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Investments - Summary of Fair Value, Including Financial Guarantee, and Weighted-Average Underlying Rating, Excluding Financial Guarantee, of Insured Securities (Detail2) - USD ($) $ in Millions | Dec. 31, 2019 | Dec. 31, 2018
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Ambac UK [Member] |  |
Investment [Line Items] |  |
Asset-backed securities fair value | $ 145 | $ 0

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- Definition Asset-backed securities guaranteed by Ambac UK. + References No definition available. + Details
Name: | ambc_AssetBackedSecuritiesFairValueDisclosure
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

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- Details
Name: | dei_LegalEntityAxis=ambc_AmbacUkMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |