Summary of Available for Sale (Details) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2012	Mar. 31, 2012
Summary of available-for-sale securities [Line Items]
Available-for-sale, adjusted cost	$ 1,265,369	$ 1,148,914
Available-for-sale, gross unrealized gains	6,303	5,048
Available-for-sale, gross unrealized losses	(995)	(2,122)
Available-for-sale, estimated fair value	1,270,677	1,151,840
Government agency bonds [Member]
Summary of available-for-sale securities [Line Items]
Available-for-sale, adjusted cost	480,028	342,025
Available-for-sale, gross unrealized gains	500	476
Available-for-sale, gross unrealized losses	(23)	(397)
Available-for-sale, estimated fair value	480,505	342,104
Municipal bonds [Member]
Summary of available-for-sale securities [Line Items]
Available-for-sale, adjusted cost	19,825	19,888
Available-for-sale, gross unrealized gains	243	234
Available-for-sale, gross unrealized losses	0	0
Available-for-sale, estimated fair value	20,068	20,122
Auction rate securities [Member]
Summary of available-for-sale securities [Line Items]
Available-for-sale, adjusted cost	34,188	10,246
Available-for-sale, gross unrealized gains	114	0
Available-for-sale, gross unrealized losses	0	0
Available-for-sale, estimated fair value	34,302	10,246
Corporate bonds and debt [Member]
Summary of available-for-sale securities [Line Items]
Available-for-sale, adjusted cost	726,058	770,891
Available-for-sale, gross unrealized gains	5,446	4,150
Available-for-sale, gross unrealized losses	(137)	(937)
Available-for-sale, estimated fair value	731,367	774,104
Equity Securities [Member]
Summary of available-for-sale securities [Line Items]
Available-for-sale, adjusted cost	5,270	5,864
Available-for-sale, gross unrealized gains	0	188
Available-for-sale, gross unrealized losses	(835)	(788)
Available-for-sale, estimated fair value	$ 4,435	$ 5,264

For an unclassified balance sheet, this item represents investments in debt and equity securities which are categorized neither as held-to-maturity nor trading.

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

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-Name Accounting Standards Codification

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

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-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009.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 4: http://www.xbrl.org/2003/role/presentationRef

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-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 5: http://www.xbrl.org/2003/role/presentationRef

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-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 6: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 115

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-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 7: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

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-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 8: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 115

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-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 9: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

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-Number 115

-Paragraph 16

-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 10: http://www.xbrl.org/2003/role/presentationRef

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+ Details

- 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+ Details

- 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+ Details