Investments (Details) (USD $)	12 Months Ended
Mar. 31, 2011	Mar. 31, 2010	Mar. 31, 2009
Summary of available-for-sale securities [Line Items]
Available-for-sale, adjusted cost	$ 1,000,947,000	$ 1,010,877,000
Available-for-sale, gross unrealized gains	4,997,000	4,488,000
Available-for-sale, gross unrealized losses	(1,534,000)	(857,000)
Available-for-sale, estimated fair value	1,004,410,000	1,014,508,000
Summary of trading securities [Line Items]
Trading securities, adjusted cost		24,900,000
Trading securities, estimated fair value		24,900,000
Amount of short-term investments	539,572,000	722,193,000
Amount of long-term investments	464,838,000	317,215,000
Amount of marketable securities	26,900,000
Amount of investment in auction rate securities	12,500,000
Amount of investment in auction rate securities failed	12,500,000
Impairment amount in value of auction rate securities failed	1,600,000	4,700,000
Amount of investments in student loan-backed auction rate securities redeemed at par by issuers.	17,800,000
Amount of reduced overall cost basis position in student loan-backed auction rate securities	7,100,000
Amount of unrealized loss on investments portfolio	1,500,000
Amount of net realized gains on sales of available-for-sale securities		1,200,000	500,000
Schedule of available-for-sale securities, debt maturities [Line Items]
Available-for-sale, adjusted cost	1,000,947,000	1,010,877,000
Available-for-sale, gross unrealized gains	4,997,000	4,488,000
Available-for-sale, gross unrealized losses	(1,534,000)	(857,000)
Available-for-sale, estimated fair value	1,004,410,000	1,014,508,000
Fair value of public companies acquired as part of Silicon Storage Technology, Inc. acquisition	47,100,000
Amount reclassified from non-marketable equity investments to marketable equity investments due to initial public offering	9,000,000
Amount of acquired investments included in short-term investments	16,600,000
Amount of acquired investments included in long-term investments	10,300,000
Amount of unrealized gains included in other comprehensive income from acquired investments	700,000
Fair value of investment in privately held companies at acquisition	18,100,000
Carrying value of acquired investments in privately held companies at time of acquisition	5,600,000
Reduction of assets due to sale	4,400,000
Reduction due to reclassification to marketable securities	8,100,000
Government agency bonds [Member]
Summary of available-for-sale securities [Line Items]
Available-for-sale, adjusted cost	431,355,000	389,801,000
Available-for-sale, gross unrealized gains	159,000	215,000
Available-for-sale, gross unrealized losses	(923,000)	(622,000)
Available-for-sale, estimated fair value	430,591,000	389,394,000
Schedule of available-for-sale securities, debt maturities [Line Items]
Available-for-sale, adjusted cost	431,355,000	389,801,000
Available-for-sale, gross unrealized gains	159,000	215,000
Available-for-sale, gross unrealized losses	(923,000)	(622,000)
Available-for-sale, estimated fair value	430,591,000	389,394,000
Municipal bonds [Member]
Summary of available-for-sale securities [Line Items]
Available-for-sale, adjusted cost	11,445,000	156,415,000
Available-for-sale, gross unrealized gains	34,000	1,290,000
Available-for-sale, gross unrealized losses	(22,000)
Available-for-sale, estimated fair value	11,457,000	157,705,000
Schedule of available-for-sale securities, debt maturities [Line Items]
Available-for-sale, adjusted cost	11,445,000	156,415,000
Available-for-sale, gross unrealized gains	34,000	1,290,000
Available-for-sale, gross unrealized losses	(22,000)
Available-for-sale, estimated fair value	11,457,000	157,705,000
Auction rate securities [Member]
Summary of available-for-sale securities [Line Items]
Available-for-sale, adjusted cost	12,475,000	14,151,000
Available-for-sale, gross unrealized gains	0
Available-for-sale, gross unrealized losses	0
Available-for-sale, estimated fair value	12,475,000	14,151,000
Schedule of available-for-sale securities, debt maturities [Line Items]
Available-for-sale, adjusted cost	12,475,000	14,151,000
Available-for-sale, gross unrealized gains	0
Available-for-sale, gross unrealized losses	0
Available-for-sale, estimated fair value	12,475,000	14,151,000
Auction rate securities [Member]
Summary of trading securities [Line Items]
Trading securities, adjusted cost		23,086,000
Trading securities, estimated fair value		23,086,000
Marketable equity securities [Member]
Summary of available-for-sale securities [Line Items]
Available-for-sale, adjusted cost	26,173,000	58,402,000
Available-for-sale, gross unrealized gains	688,000
Available-for-sale, gross unrealized losses	0
Available-for-sale, estimated fair value	26,861,000	58,402,000
Schedule of available-for-sale securities, debt maturities [Line Items]
Available-for-sale, adjusted cost	26,173,000	58,402,000
Available-for-sale, gross unrealized gains	688,000
Available-for-sale, gross unrealized losses	0
Available-for-sale, estimated fair value	26,861,000	58,402,000
Corporate bonds [Member]
Summary of available-for-sale securities [Line Items]
Available-for-sale, adjusted cost		392,108,000
Available-for-sale, gross unrealized gains		2,983,000
Available-for-sale, gross unrealized losses		(235,000)
Available-for-sale, estimated fair value		394,856,000
Schedule of available-for-sale securities, debt maturities [Line Items]
Available-for-sale, adjusted cost		392,108,000
Available-for-sale, gross unrealized gains		2,983,000
Available-for-sale, gross unrealized losses		(235,000)
Available-for-sale, estimated fair value		394,856,000
Corporate bonds and debt [Member]
Summary of available-for-sale securities [Line Items]
Available-for-sale, adjusted cost	519,499,000
Available-for-sale, gross unrealized gains	4,116,000
Available-for-sale, gross unrealized losses	(589,000)
Available-for-sale, estimated fair value	523,026,000
Schedule of available-for-sale securities, debt maturities [Line Items]
Available-for-sale, adjusted cost	519,499,000
Available-for-sale, gross unrealized gains	4,116,000
Available-for-sale, gross unrealized losses	(589,000)
Available-for-sale, estimated fair value	523,026,000
Put option on auction rate securities [Member]
Summary of trading securities [Line Items]
Trading securities, adjusted cost		1,814,000
Trading securities, estimated fair value		1,814,000
Due in one year or less [Member]
Summary of available-for-sale securities [Line Items]
Available-for-sale, adjusted cost	143,447,000	308,020,000
Available-for-sale, gross unrealized gains	903,000	1,743,000
Available-for-sale, gross unrealized losses	(2,000)	(19,000)
Available-for-sale, estimated fair value	144,348,000	309,744,000
Schedule of available-for-sale securities, debt maturities [Line Items]
Available-for-sale, adjusted cost	143,447,000	308,020,000
Available-for-sale, gross unrealized gains	903,000	1,743,000
Available-for-sale, gross unrealized losses	(2,000)	(19,000)
Available-for-sale, estimated fair value	144,348,000	309,744,000
Due after one year and through five years [Member]
Summary of available-for-sale securities [Line Items]
Available-for-sale, adjusted cost	815,352,000	688,706,000
Available-for-sale, gross unrealized gains	3,406,000	2,745,000
Available-for-sale, gross unrealized losses	(1,532,000)	(838,000)
Available-for-sale, estimated fair value	817,226,000	690,613,000
Schedule of available-for-sale securities, debt maturities [Line Items]
Available-for-sale, adjusted cost	815,352,000	688,706,000
Available-for-sale, gross unrealized gains	3,406,000	2,745,000
Available-for-sale, gross unrealized losses	(1,532,000)	(838,000)
Available-for-sale, estimated fair value	817,226,000	690,613,000
Due after ten years [Member]
Summary of available-for-sale securities [Line Items]
Available-for-sale, adjusted cost	12,475,000	14,151,000
Available-for-sale, estimated fair value	12,475,000	14,151,000
Schedule of available-for-sale securities, debt maturities [Line Items]
Available-for-sale, adjusted cost	12,475,000	14,151,000
Available-for-sale, estimated fair value	$ 12,475,000	$ 14,151,000

Amount of securities that are debt instruments (corporate or municipal bonds) that typically have long-term nominal maturities for which the interest rate is reset through an auction process. It could also refer to a preferred stock for which the dividend is reset through the same type of auction process.

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Amount of failed investment in auction rate securities. Auction rate securities are debt instruments (corporate or municipal bonds) that typically have long-term nominal maturities for which the interest rate is reset through an auction process. It could also refer to a preferred stock for which the dividend is reset through the same type of auction process.

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Amount of investments in student loan-backed auction rate securities redeemed or bought back, at par, by issuers.

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Represents investments in common and preferred stocks and other forms of securities that provide ownership interests in a corporation.

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Amount of reduced overall cost basis position in student loan-backed auction rate securities, after redemption, at par, by issuers.

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Represents the unrealized loss on investments due to fluctuations in interest rates and credit market conditions.

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The fair value of investments in public companies that were acquired in a business acquisition.

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The portion of the fair value of investments in public companies that were acquired in a business acquisition that are included in long-term investments.

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The portion of the fair value of investments in public companies that were acquired in a business acquisition that are included in short-term investments.

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Amount of equity investments acquired in business acquisition that were reclassified from non-marketable to marketable following the initial public offering of the investee.

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The amount of unrealized gains from acquired investments in marketable securities included in other comprehensive income.

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This item represents the carrying amount on the entity's balance sheet of its investment in common stock of cost and equity method investees that are privately held companies that were acquired in a business combination due to a reclassification of a portion of the investments to marketable following the initial public offering. The investments were reclassified to publically held companies.

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This item represents the change in the carrying amount on the entity's balance sheet of its investment in common stock of cost and equity method investees that are privately held companies that were acquired in a business combination due to sales of investments.

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This item represents the fair value, at acquisition, of the privately held company.

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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For an unclassified balance sheet, this item represents investments in debt and equity securities which are categorized neither as held-to-maturity nor trading. Such securities are reported at fair value, with unrealized gains and losses excluded from earnings and reported in a separate component of shareholders' equity (other comprehensive income), unless the Available-for-sale Security is designated as a hedge or is determined to have had an other than temporary decline in fair value below its amortized cost basis. All or a portion of the unrealized holding gain or loss of an Available-for-sale Security that is designated as being hedged in a fair value hedge shall be recognized in earnings during the period of the hedge, as should other than temporary declines in fair value below costs basis.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Statement of Financial Accounting Standard (FAS)
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-Paragraph 16

Reference 2: http://www.xbrl.org/2003/role/presentationRef
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Reference 3: http://www.xbrl.org/2003/role/presentationRef
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- 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.

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-Publisher FASB
-Name Statement of Financial Accounting Standard (FAS)
-Number 115
-Paragraph 12
-Subparagraph b

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This item represents the difference between the gross realized gains and losses realized on the sale of debt or equity securities categorized neither as held-to-maturity nor trading securities. Additionally, this item would include any losses recognized for other than temporary impairments of the subject investments in debt and equity securities.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Statement of Financial Accounting Standard (FAS)
-Number 115
-Paragraph 16

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This item represents the gross unrealized gains for securities which are categorized neither as held-to-maturity nor trading securities. Such gross unrealized gains are the excess of the fair value of the Available-for-sale Securities over their carrying value as of the reporting date. Such gross unrealized gains are included in other comprehensive income in the statement of shareholders' equity, unless the Available-for-sale Security is designated as a hedge. All or a portion of the unrealized holding gain of an available-for-sale security that is designated as being hedged in a fair value hedge shall be recognized in earnings during the period of the hedge.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Statement of Financial Accounting Standard (FAS)
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Reference 2: http://www.xbrl.org/2003/role/presentationRef
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-Name Statement of Financial Accounting Standard (FAS)
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This item represents the gross unrealized losses for securities which are categorized neither as held-to-maturity nor trading securities. Such gross unrealized losses are the excess of the carrying value of the Available-for-sale Securities over their fair value as of the reporting date. Such gross unrealized losses are included in other comprehensive income in the statement of shareholders' equity, unless the Available-for-sale Security is designated as a hedge or is determined to have had an other than temporary decline in fair value below its amortized cost basis. All or a portion of the unrealized holding loss of an Available-for-sale Security that is designated as being hedged in a fair value hedge shall be recognized in earnings during the period of the hedge, as should other than temporary declines in fair value below costs basis.

+ References

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This element represents the amount by which the carrying amount exceeds the fair value of the investment. The amount is charged to income if the decline in fair value is deemed to be other than temporary.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
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The total amount of investments that are intended to be held for an extended period of time (longer than one operating cycle).

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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Investments which are intended to be sold in the short term (usually less than one year or the normal operating cycle, whichever is longer) including trading securities, available-for-sale securities, held-to-maturity securities, and other short-term investments not otherwise listed in the existing taxonomy.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher SEC
-Name Regulation S-X (SX)
-Number 210
-Section 03
-Paragraph 1
-Subparagraph g
-Article 7

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The total of financial instruments that are bought and held principally for the purpose of selling them in the near term (thus held for only a short period of time) or for debt and equity securities formerly categorized as available-for-sale or held-to-maturity which the entity held as of the date it opted to account for such securities at fair value. An enterprise may also categorize such a security as trading without the intent to sell it in the near term assuming the decision to categorize the security as trading occurred at acquisition; this is the reason why the trading category of investments in debt and equity securities are bought and sold "principally" for sale in the near term. Transfers into and out of the trading category should be rare. Such financial instruments that are held as of the reporting date are measured at fair value with unrealized holding gains and losses (the difference between fair value and the previously reported carrying amount) included in earnings.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Statement of Financial Accounting Standard (FAS)
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Reference 2: http://www.xbrl.org/2003/role/presentationRef
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Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Implementation Guide (Q and A)
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Reference 4: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Statement of Financial Accounting Standard (FAS)
-Number 115
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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 Trading Securities which consist of all investments in certain debt and equity securities that are bought and held principally for the purpose of selling them in the near term (thus held for only a short period of time) or for which an entity has elected the fair value option. Trading generally reflects active and frequent buying and selling, and trading securities are generally used with the objective of generating profits on short-term differences in price, but this category of investments may also include items which are not intended to be sold in the near term or for which the fair value option has been elected.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Statement of Financial Accounting Standard (FAS)
-Number 115
-Paragraph 12
-Subparagraph a

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