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Investments (Narrative) (Detail) (USD $) In Millions, unless otherwise specified | Dec. 31, 2012 Y | Dec. 31, 2011 Y
--- | --- | ---
Schedule of Investments [Line Items] |  |
Fair value of securities on deposit with various regulatory authorities | $ 10 | $ 11
Fair value of securities pledged as collateral | 820 | 1,900
Cash pledged as collateral under investment agreements | $ 144 | $ 224
Weighted average contractual maturity period in years for securities in an unrealized loss position | 23 | 21
Number of securities, unrealized loss position for continuous twelve-month period or longer | 153 | 290
Rate that a security's Fair Value is below book value | 5.00% |
Fair Value Below Book Value By More Than 5% [Member] |  |
Schedule of Investments [Line Items] |  |
Number of securities, unrealized loss position for continuous twelve-month period or longer | 89 | 218

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- Definition Fair value of financial instrument held by the entity for its own account (proprietary securities) for trading or investment purposes that are carried at fair value and pledged to counterparties as collateral for financing transactions. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 860 -SubTopic 30 -Section 45 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6467456&loc=d3e113888-111728 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 860 -SubTopic 30 -Section 50 -Paragraph 1A -Subparagraph (b)(1)(i) -URI http://asc.fasb.org/extlink&oid=7523297&loc=SL6224234-111729 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name FASB Staff Position (FSP) -Number FAS140-4/FIN46(R)-8 -Paragraph B6 -Subparagraph b -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009.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 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 140 -Paragraph 17 -Subparagraph a(2) -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009.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. + Details
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