Net Income Per Share - Additional Information (Detail) (USD $) In Millions, except Per Share data, unless otherwise specified	3 Months Ended		9 Months Ended
	Oct. 31, 2012	Oct. 31, 2011	Oct. 31, 2012	Oct. 31, 2011
Earnings Per Share [Abstract]
Net income	$ 29.4	$ 72.8	$ 172.9	$ 213.3
Denominator for basic net income per share-weighted average shares	225.5	227.1	227.1	228.2
Effect of dilutive securities	4.4	3.6	4.3	5.5
Denominator for dilutive net income per share	229.9	230.7	231.4	233.7
Basic net income per share (in usd per share)	$ 0.13	$ 0.32	$ 0.76	$ 0.93
Diluted net income per share (in usd per share)	$ 0.13	$ 0.32	$ 0.75	$ 0.91
Potentially dilutive shares excluded from the computation of diluted net income per share	11.8	22.8	9.8	11.9