Recognized Amounts in Interest Expense in the Condensed Consolidated Statement of Operations Related to the 4.00% Debentures (Detail) (USD $) In Thousands	3 Months Ended		6 Months Ended
	Jul. 31, 2011	Jul. 31, 2010	Jul. 31, 2011	Jul. 31, 2010
Debt Instrument [Line Items]
Interest expense at the contractual interest rate	$ 4,634	$ 4,727	$ 22,074	$ 9,054
4.00% Debentures due 2031
Debt Instrument [Line Items]
Interest expense at the contractual interest rate	2,530		3,289
Amortization of debt discount	$ 1,228		$ 1,632