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Special Purpose Entities, Including Variable Interest Entities - Supplemental Information about Loans Held as Assets and Long-Term Debt Associated with Consolidated Variable Interest Entities (Detail) - Variable Interest Entities [Member] - USD ($) $ in Thousands | Mar. 31, 2019 | Dec. 31, 2018
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Variable Interest Entities [Line Items] |  |
Loans, Unpaid principal balance | $ 3,432,240 | $ 3,402,413
Long-term debt, Unpaid principal balance | $ 4,605,291 | $ 4,552,643

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- Definition This represents the amount of principal outstanding on loans receivable that are presented on the balance sheet at fair value, whether such presentation is as a separate caption or parenthetical disclosure. + References No definition available. + Details
Name: | ambc_LoansUnpaidPrincipalBalance
Namespace Prefix: | ambc_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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- Definition This represents the amount of principal outstanding on long-term debt that is presented on the balance sheet at fair value, whether such presentation is as a separate caption or parenthetical disclosure. + References No definition available. + Details
Name: | ambc_LongTermDebtUnpaidPrincipalBalance
Namespace Prefix: | ambc_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- 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_VariableInterestEntityLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Details
Name: | us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis=us-gaap_VariableInterestEntityPrimaryBeneficiaryAggregatedDisclosureMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |