Measured on recurring basis (Details) (USD $) In Thousands	Sep. 30, 2011	Mar. 31, 2011	Sep. 30, 2010	Mar. 31, 2010
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Money Market Fund Deposits Fair Value Disclosure	$ 318,820	$ 464,669
Marketable Equity Securities Fair Value Disclosure	8,078	26,861
Corporate Bonds and Debt Fair Value Disclosure	723,599	523,026
Government Agency Bonds Fair Value Disclosure	346,342	430,591
Cash Deposit Accounts Fair Value Disclosure	356,309	239,255
Municipal Bonds Fair Value Disclosure	10,577	11,457
Auction Rate Securities Fair Value Disclosure	11,693	12,475
Fair Value, Assets and Liabilities, as Classified on the Condensed Consolidated Balance Sheets [Abstract]
Cash and cash equivalents	675,129	703,924	423,721	492,130
Short-term investments	740,309	539,572
Long-term investments	359,980	464,838
Total assets measured at fair value	1,775,418	1,708,334
Fair Value, Inputs, Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Money Market Fund Deposits Fair Value Disclosure	318,820	464,669
Marketable Equity Securities Fair Value Disclosure	8,078	26,861
Corporate Bonds and Debt Fair Value Disclosure	0	0
Government Agency Bonds Fair Value Disclosure	0	0
Cash Deposit Accounts Fair Value Disclosure	0	0
Municipal Bonds Fair Value Disclosure	0	0
Auction Rate Securities Fair Value Disclosure	0	0
Fair Value, Assets and Liabilities, as Classified on the Condensed Consolidated Balance Sheets [Abstract]
Cash and cash equivalents	318,820	464,669
Short-term investments	3,786	16,553
Long-term investments	4,292	10,308
Total assets measured at fair value	326,898	491,530
Fair Value, Inputs, Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Money Market Fund Deposits Fair Value Disclosure	0	0
Marketable Equity Securities Fair Value Disclosure	0	0
Corporate Bonds and Debt Fair Value Disclosure	718,974	519,526
Government Agency Bonds Fair Value Disclosure	346,342	430,591
Cash Deposit Accounts Fair Value Disclosure	356,309	239,255
Municipal Bonds Fair Value Disclosure	10,577	11,457
Auction Rate Securities Fair Value Disclosure	0	0
Fair Value, Assets and Liabilities, as Classified on the Condensed Consolidated Balance Sheets [Abstract]
Cash and cash equivalents	356,309	239,255
Short-term investments	736,523	523,019
Long-term investments	339,370	438,555
Total assets measured at fair value	1,432,202	1,200,829
Fair Value, Inputs, Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Money Market Fund Deposits Fair Value Disclosure	0	0
Marketable Equity Securities Fair Value Disclosure	0	0
Corporate Bonds and Debt Fair Value Disclosure	4,625	3,500
Government Agency Bonds Fair Value Disclosure	0	0
Cash Deposit Accounts Fair Value Disclosure	0	0
Municipal Bonds Fair Value Disclosure	0	0
Auction Rate Securities Fair Value Disclosure	11,693	12,475
Fair Value, Assets and Liabilities, as Classified on the Condensed Consolidated Balance Sheets [Abstract]
Cash and cash equivalents	0	0
Short-term investments	0	0
Long-term investments	16,318	15,975
Total assets measured at fair value	$ 16,318	$ 15,975

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Corporate Bonds and Debt Fair Value Disclosure

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Government Agency Bonds Fair Value Disclosure

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Money Market Fund Deposits Fair Value Disclosure

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This element represents the aggregate of the assets reported on the balance sheet at period end measured at fair value by the entity. This element is intended to be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements.

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Includes currency on hand as well as demand deposits with banks or financial institutions. It also includes other kinds of accounts that have the general characteristics of demand deposits in that the Entity may deposit additional funds at any time and also effectively may withdraw funds at any time without prior notice or penalty. Cash equivalents, excluding items classified as marketable securities, include short-term, highly liquid investments that are both readily convertible to known amounts of cash, and so near their maturity that they present minimal risk of changes in value because of changes in interest rates. Generally, only investments with original maturities of three months or less qualify under that definition. Original maturity means original maturity to the entity holding the investment. For example, both a three-month US Treasury bill and a three-year Treasury note purchased three months from maturity qualify as cash equivalents. However, a Treasury note purchased three years ago does not become a cash equivalent when its remaining maturity is three months. Compensating balance arrangements that do not legally restrict the withdrawal or usage of cash amounts may be reported as Cash and Cash Equivalents, while legally restricted deposits held as compensating balances against borrowing arrangements, contracts entered into with others, or company statements of intention with regard to particular deposits are not generally reported as cash and cash equivalents. Includes cash and cash equivalents associated with the entity's continuing operations. Excludes cash and cash equivalents associated with the disposal group (and discontinued operation).

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

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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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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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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.

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