Net Income per Common Share (Details) (USD $) In Millions, except Per Share data	3 Months Ended		9 Months Ended
	Sep. 30, 2011	Sep. 30, 2010	Sep. 30, 2011	Sep. 30, 2010
Basic net income per common share:
Net income	$ 32.2	$ 15.6	$ 96.0	$ 54.5
Weighted average common shares outstanding (in shares)	99.0	97.7	98.6	97.6
Basic net income per common share (in dollars per share)	$ 0.33	$ 0.16	$ 0.97	$ 0.56
Diluted net income per common share:
Net income	$ 32.2	$ 15.6	$ 96.0	$ 54.5
Weighted average common shares outstanding-Basic (in shares)	99.0	97.7	98.6	97.6
Incremental shares from assumed conversions:
Restricted stock units	0.8	0.8	0.8	1.0
Stock options	1.3	1.5	1.3	1.3
Weighted average common shares outstanding-Dilutive (in shares)	101.1	100.0	100.7	99.9
Diluted net income per common share (in dollars per share)	$ 0.32	$ 0.16	$ 0.95	$ 0.55
Antidilutive securities excluded from computation of earnings per share amount (in shares)	0.3	0.8	0.5	0.8