Net Income per Common Share (Details) (USD $) In Millions, except Per Share data, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011
Basic net income per common share:
Net income	$ 48.0	$ 37.4	$ 87.6	$ 63.8
Weighted average common shares outstanding (in shares)	100.2	98.6	100.0	98.4
Basic net income per common share (in dollars per share)	$ 0.48	$ 0.38	$ 0.88	$ 0.65
Diluted net income per common share:
Net income	$ 48.0	$ 37.4	$ 87.6	$ 63.8
Weighted average common shares outstanding-Basic (in shares)	100.2	98.6	100.0	98.4
Plus incremental shares from assumed conversions:
Restricted stock units (in shares)	0.6	0.7	0.7	0.8
Stock options (in shares)	1.2	1.4	1.2	1.3
Weighted average common shares outstanding-Dilutive (in shares)	102.0	100.7	101.9	100.5
Diluted net income per common share (in dollars per share)	$ 0.47	$ 0.37	$ 0.86	$ 0.63
Antidilutive securities excluded from computation of earnings per share amount (in shares)	0.5	1.1	0.5	1.1