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Variable Interest Entities - Summary of Carrying Amount of Assets, Liabilities and Maximum Exposure to Loss of Ambac's Variable Interests in Non-Consolidated Variable Interest Entities (Detail) - USD ($) $ in Thousands | Dec. 31, 2018 | Dec. 31, 2017
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Variable Interest Entities [Line Items] |  |
Maximum Exposure To Loss | $ 35,566,155 | $ 44,199,767
Insurance Assets | 2,257,824 | 1,069,268
Insurance Liabilities | 1,433,877 | 4,260,578
Derivative Liabilities | 5,711 |
Global Structured Finance [Member] |  |
Variable Interest Entities [Line Items] |  |
Maximum Exposure To Loss | 11,420,199 | 18,569,951
Insurance Assets | 1,948,753 | 733,921
Insurance Liabilities | 1,098,440 | 3,889,522
Derivative Liabilities | 7,168 | 10,296
Global Structured Finance [Member] | Collateralized Debt Obligations [Member] |  |
Variable Interest Entities [Line Items] |  |
Maximum Exposure To Loss | 9,787 | 35,555
Insurance Assets | 0 | 169
Insurance Liabilities | 0 | 1
Derivative Liabilities | (2) |
Global Structured Finance [Member] | Residential Mortgage-Backed Securities [Member] |  |
Variable Interest Entities [Line Items] |  |
Maximum Exposure To Loss | 6,713,437 | 12,766,685
Insurance Assets | 1,859,121 | 619,848
Insurance Liabilities | 546,682 | 3,218,356
Derivative Liabilities | 0 | 0
Global Structured Finance [Member] | Other Consumer Asset-Backed [Member] |  |
Variable Interest Entities [Line Items] |  |
Maximum Exposure To Loss | 1,700,984 | 2,266,610
Insurance Assets | 15,435 | 23,405
Insurance Liabilities | 238,234 | 328,732
Derivative Liabilities | 0 | 0
Global Structured Finance [Member] | Other Commercial Asset-Backed [Member] |  |
Variable Interest Entities [Line Items] |  |
Maximum Exposure To Loss | 873,343 | 987,797
Insurance Assets | 20,735 | 30,413
Insurance Liabilities | 12,264 | 35,976
Derivative Liabilities | 0 | 0
Global Structured Finance [Member] | Other Debt Obligations [Member] |  |
Variable Interest Entities [Line Items] |  |
Maximum Exposure To Loss | 2,122,648 | 2,513,304
Insurance Assets | 53,462 | 60,086
Insurance Liabilities | 301,260 | 306,457
Derivative Liabilities | 7,170 | 10,311
Global Public Finance [Member] |  |
Variable Interest Entities [Line Items] |  |
Maximum Exposure To Loss | 24,145,956 | 25,629,816
Insurance Assets | 309,071 | 335,347
Insurance Liabilities | 335,437 | 371,056
Derivative Liabilities | $ (1,457) | $ (551)

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- Definition The net carrying amount of derivative assets or liabilities in the reporting entity's statement of financial position that relate to the entity's variable interests in Variable Interest Entities (VIEs) where the VIEs are not consolidated. + References No definition available. + Details
Name: | ambc_VariableInterestEntityNonconsolidatedCarryingAmountDerivativeNetAssetsOrLiabilities
Namespace Prefix: | ambc_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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- Definition The carrying amount of insurance assets in the reporting entity's statement of financial position that relate to the entity's variable interests in Variable Interest Entities (VIEs) where the VIEs are not consolidated. + References No definition available. + Details
Name: | ambc_VariableInterestEntityNonconsolidatedCarryingAmountInsuranceAssets
Namespace Prefix: | ambc_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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- Definition The carrying amount of insurance liabilities in the reporting entity's statement of financial position that relate to the entity's variable interests in Variable Interest Entities (VIEs) where the VIEs are not consolidated. + References No definition available. + Details
Name: | ambc_VariableInterestEntityNonconsolidatedCarryingAmountInsuranceLiabilities
Namespace Prefix: | ambc_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- Definition The reporting entity's maximum amount of exposure to loss as a result of its involvement with the Variable Interest Entity (VIE). + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 810 -SubTopic 10 -Section 50 -Paragraph 6 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=108788376&loc=d3e5747-111685 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 810 -SubTopic 10 -Section 50 -Paragraph 4 -Subparagraph b -URI http://asc.fasb.org/extlink&oid=108788376&loc=d3e5728-111685 + Details
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Namespace Prefix: | us-gaap_
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
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Namespace Prefix: |
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Balance Type: |
Period Type: |

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- Details
Name: | us-gaap_FinancialInstrumentAxis=us-gaap_CollateralizedDebtObligationsMember
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- Details
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Data Type: | na
Balance Type: |
Period Type: |

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- Details
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- Details
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- Details
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- Details
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