Net Income per Common Share (Details) (USD $) In Millions, except Per Share data	3 Months Ended		6 Months Ended
	Jun. 30, 2011	Jun. 30, 2010	Jun. 30, 2011	Jun. 30, 2010
Basic net income per common share:
Net income	$ 37.4	$ 23.1	$ 63.8	$ 38.9
Weighted average common shares outstanding (in shares)	98.6	97.5	98.4	97.4
Basic net income per common share (in dollars per share)	$ 0.38	$ 0.24	$ 0.65	$ 0.40
Diluted net income per common share:
Net income	$ 37.4	$ 23.1	$ 63.8	$ 38.9
Weighted average common shares outstanding-Basic (in shares)	98.6	97.5	98.4	97.4
Incremental shares from assumed conversions:
Restricted stock units	0.7	0.9	0.8	1.0
Stock options	1.4	1.3	1.3	1.2
Weighted average common shares outstanding-Dilutive (in shares)	100.7	99.7	100.5	99.6
Diluted net income per common share (in dollars per share)	$ 0.37	$ 0.23	$ 0.63	$ 0.39
Antidilutive securities excluded from computation of earnings per share amount (in shares)	1.1	0.9	1.1	1.2