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Marketable Securities (Amortized Cost, Gross Unrealized Gains (Losses) And Fair Value Of Available-For-Sale Securities) (Details) (USD $) In Thousands, unless otherwise specified	Apr. 30, 2012	Jan. 31, 2012	Apr. 30, 2011
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 208,478	$ 219,036	$ 405,155
Unrealized Gains	457	627	1,258
Unrealized (Losses)	(648)	(2,896)	(3,779)
Fair Value	208,287	216,767	402,634
Short-Term Investments [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	116,945	89,736	68,637
Unrealized Gains	132	156	81
Unrealized (Losses)	(21)	(38)	(30)
Fair Value	117,056	89,854	68,688
Long-Term Investments [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	91,533	129,300	336,518
Unrealized Gains	325	471	1,177
Unrealized (Losses)	(627)	(2,858)	(3,749)
Fair Value	91,231	126,913	333,946
Corporate Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair Value	87,750	95,935	138,799
Corporate Bonds [Member] | Short-Term Investments [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	40,934	34,899	5,377
Unrealized Gains	23	7	18
Unrealized (Losses)	(14)	(28)
Fair Value	40,943	34,878	5,395
Corporate Bonds [Member] | Long-Term Investments [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	46,685	60,852	132,937
Unrealized Gains	148	277	513
Unrealized (Losses)	(26)	(72)	(46)
Fair Value	46,807	61,057	133,404
Municipal And Pre-Refunded Municipal Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair Value	55,529	60,234	133,338
Municipal And Pre-Refunded Municipal Bonds [Member] | Short-Term Investments [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	36,465	41,519	36,498
Unrealized Gains	96	135	24
Unrealized (Losses)	(5)	(10)	(30)
Fair Value	36,556	41,644	36,492
Municipal And Pre-Refunded Municipal Bonds [Member] | Long-Term Investments [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	18,809	18,410	96,748
Unrealized Gains	169	186	367
Unrealized (Losses)	(5)	(6)	(269)
Fair Value	18,973	18,590	96,846
Federal Government Agencies [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair Value		9,627	71,628
Federal Government Agencies [Member] | Short-Term Investments [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	6,615	4,513	16,284
Unrealized Gains		2	17
Unrealized (Losses)	(1)
Fair Value	6,614	4,515	16,301
Federal Government Agencies [Member] | Long-Term Investments [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		5,111	55,122
Unrealized Gains		2	205
Unrealized (Losses)		(1)
Fair Value		5,112	55,327
FDIC Insured Corporate Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair Value	6,614		10,500
FDIC Insured Corporate Bonds [Member] | Short-Term Investments [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost			10,478
Unrealized Gains			22
Fair Value			10,500
Auction Rate Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair Value	4,330	20,197	26,716
Auction Rate Securities [Member] | Long-Term Investments [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	4,925	22,975	30,150
Unrealized (Losses)	(595)	(2,778)	(3,434)
Fair Value	4,330	20,197	26,716
Treasury Bills [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair Value	24,643	14,988	19,753
Treasury Bills [Member] | Short-Term Investments [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	11,694
Unrealized (Losses)	(1)
Fair Value	11,693
Treasury Bills [Member] | Long-Term Investments [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	12,944	14,982	19,661
Unrealized Gains	6	6	92
Fair Value	12,950	14,988	19,753
Certificate Of Deposit [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair Value	21,840	12,194
Certificate Of Deposit [Member] | Short-Term Investments [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	13,665	5,225
Unrealized Gains	4
Fair Value	13,669	5,225
Certificate Of Deposit [Member] | Long-Term Investments [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	8,170	6,970
Unrealized Gains	2
Unrealized (Losses)	(1)	(1)
Fair Value	8,171	6,969
Variable Demand Notes [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair Value	4,330		1,900
Variable Demand Notes [Member] | Long-Term Investments [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost			1,900
Fair Value			1,900
Commercial Paper [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair Value	7,581	3,592
Commercial Paper [Member] | Short-Term Investments [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	7,572	3,580
Unrealized Gains	9	12
Fair Value	$ 7,581	$ 3,592

X

- Definition

This item represents the cost of debt and equity securities, which are categorized neither as held-to-maturity nor trading, net of adjustments including accretion, amortization, collection of cash, previous other-than-temporary impairments recognized in earnings (less any cumulative-effect adjustments recognized, as defined), and fair value hedge accounting adjustments, if any.

+ References

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

-Publisher FASB

-Name FASB Staff Position (FSP)

-Number FAS115-1/124-1

-Paragraph 7

-Footnote 2

-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 Statement of Financial Accounting Standard (FAS)

-Number 115

-Paragraph 19

-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

-Topic 210

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SX 210.5-02.2)

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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

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-Name Accounting Standards Codification

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- Definition

This item represents the gross unrealized gains for securities, at a point in time, which are categorized neither as held-to-maturity nor trading securities.

+ References

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

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 133

-Paragraph 22

-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 Statement of Financial Accounting Standard (FAS)

-Number 115

-Paragraph 13, 19

-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

-Topic 320

-SubTopic 10

-Section 50

-Paragraph 2

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- Definition

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+ References

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