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Financial Guarantee Insurance - Summary of Percentage Ceded to Reinsurers and Reinsurance Recoverable and Rating Levels (Detail) $ in Thousands	12 Months Ended
	Dec. 31, 2018 USD ($)
Summary of Ceded Par and Net Unsecured Reinsurance Recoverable [Line Items]
Percentage ceded Par	100.00%
Net unsecured reinsurance recoverable	$ 18,815
Assured Guaranty Re Ltd [Member]
Summary of Ceded Par and Net Unsecured Reinsurance Recoverable [Line Items]
Percentage ceded Par	60.00%
Net unsecured reinsurance recoverable	$ 0
Sompo Japan Nipponkoa Holdings, Inc [Member]
Summary of Ceded Par and Net Unsecured Reinsurance Recoverable [Line Items]
Percentage ceded Par	4.70%
Net unsecured reinsurance recoverable	$ 0
Assured Guaranty Corporation [Member]
Summary of Ceded Par and Net Unsecured Reinsurance Recoverable [Line Items]
Percentage ceded Par	7.10%
Net unsecured reinsurance recoverable	$ 0
Build America Mutual Assurance Company [Domain]
Summary of Ceded Par and Net Unsecured Reinsurance Recoverable [Line Items]
Percentage ceded Par	28.20%
Net unsecured reinsurance recoverable	$ 18,815

X

- Definition

Percentage of the cedant's total ceded financial guarantee principal outstanding.

+ References

No definition available.

+ Details

Name:	ambc_PercentageOfCededFinancialGuaranteePrincipalOutstanding
Namespace Prefix:	ambc_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

X

- Definition

Summary of Ceded Par and Net Unsecured Reinsurance Recoverable [Line Items]

+ References

No definition available.

+ Details

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

X

- Definition

Represents reinsurance recoverables on paid and unpaid losses and deferred ceded premiums, net of ceded premiums payables due to reinsurers, letters of credit, and collateral posted for the benefit of the cedant.

+ References

No definition available.

+ Details

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

X

- Details

Name:	us-gaap_CededCreditRiskAxis=ambc_AssuredGuarantyReLtdMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	us-gaap_CededCreditRiskAxis=ambc_SompoJapanNipponkoaInsuranceIncMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	us-gaap_CededCreditRiskAxis=ambc_AssuredGuarantyCorpMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	us-gaap_CededCreditRiskAxis=ambc_BuildAmericaMutualAssuranceCompanyDomain
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: