Investments (Details 3) (USD $) In Millions, unless otherwise specified	Mar. 31, 2012	Dec. 31, 2011
Fair Value, Concentration of Risk, Financial Statement Captions [Line Items]
Short-term investments, principally at estimated fair value	$ 11,801	$ 17,310
Cash equivalents	6,600	5,000
Available-for-sale Securities, Debt Securities	354,451	350,271
United States [Member] \| US Government Agencies Debt Securities [Member]
Fair Value, Concentration of Risk, Financial Statement Captions [Line Items]
Government and agency fixed maturity securities	41,416	40,012
Short-term investments, principally at estimated fair value	10,023	15,775
Cash equivalents	3,072	1,748
Available-for-sale Securities, Debt Securities	54,511	57,535
Japan [Member] \| US Government Agencies Debt Securities [Member]
Fair Value, Concentration of Risk, Financial Statement Captions [Line Items]
Government and agency fixed maturity securities	20,562	21,003
Short-term investments, principally at estimated fair value	62	0
Cash equivalents	395	0
Available-for-sale Securities, Debt Securities	$ 21,019	$ 21,003

For an unclassified balance sheet, total of debt securities categorized neither as held-to-maturity nor trading.

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+ Details

- Definition

Cash equivalents, excluding items classified as marketable securities, include short-term, highly liquid investments that are both readily convertible to known amounts of cash, and so near their maturity that they present insignificant risk of changes in value because of changes in interest rates. Generally, only investments with original maturities of three months or less qualify under that definition. Original maturity means original maturity to the entity holding the investment. For example, both a three-month US Treasury bill and a three-year Treasury note purchased three months from maturity qualify as cash equivalents. However, a Treasury note purchased three years ago does not become a cash equivalent when its remaining maturity is three months. Compensating balance arrangements that do not legally restrict the withdrawal or usage of cash equivalents may be reported as cash equivalents, while legally restricted equivalents held as compensating balances against borrowing arrangements, contracts entered into with others, or company statements of intention with regard to particular equivalents are not generally reported as part of unrestricted cash equivalents.

+ References+ Details

- Definition

This item represents disclosure of all significant concentrations of credit risk or market risk arising from the subject financial instrument (as defined), whether from an individual counterparty or groups of counterparties.

+ References

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

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-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Article 7

-Section 03

-Paragraph 1

-Subparagraph a, f, g

+ Details