Notes Payable - 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, 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
6.25% Debentures due 2026, issued 2006
Debt Instrument [Line Items]
Interest expense at the contractual interest rate	0	0	0	2,900
Amortization of debt discount	$ 0	$ 0	$ 0	$ 793