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Loss And Loss Adjustment Expense Reserves (Schedule Of Loss And Loss Adjustment Expenses Reserves) (Detail) - USD ($) $ in Millions | 3 Months Ended
--- | ---
Mar. 31, 2021 | Mar. 31, 2020
Loss And Loss Adjustment Expense Reserves [Line Items] |  |  |
Changes in unearned premium revenue |  | $ 19 | $ 19
Loss And Lae Reserves [Member] |  |  |
Loss And Loss Adjustment Expense Reserves [Line Items] |  |  |
Gross loss and LAE reserve, beginning balance | [1],[2] | 990 |
Loss payments for cases |  | (55) |
Accretion of claim liability discount |  | 3 |
Changes in discount rates |  | (11) |
Changes in assumptions |  | 57 |
Changes in unearned premium revenue |  | 0 |
Other |  | 6 |
Gross loss and LAE reserve, ending balance | [1],[2] | $ 990 |
[1] | Amounts are net of expected recoveries.
[2] | Includes changes in amount and timing of estimated payments and recoveries.

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- Definition Financial Guarantee Insurance Contracts Claim Liability Significant Components Of Change In Assumptions + References No definition available. + Details
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Balance Type: | credit
Period Type: | duration

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- Definition Changes in the timing and amount of net payments on financial guarantee contracts. + References No definition available. + Details
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Namespace Prefix: | mbi_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- Definition Loss And Loss Adjustment Expense Reserves [Line Items] + References No definition available. + Details
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- Definition Loss payments for cases + References No definition available. + Details
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Namespace Prefix: | mbi_
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Balance Type: | credit
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- Definition This element represents the amount of the change in the claim liability related to financial guarantee insurance contracts arising from the accretion of the discount of the claim liability. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 40 -Section 50 -Paragraph 9 -Subparagraph (a)(3) -URI http://asc.fasb.org/extlink&oid=99382878&loc=SL5749324-161292 + Details
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Period Type: | duration

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- Definition This element represents the amount of change in the claim liability related to financial guarantee insurance contracts arising from a change in the rate used to discount the claim liability. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 40 -Section 50 -Paragraph 9 -Subparagraph (a)(3) -URI http://asc.fasb.org/extlink&oid=99382878&loc=SL5749324-161292 + Details
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- Definition Amount of increase (decrease) unearned premiums written. Excludes portion of unearned premiums amortized into income. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 28 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=121586364&loc=d3e3602-108585 + Details
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- Definition The amount needed to reflect the estimated ultimate cost of settling claims relating to insured events that have occurred on or before the balance sheet date, whether or not reported to the insurer at that date. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 40 -Section 50 -Paragraph 3 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=116884095&loc=d3e14764-158437 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 40 -Section 50 -Paragraph 3 -Subparagraph (cc) -URI http://asc.fasb.org/extlink&oid=116884095&loc=d3e14764-158437 + Details
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- Details
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