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Financial Assets Measured at Fair Value on Recurring Basis (Detail) (USD $) In Thousands, unless otherwise specified	Oct. 31, 2012	Jan. 31, 2012	Oct. 31, 2011
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Financial assets accounted for at fair value on a recurring basis	$ 239,832	$ 216,767	$ 175,795
Fair Value, Inputs, Level 1
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Financial assets accounted for at fair value on a recurring basis	145,178	120,550	90,377
Fair Value, Inputs, Level 2
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Financial assets accounted for at fair value on a recurring basis	90,324	76,020	64,462
Fair Value, Inputs, Level 3
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Financial assets accounted for at fair value on a recurring basis	4,330	20,197	20,956
Corporate Bonds
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Financial assets accounted for at fair value on a recurring basis	111,662	95,935	81,407
Corporate Bonds \| Fair Value, Inputs, Level 1
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Financial assets accounted for at fair value on a recurring basis	111,662	95,935	81,407
Municipal And Pre-Refunded Municipal Bonds
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Financial assets accounted for at fair value on a recurring basis	51,505	60,234	64,462
Municipal And Pre-Refunded Municipal Bonds \| Fair Value, Inputs, Level 2
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Financial assets accounted for at fair value on a recurring basis	51,505	60,234	64,462
Certificates of Deposit
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Financial assets accounted for at fair value on a recurring basis	30,630	12,194	6,969
Certificates of Deposit \| Fair Value, Inputs, Level 1
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Financial assets accounted for at fair value on a recurring basis			6,969
Certificates of Deposit \| Fair Value, Inputs, Level 2
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Financial assets accounted for at fair value on a recurring basis	30,630	12,194
US Treasury Bill Securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Financial assets accounted for at fair value on a recurring basis	28,803	14,988
US Treasury Bill Securities \| Fair Value, Inputs, Level 1
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Financial assets accounted for at fair value on a recurring basis	28,803	14,988
Commercial Paper
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Financial assets accounted for at fair value on a recurring basis	8,189	3,592
Commercial Paper \| Fair Value, Inputs, Level 2
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Financial assets accounted for at fair value on a recurring basis	8,189	3,592
US Government Corporations and Agencies Securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Financial assets accounted for at fair value on a recurring basis	4,713	9,627	2,001
US Government Corporations and Agencies Securities \| Fair Value, Inputs, Level 1
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Financial assets accounted for at fair value on a recurring basis	4,713	9,627	2,001
Auction Rate Securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Financial assets accounted for at fair value on a recurring basis	4,330	20,197	20,956
Auction Rate Securities \| Fair Value, Inputs, Level 3
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Financial assets accounted for at fair value on a recurring basis	$ 4,330	$ 20,197	$ 20,956

This element represents the portion of the balance sheet assertion valued at fair value by the entity whether such amount is presented as a separate caption or as a parenthetical disclosure. Additionally, this element may be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements. The element may be used in both the balance sheet and disclosure in the same submission. This item represents Available-for-sale Securities which consist of all investments in certain debt and equity securities neither classified as trading or held-to-maturity securities. A debt security represents a creditor relationship with an enterprise. Debt securities include, among other items, US Treasury securities, US government securities, municipal securities, corporate bonds, convertible debt, commercial paper, and all securitized debt instruments. An equity security represents an ownership interest in an enterprise or the right to acquire or dispose of an ownership interest in an enterprise at fixed or determinable prices. Equity securities include, among other things, common stock, certain preferred stock, warrant rights, call options, and put options, but do not include convertible debt. An entity may opt to provide the reader with additional narrative text to better understand the nature of investments in debt and equity securities which are categorized as Available-for-sale.

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