Name:	ago_AvailableforsaleDebtSecuritiesNumberofInstruments
Namespace Prefix:	ago_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

Represents the number of custodial trusts with whom the entity entered into put agreements.

No definition available.

Name:	ago_FinancialInstrumentsSubjecttoMandatoryRedemptionSettlementTermsNumberofCustodialTrusts
Namespace Prefix:	ago_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

Number Of Fixed Maturity Debt Securities Classified As Level 3

No definition available.

Name:	ago_NumberOfFixedMaturityDebtSecuritiesClassifiedAsLevel3
Namespace Prefix:	ago_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

Percentage of CDS Contracts Fair Valued Using Minimum Premium

No definition available.

Name:	ago_PercentageOfCDSContractsFairValuedUsingMinimumPremium
Namespace Prefix:	ago_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	instant

Estimated rate of prepayment of principal on financial assets for initial fair value measurement.

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 860
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 3
-Subparagraph (c)(2)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147481326/860-20-50-3

Name:	us-gaap_AssumptionForFairValueOnSecuritizationDateOfInterestsContinuedToBeHeldByTransferorServicingAssetsOrLiabilitiesPrepaymentSpeed
Namespace Prefix:	us-gaap_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	duration

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.

No definition available.

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

Amount that would be paid, determined under the conditions specified in the contract, if the holder of the share has the right to redeem the shares.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Name Accounting Standards Codification
-Topic 480
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (a)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147481648/480-10-50-2

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

The maximum amount that the issuer could be required to pay to redeem the instrument by physical settlement, if applicable.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Name Accounting Standards Codification
-Topic 480
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147481648/480-10-50-2

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

Name:	us-gaap_CreditDerivativesByContractTypeAxis=us-gaap_CreditDefaultSwapMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

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

Name:	us-gaap_FairValueByMeasurementFrequencyAxis=us-gaap_FairValueMeasurementsRecurringMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Name:	us-gaap_FairValueByFairValueHierarchyLevelAxis=us-gaap_FairValueInputsLevel3Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Name:	us-gaap_MeasurementInputTypeAxis=us-gaap_MeasurementInputDefaultRateMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

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