Name:	ago_InsuredFinancialObligationsLossMitigationSecurities
Namespace Prefix:	ago_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

Represents the period within which the entity expects the reimbursement of the liquidity claims.

No definition available.

Name:	ago_InsuredFinancialObligationsMaximumPeriodofLiquidityClaims
Namespace Prefix:	ago_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

Represents the outstanding principal amounts of the underlying insured obligations for financial guaranty contracts, after netting out ceded contracts.

No definition available.

Name:	ago_InsuredFinancialObligationsOutstandingPrincipalAmountNet
Namespace Prefix:	ago_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

Probability that, on a present value basis, the entity will pay more claims over the life of that transaction than it will ultimately have been reimbursed.

No definition available.

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

[Line Items] for Schedule of Insured Financial Obligations [Table]

No definition available.

Name:	ago_ScheduleofInsuredFinancialObligationsLineItems
Namespace Prefix:	ago_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

Specialty Business, Gross Par

No definition available.

Name:	ago_SpecialtyBusinessGrossPar
Namespace Prefix:	ago_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

Specialty Business, Net Par

No definition available.

Name:	ago_SpecialtyBusinessNetPar
Namespace Prefix:	ago_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

Maximum potential amount of future payments (undiscounted) the guarantor could be required to make under the guarantee or each group of similar guarantees before reduction for potential recoveries under recourse or collateralization provisions.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 460
-SubTopic 10
-Section 50
-Paragraph 4
-Subparagraph (b)(1)
-URI https://asc.fasb.org/extlink&oid=124440162&loc=d3e12069-110248

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

Minimum amount of other commitment not otherwise specified in the taxonomy. Excludes commitments explicitly modeled in the taxonomy, including but not limited to, long-term and short-term purchase commitments, recorded and unrecorded purchase obligations, supply commitments, registration payment arrangements, leases, debt, product warranties, guarantees, environmental remediation obligations, and pensions.

No definition available.

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

Name:	srt_StatementGeographicalAxis=country_PR
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Name:	ago_LoansInsuredOrGuaranteeForGovernmentAuthoritiesAxis=ago_PuertoRicoExposuresSubjectToAPlanSupportAgreementMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Name:	srt_CounterpartyNameAxis=ago_CommonwealthofPuertoRicoGeneralObligationBondsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Name:	srt_CounterpartyNameAxis=ago_PuertoRicoPublicBuildingsAuthorityMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Name:	srt_CounterpartyNameAxis=ago_PuertoRicoTransportationAuthorityMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Name:	srt_CounterpartyNameAxis=ago_PuertoRicoHighwaysAuthorityMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Name:	srt_CounterpartyNameAxis=ago_PuertoRicoElectricPowerAuthorityMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Name:	ago_LoansInsuredOrGuaranteeForGovernmentAuthoritiesAxis=ago_PublicCorporationsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Name:	ago_LoansInsuredOrGuaranteeForGovernmentAuthoritiesAxis=ago_PuertoRicoHighwaysAndTransportationAuthorityMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Name:	us-gaap_UnderlyingAssetClassAxis=ago_PublicFinanceMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Name:	us-gaap_UnderlyingAssetClassAxis=us-gaap_StructuredFinanceMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Name:	us-gaap_OtherCommitmentsAxis=ago_CommitmenttoProvideGuaranteesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Name:	us-gaap_InternalCreditAssessmentAxis=us-gaap_InternalNoninvestmentGradeMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: