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Amortized Cost, Gross Unrealized Gains (Losses) and Fair Value of Available-For-Sale Securities (Detail) (USD $) In Thousands, unless otherwise specified	Jul. 31, 2012	Jan. 31, 2012	Jul. 31, 2011
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 227,773	$ 219,036	$ 383,191
Unrealized Gains	434	627	2,156
Unrealized (Losses)	(653)	(2,896)	(3,098)
Fair Value	227,554	216,767	382,249
Short-term Investments
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	135,715	89,736	59,271
Unrealized Gains	192	156	99
Unrealized (Losses)	(17)	(38)	(23)
Fair Value	135,890	89,854	59,347
Long Term Investments
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	92,058	129,300	323,920
Unrealized Gains	242	471	2,057
Unrealized (Losses)	(636)	(2,858)	(3,075)
Fair Value	91,664	126,913	322,902
Corporate Bonds
Schedule of Available-for-sale Securities [Line Items]
Fair Value	101,813	95,935	145,121
Corporate Bonds | Short-term Investments
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	53,713	34,899	10,062
Unrealized Gains	71	7	22
Unrealized (Losses)	(8)	(28)	(6)
Fair Value	53,776	34,878	10,078
Corporate Bonds | Long Term Investments
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	47,905	60,852	134,199
Unrealized Gains	150	277	857
Unrealized (Losses)	(18)	(72)	(13)
Fair Value	48,037	61,057	135,043
Municipal And Pre-Refunded Municipal Bonds
Schedule of Available-for-sale Securities [Line Items]
Fair Value	52,808	60,234	124,343
Municipal And Pre-Refunded Municipal Bonds | Short-term Investments
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	28,019	41,519	38,079
Unrealized Gains	95	135	57
Unrealized (Losses)	(7)	(10)	(17)
Fair Value	28,107	41,644	38,119
Municipal And Pre-Refunded Municipal Bonds | Long Term Investments
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	24,646	18,410	85,639
Unrealized Gains	78	186	624
Unrealized (Losses)	(23)	(6)	(39)
Fair Value	24,701	18,590	86,224
US Treasury Bill Securities
Schedule of Available-for-sale Securities [Line Items]
Fair Value	30,295	14,988	19,829
US Treasury Bill Securities | Short-term Investments
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	21,425
Unrealized Gains	5
Fair Value	21,430
US Treasury Bill Securities | Long Term Investments
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	8,855	14,982	19,639
Unrealized Gains	10	6	190
Fair Value	8,865	14,988	19,829
Certificates of Deposit
Schedule of Available-for-sale Securities [Line Items]
Fair Value	25,198	12,194	5,143
Certificates of Deposit | Short-term Investments
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	20,671	5,225
Unrealized Gains	13
Unrealized (Losses)	(2)
Fair Value	20,682	5,225
Certificates of Deposit | Long Term Investments
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	4,514	6,970	5,144
Unrealized Gains	2
Unrealized (Losses)		(1)	(1)
Fair Value	4,516	6,969	5,143
Commercial Paper
Schedule of Available-for-sale Securities [Line Items]
Fair Value	7,287	3,592
Commercial Paper | Short-term Investments
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	7,279	3,580
Unrealized Gains	8	12
Fair Value	7,287	3,592
US Government Corporations and Agencies Securities
Schedule of Available-for-sale Securities [Line Items]
Fair Value	5,823	9,627	57,404
US Government Corporations and Agencies Securities | Short-term Investments
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	4,608	4,513	7,130
Unrealized Gains		2	16
Fair Value	4,608	4,515	7,146
US Government Corporations and Agencies Securities | Long Term Investments
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	1,213	5,111	49,872
Unrealized Gains	2	2	386
Unrealized (Losses)		(1)
Fair Value	1,215	5,112	50,258
Auction Rate Securities
Schedule of Available-for-sale Securities [Line Items]
Fair Value	4,330	20,197	23,504
Auction Rate Securities | Long Term Investments
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	4,925	22,975	26,525
Unrealized (Losses)	(595)	(2,778)	(3,021)
Fair Value	4,330	20,197	23,504
FDIC Insured Corporate Bonds
Schedule of Available-for-sale Securities [Line Items]
Fair Value			4,004
FDIC Insured Corporate Bonds | Short-term Investments
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost			4,000
Unrealized Gains			4
Fair Value			4,004
Variable Demand Notes
Schedule of Available-for-sale Securities [Line Items]
Fair Value			2,901
Variable Demand Notes | Long Term Investments
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost			2,902
Unrealized (Losses)			(1)
Fair Value			$ 2,901

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 Accounting Standards Codification

-Topic 210

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SX 210.5-02.2)

-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682



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-Name FASB Staff Position (FSP)

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-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.



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-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.

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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 Accounting Standards Codification

-Topic 320

-SubTopic 10

-Section 25

-Paragraph 1

-Subparagraph (b)

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-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

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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 Accounting Standards Codification

-Topic 320

-SubTopic 10

-Section 50

-Paragraph 2

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-URI http://asc.fasb.org/extlink&oid=6872113&loc=d3e27161-111563



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-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.

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