Net Income per Common Share (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended						9 Months Ended
	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Sep. 30, 2019	Sep. 30, 2018
Basic net income per common share:
Net income	$ 80.3	$ 80.9	$ 72.2	$ 80.1	$ 68.8	$ 61.6	$ 233.4	$ 210.5
Weighted average common shares outstanding - Basic (in shares)	85.1			87.0			85.1	88.6
Basic net income per common share	$ 0.94			$ 0.92			$ 2.74	$ 2.37
Diluted net income per common share:
Net income	$ 80.3			$ 80.1			$ 233.4	$ 210.5
Weighted average common shares outstanding - Basic (in shares)	85.1			87.0			85.1	88.6
Plus incremental shares from assumed conversions:
Restricted stock units (in shares)	0.4			0.5			0.4	0.4
Stock options (in shares)	0.6			0.6			0.6	0.7
Weighted average common shares outstanding -Dilutive (in shares)	86.1			88.1			86.1	89.7
Diluted net income per common share	$ 0.93			$ 0.91			$ 2.71	$ 2.35