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Financial Assets Measured at Fair Value on Recurring Basis (Detail) (USD $)
In Thousands, unless otherwise specified
Jul. 31, 2013
Jan. 31, 2013
Jul. 31, 2012
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Financial assets accounted for at fair value on a recurring basis $ 442,565 $ 378,071 $ 227,554
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 208,218 203,361 137,931
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 234,347 170,380 85,293
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 4,330
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 192,361 116,364 52,808
Municipal And Pre-Refunded Municipal Bonds | 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 192,361 116,364 52,808
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 175,229 152,775 101,813
Corporate Bonds | 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 175,229 152,775 101,813
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 33,694 43,235 25,198
Certificates of Deposit | 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 33,694 43,235 25,198
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 24,344 41,105 30,295
US Treasury Bill Securities | 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 24,344 41,105 30,295
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 8,292 10,781 7,287
Commercial Paper | 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 8,292 10,781 7,287
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 7,793 9,481 5,823
US Government Corporations and Agencies Securities | 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 7,793 9,481 5,823
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 852    
Mutual funds, held in rabbi trust | 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 852    
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 4,330
Auction Rate Securities | 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 $ 4,330