Net Income per Common Share (Details) (USD $) In Millions, except Per Share data, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013
Basic net income per common share:
Net income	$ 55.8	$ 48.7	$ 156.5	$ 140.8
Weighted average common shares outstanding (in shares)	95.8	99.7	97.2	100.0
Basic net income per common share ( in dollars per share)	$ 0.58	$ 0.49	$ 1.61	$ 1.41
Diluted net income per common share:
Net income	$ 55.8	$ 48.7	$ 156.5	$ 140.8
Weighted average common shares outstanding (in shares)	95.8	99.7	97.2	100.0
Plus incremental shares from assumed conversions:
Restricted stock units (in shares)	0.7	0.7	0.8	0.7
Stock options (in shares)	1.2	1.3	1.2	1.4
Weighted average common shares outstanding - Dilutive (in shares)	97.7	101.7	99.2	102.1
Diluted net income per common share (in dollars per share)	$ 0.57	$ 0.48	$ 1.58	$ 1.38