XML 70 R60.htm IDEA: XBRL DOCUMENT
v3.23.1

Investments (Portion Of Unrealized Gains And Losses On Equity Investments Held) (Detail) - USD ($) $ in Millions | 3 Months Ended
--- | ---
Mar. 31, 2023 | Mar. 31, 2022
Gain (Loss) on Investments [Line Items] |  |
Realized investment gains (losses) | $ (3) | $ (3)
Equity and Trading securities [Member] | Net gains and (losses) recognized during the period on equity and trading securities [Member] |  |
Gain (Loss) on Investments [Line Items] |  |
Realized investment gains (losses) | 1 | (10)
Equity and Trading securities [Member] | Net gains and (losses) recognized during the period on equity and trading securities sold during the period [Member] |  |
Gain (Loss) on Investments [Line Items] |  |
Realized investment gains (losses) | 0 | 1
Equity and Trading securities [Member] | Unrealized gains and (losses) recognized during the period on equity and trading securities still held at the reporting date [Member] |  |
Gain (Loss) on Investments [Line Items] |  |
Realized investment gains (losses) | $ 1 | $ (11)

X
- Definition Amount of realized gain (loss) on investment. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 220 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-04.3(a)) -URI https://asc.fasb.org/extlink&oid=120400993&loc=SL114874131-224263 + Details
Name: | us-gaap_RealizedInvestmentGainsLosses
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- 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
Name: | us-gaap_ScheduleOfGainLossOnInvestmentsIncludingMarketableSecuritiesAndInvestmentsHeldAtCostIncomeStatementReportedAmountsSummaryLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Details
Name: | us-gaap_CollateralAxis=mbi_EquityAndTradingSecuritiesMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_IncomeStatementLocationAxis=us-gaap_GainLossOnInvestmentsMember1
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_IncomeStatementLocationAxis=us-gaap_SalesMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_IncomeStatementLocationAxis=mbi_HeldMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |