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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
Apr. 30, 2013
Apr. 30, 2012
Debt Instrument [Line Items]    
Interest expense at the contractual interest rate $ 4,785 $ 4,594
4.00% Debentures due 2031
   
Debt Instrument [Line Items]    
Interest expense at the contractual interest rate 2,530 2,530
Amortization of debt discount $ 1,391 $ 1,295