Intrinsic Value of Options Exercised and Weighted Average Grant Date Fair Value of Stock Options Granted (Detail) (USD $) In Millions, except Per Share data, unless otherwise specified	12 Months Ended
	Jan. 31, 2012		Jan. 31, 2011		Jan. 31, 2010
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Intrinsic value of options exercised	$ 85.7	[1]	$ 61.9	[1]	$ 18.8	[1]
Weighted average grant date fair value per share of stock options granted	$ 14.04	[2]	$ 9.30	[2]	$ 6.13	[2]

[1] The intrinsic value of options exercised is calculated as the difference between the exercise price of the option and the market value of the stock on the date of exercise.

[2] The weighted average grant date fair value per share of stock options granted is calculated, as of the stock option grant date, using the BSM option pricing model.