Consolidated Statement of Comprehensive Income (USD $) In Millions, unless otherwise specified	3 Months Ended
Jun. 30, 2013	Jun. 30, 2012
Statement Of Income And Comprehensive Income [Abstract]
Net income	$ 91	$ 477
Other comprehensive income, net of tax:
Net unrealized (losses) gains on available-for-sale marketable securities, net of tax	(54)	11
Reclassification adjustment for net (gains) losses on available-for-sale marketable securities included in net income, net of tax	16	(2)
Other comprehensive (loss) income	(38)	9
Comprehensive income	$ 53	$ 486

The change in equity [net assets] of a business enterprise during a period from transactions and other events and circumstances from non-owner sources which are attributable to the economic entity, including both controlling (parent) and noncontrolling interests. It includes all changes in equity during a period except those resulting from investments by owners and distributions to owners, including any and all transactions which are directly or indirectly attributable to that ownership interest in subsidiary equity which is not attributable to the parent.

+ References+ Details

The portion of profit or loss for the period, net of income taxes, which is attributable to the parent.

+ References

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

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-Name Statement of Financial Accounting Standard (FAS)

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

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

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-Glossary Other Comprehensive Income

-URI http://asc.fasb.org/extlink&oid=6519514

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

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-Glossary Net Income

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-Publisher AICPA

-Name Accounting Research Bulletin (ARB)

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Reference 12: http://www.xbrl.org/2003/role/presentationRef

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-Name Accounting Research Bulletin (ARB)

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-Appendix A

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-Name Accounting Research Bulletin (ARB)

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Reference 14: http://www.xbrl.org/2003/role/presentationRef

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+ Details

- Definition

Net of tax amount of the income statement impact of the reclassification adjustment for unrealized gain (loss) realized upon the sale of available-for-sale securities.

+ References

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

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+ Details