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		6 Months Ended
	Jul. 31, 2012	Jul. 31, 2011	Jul. 31, 2012	Jul. 31, 2011
Debt Instrument [Line Items]
Interest expense at the contractual interest rate	$ 4,737	$ 4,634	$ 9,331	$ 22,074
4.00% Debentures due 2031
Debt Instrument [Line Items]
Interest expense at the contractual interest rate	2,530	2,530	5,060	3,289
Amortization of debt discount	$ 1,318	$ 1,228	$ 2,613	$ 1,632