Net Income per Common Share (Details) (USD $) In Millions, except Per Share data, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011
Basic net income per common share:
Net income	$ 39.8	$ 32.2	$ 127.4	$ 96.0
Weighted average common shares outstanding (in shares)	100.3	99.0	100.1	98.6
Basic net income per common share (in dollars per share)	$ 0.40	$ 0.33	$ 1.27	$ 0.97
Diluted net income per common share:
Net income	$ 39.8	$ 32.2	$ 127.4	$ 96.0
Weighted average common shares outstanding-Basic (in shares)	100.3	99.0	100.1	98.6
Plus incremental shares from assumed conversions:
Restricted stock units (in shares)	0.7	0.8	0.8	0.8
Stock options (in shares)	1.1	1.3	1.1	1.3
Weighted average common shares outstanding-Dilutive (in shares)	102.1	101.1	102.0	100.7
Diluted net income per common share (in dollars per share)	$ 0.39	$ 0.32	$ 1.25	$ 0.95
Antidilutive securities excluded from computation of earnings per share amount (in shares)	0.5	0.3	0.5	0.5