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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
12 Months Ended
Jan. 31, 2012
Jan. 31, 2011
Jan. 31, 2010
Debt Instrument [Line Items]      
Interest expense at the contractual interest rate $ 31,444 $ 18,411 $ 17,546
4.00% Debentures due 2031
     
Debt Instrument [Line Items]      
Interest expense at the contractual interest rate 8,349    
Amortization of debt discount $ 4,154