Net Income per Common Share (Details) (USD $) In Millions, except Per Share data, unless otherwise specified	3 Months Ended
	Mar. 31, 2012	Mar. 31, 2011
Basic net income per common share:
Net income	$ 39.6	$ 26.4
Weighted average common shares outstanding (in shares)	99.8	98.2
Basic net income per common share (in dollars per share)	$ 0.40	$ 0.27
Diluted net income per common share:
Weighted average common shares outstanding-Basic (in shares)	99.8	98.2
Plus incremental shares from assumed conversions:
Restricted stock units (in shares)	0.7	0.8
Stock options (in shares)	1.2	1.4
Weighted average common shares outstanding-Dilutive (in shares)	101.7	100.4
Diluted net income per common share (in dollars per share)	$ 0.39	$ 0.26
Antidilutive securities excluded from computation of earnings per share amount (in shares)	0.5	1.0