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Special Purpose Entities, Including Variable Interest Entities - Additional Information (Detail) (USD $) In Thousands, unless otherwise specified | 3 Months Ended |
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Mar. 31, 2015 | Dec. 31, 2014
Variable Interest Entities [Line Items] |  |
Fair value of special purpose entities | 10,036 us-gaap_EquityMethodInvestmentsFairValueDisclosure | $ 12,036 us-gaap_EquityMethodInvestmentsFairValueDisclosure
Consolidated VIE assets | 15,506,746 us-gaap_VariableInterestEntityConsolidatedCarryingAmountAssets | 15,126,110 us-gaap_VariableInterestEntityConsolidatedCarryingAmountAssets
Consolidated VIE liabilities | 15,473,572 us-gaap_VariableInterestEntityConsolidatedCarryingAmountLiabilities | 15,085,685 us-gaap_VariableInterestEntityConsolidatedCarryingAmountLiabilities
Asset-Backed Securities and Utility Obligations [Member] |  |
Variable Interest Entities [Line Items] |  |
Weighted average life | 6 years 9 months 12 days |
Average rating of assets held by sponsored special purpose entities | BBB |
Sponsored Variable Interest Entities [Member] |  |
Variable Interest Entities [Line Items] |  |
Number of individual transactions with special purpose entities | 15 ambc_NumberOfTransactionsWithSponsoredSpecialPurposeEntitiesInceptionToDate / us-gaap_GuaranteeObligationsByNatureAxis = ambc_SponsoredVariableInterestEntitiesMember |
Number of transactions outstanding with special purpose entities | 3 ambc_NumberOfTransactionsOutstandingWithSponsoredSpecialPurposeEntities / us-gaap_GuaranteeObligationsByNatureAxis = ambc_SponsoredVariableInterestEntitiesMember |
Total principal amount of debt outstanding | 457,960 ambc_FaceAmountOfDebtOutstandingIssuedBySponsoredSpecialPurposeEntities / us-gaap_GuaranteeObligationsByNatureAxis = ambc_SponsoredVariableInterestEntitiesMember | $ 457,960 ambc_FaceAmountOfDebtOutstandingIssuedBySponsoredSpecialPurposeEntities / us-gaap_GuaranteeObligationsByNatureAxis = ambc_SponsoredVariableInterestEntitiesMember
Consolidated Entities [Member] |  |
Variable Interest Entities [Line Items] |  |
Number of consolidated Variable Interest Entities | 14 ambc_NumberOfConsolidatedVariableInterestEntities / dei_LegalEntityAxis = us-gaap_ConsolidatedEntitiesMember | 15 ambc_NumberOfConsolidatedVariableInterestEntities / dei_LegalEntityAxis = us-gaap_ConsolidatedEntitiesMember

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- Definition The total principal amount of outstanding debt issued by Ambac's sponsored medium term note special purpose entities. + References No definition available. + Details
Name: | ambc_FaceAmountOfDebtOutstandingIssuedBySponsoredSpecialPurposeEntities
Namespace Prefix: | ambc_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Number of consolidated variable interest entities. + References No definition available. + Details
Name: | ambc_NumberOfConsolidatedVariableInterestEntities
Namespace Prefix: | ambc_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | instant

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- Definition The number of asset transfer transactions executed with Ambac's sponsored medium term note special purpose entities that are still outstanding at the reporting date. + References No definition available. + Details
Name: | ambc_NumberOfTransactionsOutstandingWithSponsoredSpecialPurposeEntities
Namespace Prefix: | ambc_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | instant

X
- Definition The number of asset transfer transactions executed with Ambac's sponsored medium term note special purpose entities since the inception of those entities. + References No definition available. + Details
Name: | ambc_NumberOfTransactionsWithSponsoredSpecialPurposeEntitiesInceptionToDate
Namespace Prefix: | ambc_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | instant

X
- Definition The weighted average remaining life of assets held by Ambac's sponsored medium term note special purpose entities at the reporting date. + References No definition available. + Details
Name: | ambc_WeightedAverageLifeOfAssetsHeldBySponsoredSpecialPurposeEntities
Namespace Prefix: | ambc_
Data Type: | xbrli:durationItemType
Balance Type: | na
Period Type: | duration

X
- Definition The weighted average credit rating of assets held by Ambac's sponsored medium term note special purpose entities at the reporting date. + References No definition available. + Details
Name: | ambc_WeightedAverageRatingOfAssetsHeldBySponsoredSpecialPurposeEntities
Namespace Prefix: | ambc_
Data Type: | us-types:durationStringItemType
Balance Type: | na
Period Type: | duration

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- Definition Fair value portion of investments accounted under the equity method. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 323 -SubTopic 10 -Section 25 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=35740979&loc=d3e32014-111567 + Details
Name: | us-gaap_EquityMethodInvestmentsFairValueDisclosure
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition The carrying amount of the consolidated Variable Interest Entity's assets included in the reporting entity's statement of financial position. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 810 -SubTopic 10 -Section 50 -Paragraph 3 -Subparagraph (bb) -URI http://asc.fasb.org/extlink&oid=28200181&loc=d3e5710-111685 + Details
Name: | us-gaap_VariableInterestEntityConsolidatedCarryingAmountAssets
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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- Definition The carrying amount of the consolidated Variable Interest Entity's liabilities included in the reporting entity's statement of financial position. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 810 -SubTopic 10 -Section 50 -Paragraph 3 -Subparagraph (bb) -URI http://asc.fasb.org/extlink&oid=28200181&loc=d3e5710-111685 + Details
Name: | us-gaap_VariableInterestEntityConsolidatedCarryingAmountLiabilities
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- 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: | dei_LegalEntityAxis=us-gaap_ConsolidatedEntitiesMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
Name: | us-gaap_DebtInstrumentAxis=ambc_AssetBackedSecuritiesAndUtilityObligationsMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
Name: | us-gaap_GuaranteeObligationsByNatureAxis=ambc_SponsoredVariableInterestEntitiesMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |