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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	Apr. 30, 2013	Jan. 31, 2013	Apr. 30, 2012
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 364,396	$ 378,393	$ 208,478
Unrealized Gains	389	438	457
Unrealized (Losses)	(76)	(760)	(648)
Fair Value	364,709	378,071	208,287
Short-term Investments
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	214,828	228,286	116,945
Unrealized Gains	137	242	132
Unrealized (Losses)	(27)	(42)	(21)
Fair Value	214,938	228,486	117,056
Long Term Investments
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	149,568	150,107	91,533
Unrealized Gains	252	196	325
Unrealized (Losses)	(49)	(718)	(627)
Fair Value	149,771	149,585	91,231
Corporate Bonds
Schedule of Available-for-sale Securities [Line Items]
Fair Value	153,376	152,775	87,750
Corporate Bonds | Short-term Investments
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	76,563	88,432	40,934
Unrealized Gains	49	106	23
Unrealized (Losses)	(26)	(23)	(14)
Fair Value	76,586	88,515	40,943
Corporate Bonds | Long Term Investments
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	76,717	64,219	46,685
Unrealized Gains	117	102	148
Unrealized (Losses)	(44)	(61)	(26)
Fair Value	76,790	64,260	46,807
Municipal And Pre-Refunded Municipal Bonds
Schedule of Available-for-sale Securities [Line Items]
Fair Value	126,594	116,364	55,529
Municipal And Pre-Refunded Municipal Bonds | Short-term Investments
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	75,507	63,355	36,465
Unrealized Gains	58	85	96
Unrealized (Losses)	(1)	(17)	(5)
Fair Value	75,564	63,423	36,556
Municipal And Pre-Refunded Municipal Bonds | Long Term Investments
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	50,913	52,925	18,809
Unrealized Gains	121	76	169
Unrealized (Losses)	(4)	(60)	(5)
Fair Value	51,030	52,941	18,973
Certificates of Deposit
Schedule of Available-for-sale Securities [Line Items]
Fair Value	42,345	43,235	21,840
Certificates of Deposit | Short-term Investments
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	40,222	40,870	13,665
Unrealized Gains	23	25	4
Fair Value	40,245	40,895	13,669
Certificates of Deposit | Long Term Investments
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	2,099	2,340	8,170
Unrealized Gains	1		2
Unrealized (Losses)			(1)
Fair Value	2,100	2,340	8,171
US Treasury Bill Securities
Schedule of Available-for-sale Securities [Line Items]
Fair Value	21,731	41,105	24,643
US Treasury Bill Securities | Short-term Investments
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	12,060	21,354	11,694
Unrealized Gains	1	14
Unrealized (Losses)			(1)
Fair Value	12,061	21,368	11,693
US Treasury Bill Securities | Long Term Investments
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	9,666	19,724	12,944
Unrealized Gains	4	13	6
Fair Value	9,670	19,737	12,950
US Government Corporations and Agencies Securities
Schedule of Available-for-sale Securities [Line Items]
Fair Value	11,844	9,481	6,614
US Government Corporations and Agencies Securities | Short-term Investments
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	2,000	3,500	6,615
Unrealized Gains		4
Unrealized (Losses)			(1)
Fair Value	2,000	3,504	6,614
US Government Corporations and Agencies Securities | Long Term Investments
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	9,845	5,974
Unrealized Gains		5
Unrealized (Losses)	(1)	(2)
Fair Value	9,844	5,977
Commercial Paper
Schedule of Available-for-sale Securities [Line Items]
Fair Value	8,482	10,781	7,581
Commercial Paper | Short-term Investments
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	8,476	10,775	7,572
Unrealized Gains	6	8	9
Unrealized (Losses)		(2)
Fair Value	8,482	10,781	7,581
Mutual funds, held in rabbi trust
Schedule of Available-for-sale Securities [Line Items]
Fair Value	337
Mutual funds, held in rabbi trust | Long Term Investments
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	328
Unrealized Gains	9
Fair Value	337
Auction Rate Securities
Schedule of Available-for-sale Securities [Line Items]
Fair Value		4,330	4,330
Auction Rate Securities | Long Term Investments
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		4,925	4,925
Unrealized (Losses)		(595)	(595)
Fair Value		$ 4,330	$ 4,330

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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-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 3: 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.

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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)

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



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-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

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

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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

-Subparagraph (b)

-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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+ References

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