Financial Instruments (Details) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2012	Mar. 31, 2012
Available-for-sale securities
Amortized Cost	$ 3,060,877	$ 3,048,860
Gross Unrealized Gains	22,353	19,590
Gross Unrealized Losses	(6,254)	(5,412)
Estimated Fair Value	3,076,976	3,063,038
Money Market Funds [Member]
Available-for-sale securities
Amortized Cost	174,888	232,017
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Estimated Fair Value	174,888	232,017
Bank Certificates of Deposit [Member]
Available-for-sale securities
Amortized Cost	189,914	159,972
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Estimated Fair Value	189,914	159,972
Commercial Paper [Member]
Available-for-sale securities
Amortized Cost	489,826	594,867
Gross Unrealized Gains	0	1
Gross Unrealized Losses	0	(1)
Estimated Fair Value	489,826	594,867
Corporate Bonds [Member]
Available-for-sale securities
Amortized Cost	207,372	186,455
Gross Unrealized Gains	3,713	3,401
Gross Unrealized Losses	(213)	(184)
Estimated Fair Value	210,872	189,672
Auction Rate Securities [Member]
Available-for-sale securities
Amortized Cost	32,250	32,600
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(3,435)	(3,671)
Estimated Fair Value	28,815	28,929
Municipal Bonds [Member]
Available-for-sale securities
Amortized Cost	19,598	25,454
Gross Unrealized Gains	677	734
Gross Unrealized Losses	(61)	(28)
Estimated Fair Value	20,214	26,160
U.S. Government and Agency Securities [Member]
Available-for-sale securities
Amortized Cost	668,759	668,702
Gross Unrealized Gains	382	360
Gross Unrealized Losses	(92)	(149)
Estimated Fair Value	669,049	668,913
Foreign Government and Agency Securities [Member]
Available-for-sale securities
Amortized Cost	228,947	249,951
Gross Unrealized Gains	1	0
Gross Unrealized Losses	0	0
Estimated Fair Value	228,948	249,951
Mortgage-Backed Securities [Member]
Available-for-sale securities
Amortized Cost	1,008,823	878,842
Gross Unrealized Gains	17,580	15,094
Gross Unrealized Losses	(2,193)	(1,160)
Estimated Fair Value	1,024,210	892,776
Debt Mutual Fund [Member]
Available-for-sale securities
Amortized Cost	40,500	20,000
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(260)	(219)
Estimated Fair Value	$ 40,240	$ 19,781

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 320

-SubTopic 10

-Section 45

-Paragraph 1

-URI http://asc.fasb.org/extlink&oid=6871852&loc=d3e26610-111562

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

-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 FASB Staff Position (FSP)

-Number FAS115-1/124-1

-Paragraph 15E

-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

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

-Publisher FASB

-Name FASB Staff Position (FSP)

-Number FAS115-1/124-1

-Paragraph 15D

-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

-Paragraph 13

-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

-Name FASB Staff Position (FSP)

-Number FAS115-1/124-1

-Paragraph 15C

-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

-Paragraph 14

-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

-Name Statement of Financial Accounting Standard (FAS)

-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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 320

-SubTopic 10

-Section 25

-Paragraph 1

-Subparagraph (b)

-URI http://asc.fasb.org/extlink&oid=6379932&loc=d3e22054-111558

+ Details

- 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+ 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