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Derivative Instruments (Credit Derivatives Sold) (Detail) - Insurance Operations [Member] - USD ($) $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2017 | Dec. 31, 2016
Derivative [Line Items] |  |
Derivative notional amount | $ 2,928 | $ 4,043
Total fair value of credit derivatives | $ (65) | $ (66)
Credit Default Swap [Member] |  |
Derivative [Line Items] |  |
Weighted average remaining expected maturity | 1 year | 3 years 9 months
Derivative notional amount | $ 127 | $ 588
Total fair value of credit derivatives | $ (63) | $ (64)
Insured Swaps [Member] |  |
Derivative [Line Items] |  |
Weighted average remaining expected maturity | 15 years 6 months | 15 years 6 months
Derivative notional amount | $ 2,801 | $ 3,035
Total fair value of credit derivatives | (2) | $ (2)
Insured Swaps - Held for Sale [Member] |  |
Derivative [Line Items] |  |
Weighted average remaining expected maturity |  | 14 years 3 months
Derivative notional amount | 0 | $ 420
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 | 117 | 137
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 | 117 | 137
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 | 1,818 | 2,261
Total fair value of credit derivatives | (1) | (1)
Credit Rating A [Member] | Credit Default Swap [Member] |  |
Derivative [Line Items] |  |
Derivative notional amount | 0 | 115
Credit Rating A [Member] | Insured Swaps [Member] |  |
Derivative [Line Items] |  |
Derivative notional amount | 1,818 | 2,146
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 | 846 | 1,152
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 | 846 | 732
Credit Rating Bbb [Member] | Insured Swaps - Held for Sale [Member] |  |
Derivative [Line Items] |  |
Derivative notional amount | 0 | 420
Credit Rating Below Investment Grade [Member] |  |
Derivative [Line Items] |  |
Derivative notional amount | 147 | 493
Total fair value of credit derivatives | (63) | (64)
Credit Rating Below Investment Grade [Member] | Credit Default Swap [Member] |  |
Derivative [Line Items] |  |
Derivative notional amount | 127 | 473
Credit Rating Below Investment Grade [Member] | Insured Swaps [Member] |  |
Derivative [Line Items] |  |
Derivative notional amount | $ 20 | $ 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=73719730&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=64837297&loc=SL5708775-113959 + Details
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- Definition Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. + References No definition available. + Details
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