Recognized Amounts in Interest Expense in the Condensed Consolidated Statement of Operations Related to the 6.25% Debentures (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Jan. 31, 2012	Jan. 31, 2011	Jan. 31, 2010
Debt Instrument [Line Items]
Interest expense at the contractual interest rate	$ 31,444	$ 18,411	$ 17,546
6.25% Debentures due 2026, issued 2006
Debt Instrument [Line Items]
Interest expense at the contractual interest rate	2,900	10,322	10,312
Amortization of debt discount	$ 793	$ 3,010	$ 2,764