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Financial Assets Measured at Fair Value on Recurring Basis (Detail) (USD $)
In Thousands, unless otherwise specified
Jul. 31, 2014
Jan. 31, 2014
Jul. 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 $ 263,867 $ 648,235 $ 442,565
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 148,923 309,423 175,229
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 74,526 211,437 192,361
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 16,850 39,854 33,694
Treasury Bills
      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Financial assets accounted for at fair value on a recurring basis 15,803 46,455 24,344
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 3,089 1,666 852
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 2,896 35,107 8,292
Federal Government Agencies
      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Financial assets accounted for at fair value on a recurring basis 1,780 4,293 7,793
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 169,595 361,837 208,218
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 148,923 309,423 175,229
Fair Value, Inputs, Level 1 | Treasury Bills
      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Financial assets accounted for at fair value on a recurring basis 15,803 46,455 24,344
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 3,089 1,666 852
Fair Value, Inputs, Level 1 | Federal Government Agencies
      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Financial assets accounted for at fair value on a recurring basis 1,780 4,293 7,793
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 94,272 286,398 234,347
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 74,526 211,437 192,361
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 16,850 39,854 33,694
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 $ 2,896 $ 35,107 $ 8,292