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Financial Assets Measured at Fair Value on Recurring Basis (Detail) (USD $)
In Thousands, unless otherwise specified
Oct. 31, 2014
Jan. 31, 2014
Oct. 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 $ 229,605 $ 648,235 $ 530,747
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 130,362 309,423 234,041
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 65,155 211,437 207,397
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 15,668 39,854 28,752
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 8,726 46,455 22,274
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 3,593 35,107 28,167
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 3,529 1,666 1,328
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 2,572 4,293 8,788
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 145,189 361,837 266,431
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 130,362 309,423 234,041
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 8,726 46,455 22,274
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 3,529 1,666 1,328
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 2,572 4,293 8,788
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 84,416 286,398 264,316
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 65,155 211,437 207,397
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 15,668 39,854 28,752
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 $ 3,593 $ 35,107 $ 28,167