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	3 Months Ended		9 Months Ended
	Oct. 31, 2011	Oct. 31, 2010	Oct. 31, 2011	Oct. 31, 2010
Debt Instrument [Line Items]
Interest expense at the contractual interest rate	$ 4,615	$ 4,324	$ 26,689	$ 13,378
6.25% Debentures due 2026, issued 2006
Debt Instrument [Line Items]
Interest expense at the contractual interest rate		2,578	2,900	7,744
Amortization of debt discount		$ 760	$ 793	$ 2,233