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Derivative Instruments (Credit Derivatives Sold) (Detail) - Insurance Operations [Member] - USD ($) $ in Millions | 9 Months Ended | 12 Months Ended
--- | --- | ---
Sep. 30, 2018 | Dec. 31, 2017
Derivative [Line Items] |  |
Derivative notional amount | $ 2,638 | $ 2,928
Total fair value of credit derivatives | $ (29) | $ (65)
Credit Default Swap [Member] |  |
Derivative [Line Items] |  |
Weighted average remaining expected maturity | 3 months 18 days | 1 year
Derivative notional amount | $ 77 | $ 127
Total fair value of credit derivatives | $ (27) | $ (63)
Insured Swaps [Member] |  |
Derivative [Line Items] |  |
Weighted average remaining expected maturity | 15 years 6 months | 15 years 6 months
Derivative notional amount | $ 2,561 | $ 2,801
Total fair value of credit derivatives | (2) | (2)
Insured Swaps - Held for Sale [Member] |  |
Derivative [Line Items] |  |
Derivative notional amount | 0 | 0
Credit Rating Aaa [Member] |  |
Derivative [Line Items] |  |
Derivative notional amount | 0 | 0
Total fair value of credit derivatives | 0 | 0
Credit Rating Aaa [Member] | Credit Default Swap [Member] |  |
Derivative [Line Items] |  |
Derivative notional amount | 0 | 0
Credit Rating Aaa [Member] | Insured Swaps [Member] |  |
Derivative [Line Items] |  |
Derivative notional amount | 0 | 0
Credit Rating Aaa [Member] | Other Credit Derivatives [Member] |  |
Derivative [Line Items] |  |
Derivative notional amount | 0 |
Credit Rating Aa [Member] |  |
Derivative [Line Items] |  |
Derivative notional amount | 107 | 117
Total fair value of credit derivatives | 0 | 0
Credit Rating Aa [Member] | Credit Default Swap [Member] |  |
Derivative [Line Items] |  |
Derivative notional amount | 0 | 0
Credit Rating Aa [Member] | Insured Swaps [Member] |  |
Derivative [Line Items] |  |
Derivative notional amount | 107 | 117
Credit Rating Aa [Member] | Other Credit Derivatives [Member] |  |
Derivative [Line Items] |  |
Derivative notional amount | 0 |
Credit Rating A [Member] |  |
Derivative [Line Items] |  |
Derivative notional amount | 1,539 | 1,818
Total fair value of credit derivatives | (1) | (1)
Credit Rating A [Member] | Credit Default Swap [Member] |  |
Derivative [Line Items] |  |
Derivative notional amount | 0 | 0
Credit Rating A [Member] | Insured Swaps [Member] |  |
Derivative [Line Items] |  |
Derivative notional amount | 1,539 | 1,818
Credit Rating A [Member] | Other Credit Derivatives [Member] |  |
Derivative [Line Items] |  |
Derivative notional amount | 0 |
Credit Rating Bbb [Member] |  |
Derivative [Line Items] |  |
Derivative notional amount | 915 | 846
Total fair value of credit derivatives | (1) | (1)
Credit Rating Bbb [Member] | Credit Default Swap [Member] |  |
Derivative [Line Items] |  |
Derivative notional amount | 0 | 0
Credit Rating Bbb [Member] | Insured Swaps [Member] |  |
Derivative [Line Items] |  |
Derivative notional amount | 915 | 846
Credit Rating Bbb [Member] | Insured Swaps - Held for Sale [Member] |  |
Derivative [Line Items] |  |
Derivative notional amount | 0 | 0
Credit Rating Below Investment Grade [Member] |  |
Derivative [Line Items] |  |
Derivative notional amount | 77 | 147
Total fair value of credit derivatives | (27) | (63)
Credit Rating Below Investment Grade [Member] | Credit Default Swap [Member] |  |
Derivative [Line Items] |  |
Derivative notional amount | 77 | 127
Credit Rating Below Investment Grade [Member] | Insured Swaps [Member] |  |
Derivative [Line Items] |  |
Derivative notional amount | $ 0 | $ 20

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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 Weighted Average Remaining Maturity Credit Derivatives In Years + References No definition available. + Details
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- Definition Fair value of credit risk derivative liability. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 825 -SubTopic 10 -Section 50 -Paragraph 10 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=77997519&loc=d3e13433-108611 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 4K -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=84234895&loc=SL5708775-113959 + Details
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