Net Income per Common Share (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Basic net income per common share:
Net income	$ 68.8	$ 61.6	$ 130.4	$ 126.2
Weighted average common shares outstanding — Basic (in shares)	88.8	90.7	89.4	91.1
Basic net income per common share	$ 0.77	$ 0.68	$ 1.45	$ 1.39
Diluted net income per common share:
Net income	$ 68.8	$ 61.6	$ 130.4	$ 126.2
Weighted average common shares outstanding — Basic (in shares)	88.8	90.7	89.4	91.1
Plus incremental shares from assumed conversions:
Restricted stock units (in shares)	0.4	0.4	0.4	0.4
Stock options (in shares)	0.7	0.9	0.7	0.9
Weighted average common shares outstanding — Dilutive (in shares)	89.9	92.0	90.5	92.4
Diluted net income per common share	$ 0.76	$ 0.67	$ 1.44	$ 1.37