Investments (Changes in Fixed Maturity Securities with Noncredit OTTI Losses) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended	12 Months Ended
Mar. 31, 2013	Dec. 31, 2012
Changes in fixed maturity securities with noncredit OTTI losses in accumulated other comprehensive income (loss)
Balance, beginning of period	$ (361)	$ (724)
Noncredit OTTI losses recognized	31	(29)
Securities sold with a previous noncredit OTTI loss	54	177
Subsequent increases in estimated fair value	16	215
Balance, end of period	$ (260)	$ (361)

Before tax amount of the income statement impact of the reclassification adjustment 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

-Glossary Reclassification Adjustments

-URI http://asc.fasb.org/extlink&oid=6522872

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

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-Name Accounting Standards Codification

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Reference 3: http://www.xbrl.org/2003/role/presentationRef

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-Name Accounting Research Bulletin (ARB)

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Reference 4: http://www.xbrl.org/2003/role/presentationRef

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-Name Statement of Financial Accounting Standard (FAS)

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-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009.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

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

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Reference 7: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

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+ Details