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Fair Value (Financial Assets Measured At Fair Value On A Recurring Basis) (Details) (USD $) In Thousands, unless otherwise specified	Apr. 30, 2012	Jan. 31, 2012	Apr. 30, 2011
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Financial assets accounted for at fair value on a recurring basis	$ 208,287	$ 216,767	$ 402,634
Level 1 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Financial assets accounted for at fair value on a recurring basis	119,007	120,550	240,680
Level 2 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Financial assets accounted for at fair value on a recurring basis	84,950	76,020	135,238
Level 3 [Member]
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	26,716
Corporate Bonds [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Financial assets accounted for at fair value on a recurring basis	87,750	95,935	138,799
Corporate Bonds [Member] | Level 1 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Financial assets accounted for at fair value on a recurring basis	87,750	95,935	138,799
Municipal And Pre-Refunded Municipal Bonds [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Financial assets accounted for at fair value on a recurring basis	55,529	60,234	133,338
Municipal And Pre-Refunded Municipal Bonds [Member] | Level 2 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Financial assets accounted for at fair value on a recurring basis	55,529	60,234	133,338
Federal Government Agencies [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Financial assets accounted for at fair value on a recurring basis		9,627	71,628
Federal Government Agencies [Member] | Level 1 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Financial assets accounted for at fair value on a recurring basis		9,627	71,628
FDIC Insured Corporate Bonds [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Financial assets accounted for at fair value on a recurring basis	6,614		10,500
FDIC Insured Corporate Bonds [Member] | Level 1 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Financial assets accounted for at fair value on a recurring basis	6,614		10,500
Auction Rate Securities [Member]
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	26,716
Auction Rate Securities [Member] | Level 3 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Financial assets accounted for at fair value on a recurring basis		20,197	26,716
Treasury Bills [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Financial assets accounted for at fair value on a recurring basis	24,643	14,988	19,753
Treasury Bills [Member] | Level 1 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Financial assets accounted for at fair value on a recurring basis	24,643	14,988	19,753
Certificate Of Deposit [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Financial assets accounted for at fair value on a recurring basis	21,840	12,194
Certificate Of Deposit [Member] | Level 2 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Financial assets accounted for at fair value on a recurring basis	21,840	12,194
Variable Demand Notes [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Financial assets accounted for at fair value on a recurring basis	4,330		1,900
Variable Demand Notes [Member] | Level 2 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Financial assets accounted for at fair value on a recurring basis			1,900
Variable Demand Notes [Member] | Level 3 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Financial assets accounted for at fair value on a recurring basis	4,330
Commercial Paper [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Financial assets accounted for at fair value on a recurring basis	7,581	3,592
Commercial Paper [Member] | Level 2 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Financial assets accounted for at fair value on a recurring basis	$ 7,581	$ 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 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 2: 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=6379932&loc=d3e22054-111558



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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Balance Type:	debit
Period Type:	instant

X

- Details

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