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Financial Assets Measured at Fair Value on Recurring Basis (Detail) (USD $)
In Thousands, unless otherwise specified
Jan. 31, 2014
Jan. 31, 2013
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Financial assets accounted for at fair value on a recurring basis $ 648,235 $ 378,071
Corporate Bonds
    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Financial assets accounted for at fair value on a recurring basis 309,423 152,775
Municipal And Pre-Refunded Municipal Bonds
    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Financial assets accounted for at fair value on a recurring basis 211,437 116,364
US Treasury Bill Securities
    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Financial assets accounted for at fair value on a recurring basis 46,455 41,105
Certificates of Deposit
    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Financial assets accounted for at fair value on a recurring basis 39,854 43,235
Commercial Paper
    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Financial assets accounted for at fair value on a recurring basis 35,107 10,781
US Government Corporations and Agencies Securities
    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Financial assets accounted for at fair value on a recurring basis 4,293 9,481
Auction Rate Securities
    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Financial assets accounted for at fair value on a recurring basis   4,330
Mutual funds, held in rabbi trust
    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Financial assets accounted for at fair value on a recurring basis 1,666  
Fair Value, Inputs, Level 1
    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Financial assets accounted for at fair value on a recurring basis 361,837 203,361
Fair Value, Inputs, Level 1 | Corporate Bonds
    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Financial assets accounted for at fair value on a recurring basis 309,423 152,775
Fair Value, Inputs, Level 1 | US Treasury Bill Securities
    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Financial assets accounted for at fair value on a recurring basis 46,455 41,105
Fair Value, Inputs, Level 1 | US Government Corporations and Agencies Securities
    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Financial assets accounted for at fair value on a recurring basis 4,293 9,481
Fair Value, Inputs, Level 1 | Mutual funds, held in rabbi trust
    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Financial assets accounted for at fair value on a recurring basis 1,666  
Fair Value, Inputs, Level 2
    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Financial assets accounted for at fair value on a recurring basis 286,398 170,380
Fair Value, Inputs, Level 2 | Municipal And Pre-Refunded Municipal Bonds
    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Financial assets accounted for at fair value on a recurring basis 211,437 116,364
Fair Value, Inputs, Level 2 | Certificates of Deposit
    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Financial assets accounted for at fair value on a recurring basis 39,854 43,235
Fair Value, Inputs, Level 2 | Commercial Paper
    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Financial assets accounted for at fair value on a recurring basis 35,107 10,781
Fair Value, Inputs, Level 3
    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Financial assets accounted for at fair value on a recurring basis   4,330
Fair Value, Inputs, Level 3 | Auction Rate Securities
    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Financial assets accounted for at fair value on a recurring basis   $ 4,330