Net Income per Common Share (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Basic net income per common share:
Net income	$ 61.6	$ 64.6
Weighted average common shares outstanding — Basic (in shares)	90.0	91.4
Basic net income per common share	$ 0.68	$ 0.71
Diluted net income per common share:
Net income	$ 61.6	$ 64.6
Weighted average common shares outstanding — Basic (in shares)	90.0	91.4
Plus incremental shares from assumed conversions:
Restricted stock units (in shares)	0.5	0.5
Stock options (in shares)	0.7	1.0
Weighted average common shares outstanding — Dilutive (in shares)	91.2	92.9
Diluted net income per common share	$ 0.68	$ 0.70