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Investments - Summary of Sources of Collateral Received and Various Investment Agreement in which Collateral Pledged (Detail) - USD ($) $ in Millions | Dec. 31, 2019 | Dec. 31, 2018
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Investment [Line Items] |  |
Fair Value Of Securities Deposited With Governmental Authorities | $ 6 | $ 6
Market Value of Secured Note Collateral | $ 197 | $ 210

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- Definition Fair value of securities deposited with governmental authorities. + References No definition available. + Details
Name: | ambc_FairValueOfSecuritiesDepositedWithGovernmentalAuthorities
Namespace Prefix: | ambc_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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- Definition Market Value of Secured Note Collateral + References No definition available. + Details
Name: | ambc_MarketValueofSecuredNoteCollateral
Namespace Prefix: | ambc_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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- References No definition available. + Details
Name: | invest_InvestmentLineItems
Namespace Prefix: | invest_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration