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Investments - Summary of Sources of Collateral Received and Various Investment Agreement in which Collateral Pledged (Detail) - USD ($) $ in Thousands | Dec. 31, 2018 | Dec. 31, 2017
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Investment [Line Items] |  |
Fair Value Of Securities Deposited With Governmental Authorities | $ 5,975 | $ 5,974
Securities Held To Collateralize And Fund Repayment Of Debt Issued Through Resecuritization Transaction Fair Value | $ 0 | $ 346,212

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

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- Definition Fair Value of securities held to collateralize and fund repayment of debt issued through a resecuritization transaction. + References No definition available. + Details
Name: | ambc_SecuritiesHeldToCollateralizeAndFundRepaymentOfDebtIssuedThroughResecuritizationTransactionFairValue
Namespace Prefix: | ambc_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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- Definition Line items represent reporting 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 Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Article 12 -Section 12 -Sentence Column A + Details
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Namespace Prefix: | invest_
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Balance Type: | na
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