.4		0.4		0.4	0.4
Stock options (in shares)	0.6		0.7		0.6	0.7
Weighted average common shares outstanding -Dilutive (in shares)	86.2		89.9		86.1	90.5
Diluted net income per common share	$ 0.94		$ 0.76		$ 1.78	$ 1.44