Net Income per Common Share (Details) (USD $) In Millions, except Per Share data, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013
Basic net income per common share:
Net income	$ 50.6	$ 48.5	$ 100.7	$ 92.1
Weighted average common shares outstanding (in shares)	97.3	100.0	97.9	100.2
Basic net income per common share (in dollars per share)	$ 0.52	$ 0.49	$ 1.03	$ 0.92
Diluted net income per common share:
Net income	$ 50.6	$ 48.5	$ 100.7	$ 92.1
Weighted average common shares outstanding - Basic (in shares)	97.3	100.0	97.9	100.2
Plus incremental shares from assumed conversions:
Restricted stock units (in shares)	0.7	0.7	0.7	0.7
Stock options (in shares)	1.2	1.1	1.3	1.1
Weighted average common shares outstanding - Dilutive (in shares)	99.2	101.8	99.9	102.0
Diluted net income per common share (in dollars per share)	$ 0.51	$ 0.48	$ 1.01	$ 0.90
Anti-dilutive securities excluded from computation of earnings per share amount (in shares)	0.2	0.3	0.2	0.3