Weighted average discounted rate of estimated future premium payments to be paid by the VIEs.

No definition available.

Name:	ambc_EstimatedFuturePremiumPaymentsWeightedAverageDiscountedRate
Namespace Prefix:	ambc_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

The amount of gains or losses included in earnings attributable to unrealized gains or losses on financial instruments measured on a recurring basis using unobservable inputs that are still held at the reporting date.

+ References

No definition available.

+ Details

Name:	ambc_FairValueAssetOrLiabilityMeasuredOnRecurringBasisChangeInUnrealizedGainsLossesStillHeld
Namespace Prefix:	ambc_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Fair value inputs coupon rate.

+ References

No definition available.

+ Details

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Namespace Prefix:	ambc_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Fair value inputs maturity.

+ References

No definition available.

+ Details

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Namespace Prefix:	ambc_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- 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

- Definition

Fair value of financial instruments measured on a recurring basis using unobservable inputs removed as a result of the deconsolidation of variable interest entities.

+ References

No definition available.

+ Details

Name:	ambc_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetOrLiabilityDeconsolidationOfVies
Namespace Prefix:	ambc_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Fair Value of consolidated VIE debt obligations valued using unobservable inputs by asset class

+ References

No definition available.

+ Details

Name:	ambc_FairValueOfConsolidatedVariableInterestEntityDebtObligationsValuedUsingUnobservableInputsByAssetClass
Namespace Prefix:	ambc_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- 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

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

- Details

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Namespace Prefix:
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- Details

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Balance Type:
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- Details

Name:	us-gaap_FairValueByAssetClassAxis=us-gaap_InvestmentsMember
Namespace Prefix:
Data Type:	na
Balance Type:
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- Details

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Namespace Prefix:
Data Type:	na
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- Details

Name:	us-gaap_FairValueByAssetClassAxis=us-gaap_OtherAssetsMember
Namespace Prefix:
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- Details

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- Details

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- Details

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- Details

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- Details

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Fair Value Measurements - Information about Valuation Inputs for Variable Interest Entity Assets and Liabilities Classified as Level 3 (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Fair Value Inputs, Liabilities, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Estimated Future Premium Payments Weighted Average Discounted Rate	2.70%	2.50%
Asset-backed Securities [Member]
Fair Value Inputs, Liabilities, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Fair Value Inputs Coupon Rate	5.97%	5.97%
Fair Value Inputs Maturity	16 years 3 months 15 days	17 years 9 days
Fair Value Inputs Yield	12.00%	12.00%
Fair Value, Inputs, Level 3 [Member]
Fair Value Inputs, Liabilities, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability Deconsolidation Of Vies	$ 3,709,113
Fair Value, Inputs, Level 3 [Member] \| Investments [Member]
Fair Value Inputs, Liabilities, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability Deconsolidation Of Vies	0
Fair Value, Inputs, Level 3 [Member] \| European ABS Transactions [Member]
Fair Value Inputs, Liabilities, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Fair Value Of Consolidated Variable Interest Entity Debt Obligations Valued Using Unobservable Inputs By Asset Class	217,092	$ 2,757,688
Fair Value, Inputs, Level 3 [Member] \| Other Assets [Member]
Fair Value Inputs, Liabilities, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability Deconsolidation Of Vies	0
Fair Value, Inputs, Level 3 [Member] \| Derivative Financial Instruments, Liabilities [Member]
Fair Value Inputs, Liabilities, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability Deconsolidation Of Vies	0
Loan Origination Commitments [Member] \| Fair Value, Inputs, Level 3 [Member]
Fair Value Inputs, Liabilities, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability Deconsolidation Of Vies	5,946,465
Long-term Debt [Member] \| Fair Value, Inputs, Level 3 [Member]
Fair Value Inputs, Liabilities, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability Deconsolidation Of Vies	(2,237,352)
Investment Contracts [Member] \| Fair Value, Inputs, Level 3 [Member]
Fair Value Inputs, Liabilities, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Or Liability Deconsolidation Of Vies	$ 0
Variable Interest Entity, Primary Beneficiary [Member]
Fair Value Inputs, Liabilities, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Fair Value Asset Or Liability Measured On Recurring Basis Change In Unrealized Gains Losses Still Held			$ 833,023
Estimated Future Premium Payments Weighted Average Discounted Rate	3.10%	3.10%
Variable Interest Entity, Primary Beneficiary [Member] \| European ABS Transactions [Member]
Fair Value Inputs, Liabilities, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Fair Value Inputs Coupon Rate	2.20%	0.40%
Fair Value Inputs Maturity	18 years 11 months 6 days	15 years 3 months 11 days
Fair Value Inputs Yield	3.18%	4.82%