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Financial Assets Measured at Fair Value on Recurring Basis (Detail) (USD $)
In Thousands, unless otherwise specified
Apr. 30, 2014
Jan. 31, 2014
Apr. 30, 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 $ 325,490 $ 648,235 $ 364,709
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 140,411 309,423 153,376
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 113,238 211,437 126,594
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 24,857 39,854 42,345
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 24,684 35,107 8,482
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 17,914 46,455 21,731
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 2,600 1,666 337
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 1,786 4,293 11,844
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 162,711 361,837 187,288
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 140,411 309,423 153,376
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 17,914 46,455 21,731
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 2,600 1,666 337
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 1,786 4,293 11,844
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 162,779 286,398 177,421
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 113,238 211,437 126,594
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 24,857 39,854 42,345
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 $ 24,684 $ 35,107 $ 8,482