Investments AFS (Details) (USD $)	12 Months Ended
Mar. 31, 2015	Mar. 31, 2014	Mar. 31, 2013
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities	$ 1,734,380,000us-gaap_AvailableForSaleSecurities	$ 1,676,894,000us-gaap_AvailableForSaleSecurities
Amount of short-term investments	1,351,054,000us-gaap_ShortTermInvestments	878,182,000us-gaap_ShortTermInvestments
Amount of long-term investments	383,326,000us-gaap_LongTermInvestments	798,712,000us-gaap_LongTermInvestments
Carrying value of available-for-sale investment previously classified as non-marketable cost-method investment	3,600,000
Proceeds for sale of available-for-sale securities	273,900,000	135,300,000	40,500,000
Realized gain on available-for-sale investments	18,469,000	0	0
Corporate bonds and debt [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities	926,929,000	943,966,000
Amount of corporate debt securities with no contractual maturity excluded from estimated fair value of available-for-sale securities	6,200,000	6,200,000
Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities	$ 13,166,000

Carrying value of available-for-sale investment previously classified as non-marketable cost-method investment

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- Definition

Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

+ 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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-Topic 320

-SubTopic 10

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- Definition

Amount of available-for-sale debt securities at fair value that do not have a single maturity date and which the entity has decided to disclose separately rather than allocating the fair value over several maturity groupings.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 320

-SubTopic 10

-Section 50

-Paragraph 3

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- Definition

This item represents the gross profit realized on the sale of debt or equity securities categorized neither as held-to-maturity nor trading securities.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 320

-SubTopic 10

-Section 50

-Paragraph 9

-Subparagraph (a)

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- Definition

The total amount of proceeds received for the sale of securities, categorized neither as held-to-maturity nor trading securities, during the reporting period.

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- Definition

The total amount of investments that are intended to be held for an extended period of time (longer than one operating cycle).

+ References

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- Definition

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 FASB

-Name Accounting Standards Codification

-Topic 210

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SX 210.5-02.2)

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-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 03

-Paragraph 1

-Subparagraph g

-Article 7

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