Notes Payable - Recognized Amounts in Interest Expense in the Condensed Consolidated Statement of Operations Related to the 4.00% Debentures (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	Oct. 31, 2012	Oct. 31, 2011	Oct. 31, 2012	Oct. 31, 2011
Debt Instrument [Line Items]
Interest expense at the contractual interest rate	$ 4,652	$ 4,615	$ 13,983	$ 26,689
4.00% Debentures due 2031
Debt Instrument [Line Items]
Interest expense at the contractual interest rate	2,530	2,530	7,590	5,819
Amortization of debt discount	$ 1,342	$ 1,250	$ 3,955	$ 2,882