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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
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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

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