Net Income per Common Share (Details) (USD $) In Millions, except Per Share data, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Basic net income per common share:
Net income	$ 48.5	$ 48.0	$ 92.1	$ 87.6
Weighted average common shares outstanding (in shares)	100.0	100.2	100.2	100.0
Basic net income per common share (in dollars per share)	$ 0.49	$ 0.48	$ 0.92	$ 0.88
Diluted net income per common share:
Net income	$ 48.5	$ 48.0	$ 92.1	$ 87.6
Weighted average common shares outstanding - Basic (in shares)	100.0	100.2	100.2	100.0
Plus incremental shares from assumed conversions:
Restricted stock units (in shares)	0.7	0.6	0.7	0.7
Stock options (in shares)	1.1	1.2	1.1	1.2
Weighted average common shares outstanding - Dilutive (in shares)	101.8	102.0	102.0	101.9
Diluted net income per common share (in dollars per share)	$ 0.48	$ 0.47	$ 0.90	$ 0.86
Anti-dilutive securities excluded from computation of earnings per share amount (in shares)	0.3	0.5	0.3	0.5