Computation of the Numerators and Denominators Used in the Basic and Diluted Net Income Per Share Amounts (Detail) (USD $) In Millions, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Jan. 31, 2012	Oct. 31, 2011	Jul. 31, 2011	Apr. 30, 2011	Jan. 31, 2011	Oct. 31, 2010	Jul. 31, 2010	Apr. 30, 2010	Jan. 31, 2012	Jan. 31, 2011	Jan. 31, 2010
Numerator:
Net income	$ 72.0	$ 72.8	$ 71.2	$ 69.3	$ 61.6	$ 53.6	$ 59.9	$ 36.9	$ 285.3	$ 212.0	$ 58.0
Denominator:
Denominator for basic net income per share-weighted average shares									227.7	227.6	228.7
Effect of dilutive securities									5.6	6.6	3.4
Denominator for dilutive net income per share									233.3	234.2	232.1
Basic net income per share	$ 0.32	$ 0.32	$ 0.31	$ 0.30	$ 0.27	$ 0.24	$ 0.26	$ 0.16	$ 1.25	$ 0.93	$ 0.25
Diluted net income per share	$ 0.31	$ 0.32	$ 0.30	$ 0.29	$ 0.26	$ 0.23	$ 0.25	$ 0.16	$ 1.22	$ 0.90	$ 0.25