Net Income per Common Share (Details) (USD $) In Millions, except Per Share data, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Basic net income per common share:
Net income	$ 48.7	$ 39.8	$ 140.8	$ 127.4
Weighted average common shares outstanding (in shares)	99.7	100.3	100.0	100.1
Basic net income per common share (in dollars per share)	$ 0.49	$ 0.40	$ 1.41	$ 1.27
Diluted net income per common share:
Net income	$ 48.7	$ 39.8	$ 140.8	$ 127.4
Weighted average common shares outstanding - Basic (in shares)	99.7	100.3	100.0	100.1
Plus incremental shares from assumed conversions:
Restricted stock units (in shares)	0.7	0.7	0.7	0.8
Stock options (in shares)	1.3	1.1	1.4	1.1
Weighted average common shares outstanding - Dilutive (in shares)	101.7	102.1	102.1	102.0
Diluted net income per common share (in dollars per share)	$ 0.48	$ 0.39	$ 1.38	$ 1.25
Anti-dilutive securities excluded from computation of earnings per share amount (in shares)	0	0.5	0	0.5