Stock-based Compensation Expense Included in the Results of Operations and the Tax Benefit Associated with the Exercise of Stock Options (Parenthetical) (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Jan. 31, 2012		Jan. 31, 2011		Jan. 31, 2010
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Stock-based compensation	$ 21,658	[1]	$ 20,511	[1]	$ 26,159	[1]
Equity Compensation Plans
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Stock-based compensation					25,657
Founders
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Stock-based compensation					$ 502

[1] All of equity plan-related compensation expense for the years ended January 31, 2012 and 2011 relates to stock options, stock awards, and the ESPPs. Equity plan-related compensation expense for the year ended January 31, 2010 includes $25,657 relating to stock options, stock awards, and the ESPPs and $502 relating to the amortization of expense associated with stock issued to the founders of a company we acquired during the year ended January 31, 2008.