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Derivative Financial Instruments and Off-balance sheet Financial Instruments - CDS notional amounts by credit rating (Details) $ in Millions | 12 Months Ended |
--- | --- | ---
Dec. 31, 2017 USD ($) entity | Dec. 31, 2016 USD ($)
Derivative Financial Instruments and Off-balance sheet Financial Instruments |  |
Term of credit default swaps | 5 years |
Credit Derivatives |  |
Notional amount | $ 3,388 | $ 3,776
The number of reference entities generally included in a CDX index | entity | 125 |
Credit default contracts |  |
Credit Derivatives |  |
Notional amount | $ 105 | 245
Fair value | 1 | (1)
Credit default contracts | Single name | Corporate |  |
Credit Derivatives |  |
Notional amount | 25 | 65
Fair value | 0 | 1
Credit default contracts | First-to-default Basket | Municipal |  |
Credit Derivatives |  |
Notional amount |  | 100
Fair value |  | (3)
Credit default contracts | Index | Corporate |  |
Credit Derivatives |  |
Notional amount | 80 | 80
Fair value | 1 | 1
AA | Credit default contracts |  |
Credit Derivatives |  |
Notional amount | 1 | 21
AA | Credit default contracts | Single name | Corporate |  |
Credit Derivatives |  |
Notional amount | 0 | 20
AA | Credit default contracts | First-to-default Basket | Municipal |  |
Credit Derivatives |  |
Notional amount |  | 0
AA | Credit default contracts | Index | Corporate |  |
Credit Derivatives |  |
Notional amount | 1 | 1
A | Credit default contracts |  |
Credit Derivatives |  |
Notional amount | 29 | 29
A | Credit default contracts | Single name | Corporate |  |
Credit Derivatives |  |
Notional amount | 10 | 10
A | Credit default contracts | First-to-default Basket | Municipal |  |
Credit Derivatives |  |
Notional amount |  | 0
A | Credit default contracts | Index | Corporate |  |
Credit Derivatives |  |
Notional amount | 19 | 19
BBB | Credit default contracts |  |
Credit Derivatives |  |
Notional amount | 55 | 185
BBB | Credit default contracts | Single name | Corporate |  |
Credit Derivatives |  |
Notional amount | 10 | 35
BBB | Credit default contracts | First-to-default Basket | Municipal |  |
Credit Derivatives |  |
Notional amount |  | 100
BBB | Credit default contracts | Index | Corporate |  |
Credit Derivatives |  |
Notional amount | 45 | 50
BB and lower | Credit default contracts |  |
Credit Derivatives |  |
Notional amount | 20 | 10
BB and lower | Credit default contracts | Single name | Corporate |  |
Credit Derivatives |  |
Notional amount | 5 | 0
BB and lower | Credit default contracts | First-to-default Basket | Municipal |  |
Credit Derivatives |  |
Notional amount |  | 0
BB and lower | Credit default contracts | Index | Corporate |  |
Credit Derivatives |  |
Notional amount | $ 15 | $ 10

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- Definition Aggregate notional amount specified by the derivative(s). Expressed as an absolute value. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Article 12 -Section 13 -Sentence Column B + Details
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- Definition Term of the credit risk derivative contract, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 4K -Subparagraph (a)(1) -URI http://asc.fasb.org/extlink&oid=84234895&loc=SL5708775-113959 + Details
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