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Financial Assets Measured at Fair Value on Recurring Basis (Detail) (USD $) In Thousands, unless otherwise specified	Jul. 31, 2012	Jan. 31, 2012	Jul. 31, 2011
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Financial assets accounted for at fair value on a recurring basis	$ 227,554	$ 216,767	$ 382,249
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	137,931	120,550	231,501
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	85,293	76,020	127,244
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	23,504
Corporate Bonds
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Financial assets accounted for at fair value on a recurring basis	101,813	95,935	145,121
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	101,813	95,935	145,121
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	52,808	60,234	124,343
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	52,808	60,234	124,343
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	30,295	14,988	19,829
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	30,295	14,988	19,829
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	5,823	9,627	57,404
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	5,823	9,627	57,404
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	23,504
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	23,504
FDIC Insured Corporate Bonds
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Financial assets accounted for at fair value on a recurring basis			4,004
FDIC Insured 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			4,004
Variable Demand Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Financial assets accounted for at fair value on a recurring basis			2,901
Variable Demand Notes | 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			2,901
Certificates of Deposit
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Financial assets accounted for at fair value on a recurring basis	25,198	12,194	5,143
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			5,143
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	25,198	12,194
Commercial Paper
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Financial assets accounted for at fair value on a recurring basis	7,287	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	$ 7,287	$ 3,592

X

- Definition

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.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 320

-SubTopic 10

-Section 25

-Paragraph 1

-Subparagraph (b)

-URI http://asc.fasb.org/extlink&oid=7534914&loc=d3e22054-111558



Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 115

-Paragraph 12

-Subparagraph b

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.



Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Glossary Available-for-Sale Securities

-URI http://asc.fasb.org/extlink&oid=6505594

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

X

- Details

Name:	us-gaap_FairValueBalanceSheetGroupingFinancialStatementCaptionsLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration