Net Income Per Common Share (Details) (USD $) In Millions, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2010	Sep. 30, 2010	Jun. 30, 2010	Mar. 31, 2010	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Net Income Per Common Share
Net income									$ 135.5	$ 77.4	$ 56.3
Basic net income per common share:
Weighted average common shares outstanding (in shares)									98.8	97.6	96.9
Basic net income per common share (in dollars per share)	$ 0.40	$ 0.33	$ 0.38	$ 0.27	$ 0.23	$ 0.16	$ 0.24	$ 0.16	$ 1.37	$ 0.79	$ 0.58
Diluted net income per common share:
Weighted average common shares outstanding-Basic (in shares)									98.8	97.6	96.9
Incremental shares from assumed conversions:
Restricted stock units									0.9	1.0	0.6
Stock options									1.0	1.3	0.7
Weighted average common shares outstanding-Dilutive (in shares)									100.7	99.9	98.2
Diluted net income per common share (in dollars per share)	$ 0.39	$ 0.32	$ 0.37	$ 0.26	$ 0.23	$ 0.16	$ 0.23	$ 0.16	$ 1.35	$ 0.77	$ 0.57
Antidilutive securities excluded from computation of earnings per share amount (in shares)									0.3	0.8	2.1