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Investments (Securities Held In Unrealized Loss Position And Insured By Financial Guarantor) (Detail) $ in Millions | Mar. 31, 2018 USD ($)
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Schedule Of Investments [Line Items] |  |
Total Available-For-Sale, Fair Value | $ 2,688 |
Wrapped Securities [Member] |  |
Schedule Of Investments [Line Items] |  |
Total Available-For-Sale, Fair Value | 74 | [1]
Gross unrealized losses | (4) | [1]
Loss and loss adjustment expense reserves | 14 | [2]
Asset-backed [Member] | Wrapped Securities [Member] | Mbia Corp And National [Member] |  |
Schedule Of Investments [Line Items] |  |
Total Available-For-Sale, Fair Value | 0 | [1]
Gross unrealized losses | 0 | [1]
Loss and loss adjustment expense reserves | 0 | [2]
Mortgage-backed [Member] | Wrapped Securities [Member] | Mbia Corp And National [Member] |  |
Schedule Of Investments [Line Items] |  |
Total Available-For-Sale, Fair Value | 16 | [1]
Gross unrealized losses | (3) | [1]
Loss and loss adjustment expense reserves | 14 | [2]
Corporate Obligations [Member] | Wrapped Securities [Member] | Mbia Corp And National [Member] |  |
Schedule Of Investments [Line Items] |  |
Total Available-For-Sale, Fair Value | 51 | [1]
Gross unrealized losses | (1) | [1]
Loss and loss adjustment expense reserves | 0 | [2]
All Other Securities [Member] | Wrapped Securities [Member] |  |
Schedule Of Investments [Line Items] |  |
Total Available-For-Sale, Fair Value | 7 | [1]
Gross unrealized losses | 0 | [1]
Loss and loss adjustment expense reserves | 0 | [2]
All Other Securities [Member] | Wrapped Securities [Member] | Mbia Corp And National [Member] |  |
Schedule Of Investments [Line Items] |  |
Total Available-For-Sale, Fair Value | 6 | [1]
Gross unrealized losses | 0 | [1]
Loss and loss adjustment expense reserves | 0 | [2]
All Other Securities [Member] | Wrapped Securities [Member] | Other Insurers [Member] |  |
Schedule Of Investments [Line Items] |  |
Total Available-For-Sale, Fair Value | 1 | [1]
Gross unrealized losses | 0 | [1]
Loss and loss adjustment expense reserves | $ 0 | [2]
[1] | Includes investments insured by MBIA Corp. and National.
[2] | Insurance loss reserve estimates are based on the proportion of par value owned to the total amount of par value insured.

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- Definition Amount of accumulated pre-tax unrealized losses before deducting pre-tax unrealized gains on investments in available-for-sale securities impacting investments. + References No definition available. + Details
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- Definition The amount needed to reflect the estimated ultimate cost of settling claims relating to insured events by financial guarantor that have occurred on or before the balance sheet date, whether or not reported to the insurer at that date. + References No definition available. + Details
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- Definition This item represents the aggregate fair value of investments in debt and equity securities in an unrealized loss position which are categorized neither as held-to-maturity nor trading securities. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 6 -Subparagraph (a)(1) -URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27290-111563 + Details
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