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v3.20.4

Fair Value Measurements - Information about Valuation Inputs for Variable Interest Entity Assets and Liabilities Classified as Level 3 (Detail) $ in Millions | 12 Months Ended
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Dec. 31, 2020
Fair Value Inputs, Liabilities, Quantitative Information [Line Items] (Deprecated 2018-01-31) |
Estimated Future Premium Payments Weighted Average Discounted Rate | 2.20%
Variable Interest Entity [Member] |
Fair Value Inputs, Liabilities, Quantitative Information [Line Items] (Deprecated 2018-01-31) |
Estimated Future Premium Payments Minimum Discounted Rate | 1.80%
Estimated Future Premium Payments Maximum Discounted Rate | 3.90%
Asset-backed Securities [Member] |
Fair Value Inputs, Liabilities, Quantitative Information [Line Items] (Deprecated 2018-01-31) |
Fair Value Inputs Coupon Rate | 5.97%
Fair Value Inputs Maturity | 14 years 9 months 29 days
Fair Value Inputs Yield | 10.50%
Variable Interest Entity [Member] |
Fair Value Inputs, Liabilities, Quantitative Information [Line Items] (Deprecated 2018-01-31) |
Estimated Future Premium Payments Weighted Average Discounted Rate | 2.40%

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- Definition Estimated Future Premium Payments Maximum Discounted Rate + References No definition available. + Details
Name: | ambc_EstimatedFuturePremiumPaymentsMaximumDiscountedRate
Namespace Prefix: | ambc_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | instant

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- Definition Estimated Future Premium Payments Minimum Discounted Rate + References No definition available. + Details
Name: | ambc_EstimatedFuturePremiumPaymentsMinimumDiscountedRate
Namespace Prefix: | ambc_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | instant

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- Definition Weighted average discounted rate of estimated future premium payments to be paid by the VIEs. + References No definition available. + Details
Name: | ambc_EstimatedFuturePremiumPaymentsWeightedAverageDiscountedRate
Namespace Prefix: | ambc_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | instant

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- Definition Fair value inputs coupon rate. + References No definition available. + Details
Name: | ambc_FairValueInputsCouponRate
Namespace Prefix: | ambc_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | instant

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- Definition Fair value inputs maturity. + References No definition available. + Details
Name: | ambc_FairValueInputsMaturity
Namespace Prefix: | ambc_
Data Type: | xbrli:durationItemType
Balance Type: | na
Period Type: | duration

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- Definition Fair value inputs yield. + References No definition available. + Details
Name: | ambc_FairValueInputsYield
Namespace Prefix: | ambc_
Data Type: | num:percentItemType
Balance Type: | na
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_FairValueOptionQuantitativeDisclosuresLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Details
Name: | srt_ConsolidatedEntitiesAxis=us-gaap_VariableInterestEntityPrimaryBeneficiaryMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
Name: | us-gaap_FairValueByAssetClassAxis=us-gaap_AssetBackedSecuritiesMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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