Consolidated Statement of Comprehensive Income (Parenthetical) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Statement Of Income And Comprehensive Income [Abstract]
Net unrealized (losses) gains on available-for-sale marketable securities, tax benefit (provision)	$ 20	$ 5	$ 41	$ (22)
Reclassification adjustment for net losses (gains) on available-for-sale marketable securities included in investment and other income, tax (benefit) provision	$ (8)	$ 11	$ (4)	$ 16

Amount of tax expense (benefit) of reclassification adjustment from accumulated other comprehensive income for unrealized gain (loss) realized upon the sale of available-for-sale securities.

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 320

-SubTopic 10

-Section 50

-Paragraph 9

-Subparagraph (d)

-URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27357-111563

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 220

-SubTopic 10

-Section 45

-Paragraph 12

-URI http://asc.fasb.org/extlink&oid=36458714&loc=d3e640-108580

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Data Type:	xbrli:monetaryItemType
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Period Type:	duration