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Insurance Premiums (Narrative) (Detail) - USD ($) $ in Millions | 12 Months Ended
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Dec. 31, 2024 | Dec. 31, 2023
Insurance Premiums [Line Items] |  |  |
Weighted average risk-free rate |  | 3.00% | 3.00%
Weighted average expected collection term of the premiums receivable, years |  | 8 years 6 months 25 days | 8 years 9 months
Reinsurance premiums payable | [1] | $ 14 |
Premiums [Member] |  |  |
Insurance Premiums [Line Items] |  |  |
Reinsurance premiums payable |  | $ 3 | $ 4
[1] | Represents a withdrawal of ratings.

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- Definition Insurance Premiums Line Items + References No definition available. + Details
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Namespace Prefix: | mbi_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Definition Weighted average period over which premium receivable for financial guarantee insurance contracts is collected where premium is received over the contract period, 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/disclosureRef -Topic 944 -SubTopic 310 -Name Accounting Standards Codification -Section 50 -Paragraph 3 -Subparagraph (a)(5) -Publisher FASB -URI https://asc.fasb.org/1943274/2147477363/944-310-50-3 + Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:durationItemType
Balance Type: | na
Period Type: | duration

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- Definition Weighted-average risk-free rate used to discount amount of premium expected to be collected for financial guarantee insurance contract where premium is to be received over contract period. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Topic 944 -SubTopic 310 -Name Accounting Standards Codification -Section 50 -Paragraph 3 -Subparagraph (a)(4) -Publisher FASB -URI https://asc.fasb.org/1943274/2147477363/944-310-50-3 + Details
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Namespace Prefix: | us-gaap_
Data Type: | dtr-types:percentItemType
Balance Type: | na
Period Type: | duration

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- Definition The carrying amount as of the balance sheet date of the known and estimated amounts owed to insurers under reinsurance treaties or other arrangements. + References No definition available. + Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- Details
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Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |