Insurance Regulatory Restrictions Additional Details (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2020	Dec. 31, 2019
Average Annual Rate Of Return	4.56%	5.43%
Prescribed Discount Rate Percentage	5.10%
Quarterly Dividend Increase Limitation Percentage	15.00%
WISCONSIN
Percentage Of Policy Holders Surplus	10.00%
ARIZONA
Percentage Of Policy Holders Surplus	10.00%
Minimum [Member]
Settlement Agreement Annual Restricted Payment Amount	$ 5
Maximum [Member]
Settlement Agreement Annual Restricted Payment Amount	8
Ambac Assurance [Member]
Statutory Accounting Practices, Statutory Capital and Surplus, Balance	865	$ 1,088
Ambac Assurance [Member] \| Prescribed Or Permitted Additional Accounting Practices [Member]
Statutory Accounting Practices, Prescribed Practice, Amount	40	$ (12)
Everspan Indemnity
Statutory Accounting Practices, Statutory Capital and Surplus, Balance	$ 26
5.1 % Surplus Notes Due 2020 [Member] \| Ambac Assurance Corporation [Member]
Debt Instrument, Interest Rate, Stated Percentage	5.10%	5.10%

Average Annual Rate Of Return

No definition available.

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

- Definition

Percentage Of Policy Holders Surplus

+ References

No definition available.

+ Details

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

- Definition

Discount rate percentage for the purpose of discounting statutory loss reserves and estimated impairment losses on subsidiary guarantees prescribed the regulator.

+ References

No definition available.

+ Details

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

- Definition

No quarterly dividend may exceed the dividend paid in the corresponding quarter by more than this percentage without notification to the regulator.

+ References

No definition available.

+ Details

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

- Definition

Settlement Agreement Annual Restricted Payment Amount

+ References

No definition available.

+ Details

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

- Definition

Contractual interest rate for funds borrowed, under the debt agreement.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.22(a)(1))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

+ Details

Name:	us-gaap_DebtInstrumentInterestRateStatedPercentage
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Amount as of the balance sheet date of each item listed to reconcile the differences resulting from applying state prescribed statutory accounting practices rather than statutory accounting practices prescribed by National Association of Insurance Commissioners.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 505
-Section 50
-Paragraph 3
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=6486098&loc=d3e24746-158529

+ Details

Name:	us-gaap_StatutoryAccountingPracticesPrescribedPracticeAmount
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

The amount of statutory capital and surplus (stockholders' equity) as of the balance sheet date using prescribed or permitted statutory accounting practices (rather than GAAP, if different) of the state or country.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03(a)(23)(c))
-URI http://asc.fasb.org/extlink&oid=119400593&loc=d3e572229-122910

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 505
-Section 50
-Paragraph 1
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=6486098&loc=d3e24708-158529

+ Details

Name:	us-gaap_StatutoryAccountingPracticesStatutoryCapitalAndSurplusBalance
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Details

Name:	us-gaap_StatutoryAccountingPracticesByJurisdictionAxis=stpr_WI
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_StatutoryAccountingPracticesByJurisdictionAxis=stpr_AZ
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_RangeAxis=srt_MinimumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_RangeAxis=srt_MaximumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	dei_LegalEntityAxis=ambc_AmbacAssuranceMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_PublicUtilitiesRegulatoryProceedingAxis=ambc_PrescribedOrPermittedAdditionalAccountingPracticesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	dei_LegalEntityAxis=ambc_EverspanIndemnityMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=ambc_A5.1SurplusNotesDue2020Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	dei_LegalEntityAxis=ambc_AmbacAssuranceCorporationMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: