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Financial Guarantee Insurance Contracts - Summary of Rollforward of RMBS Subrogation, by Estimation Approach (Detail) - Successor [Member] - USD ($) $ in Thousands	9 Months Ended		12 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Subrogation By Estimation Approach [Roll Forward]
Discounted RMBS subrogation (gross of reinsurance), beginning balance	$ 1,834,387
Changes recognized
Discounted RMBS subrogation (gross of reinsurance), ending balance	1,776,248		$ 1,834,387
Random Samples [Member]
Subrogation By Estimation Approach [Roll Forward]
Discounted RMBS subrogation (gross of reinsurance), beginning balance	1,834,387	$ 1,907,035	1,907,035
Changes recognized
Impact of sponsor actions			0
Other Changes Rmbs Subrogation	(58,139)	(62,919)
Discounted RMBS subrogation (gross of reinsurance), ending balance	$ 1,776,248	$ 1,844,116	$ 1,834,387

X

- Definition

Changes Recognized in Residential Mortgage Backed Security [Abstract]

+ References

No definition available.

+ Details

Name:	ambc_ChangesRecognizedInResidentialMortgageBackedSecurityAbstract
Namespace Prefix:	ambc_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

X

- Definition

The change in RMBS subrogation due to the impact of sponsor actions which includes loan repurchases, direct payments, and other contributions from sponsors.

+ References

No definition available.

+ Details

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

X

- Definition

This element represents the changes in RMBS subrogation for reasons other than additional transactions being reviewed, adverse loans being reviewed or due to adverse loans being repurchased by the sponsor.

+ References

No definition available.

+ Details

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

X

- Definition

This element represents the reduction to gross claim liability for subrogation on insured RMBS financial obligations with credit deterioration.

+ References

No definition available.

+ Details

Name:	ambc_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationReductionsOfGrossClaimLiabilityRmbsSubrogation
Namespace Prefix:	ambc_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

X

- Definition

Subrogation By Estimation Approach [Roll Forward]

+ References

No definition available.

+ Details

Name:	ambc_SubrogationByEstimationApproachRollForward
Namespace Prefix:	ambc_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

X

- Details

Name:	us-gaap_StatementScenarioAxis=us-gaap_SuccessorMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	us-gaap_MortgageLoansOnRealEstateWriteDownOrReserveByLoanAxis=ambc_RandomSamplesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: