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Financial Guaranty Insurance Losses - BIG Transaction Loss Summary (Details) (USD $)
In Millions, unless otherwise specified
3 Months Ended 12 Months Ended
Mar. 31, 2015
risk
Dec. 31, 2014
risk
Discount    
Total $ (389)ago_ScheduleofInsuredFinancialObligationswithCreditDeteriorationReductionsofGrossClaimLiabilityNetofReinsuranceRecoverableDiscount  
Reserves (salvage)    
Total 582ago_LiabilityforUnpaidClaimsandClaimsAdjustmentExpenseNetofReinsuranceandSalvageSubrogationandOtherRecoveries 567ago_LiabilityforUnpaidClaimsandClaimsAdjustmentExpenseNetofReinsuranceandSalvageSubrogationandOtherRecoveries
BIG [Member]    
Number of risks    
Total (in contracts) 356us-gaap_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationNumberOfContractsIssuedAndOutstanding
/ us-gaap_InternalCreditAssessmentAxis
= ago_InternalNoninvestmentGradeMember
[1] 358us-gaap_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationNumberOfContractsIssuedAndOutstanding
/ us-gaap_InternalCreditAssessmentAxis
= ago_InternalNoninvestmentGradeMember
[1]
Remaining weighted average contract period    
Total (in years) 10 years 1 month 6 days 10 years 3 months 18 days
Principal    
Total 14,937ago_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationInsuredContractualPaymentsOutstandingPrincipalNet
/ us-gaap_InternalCreditAssessmentAxis
= ago_InternalNoninvestmentGradeMember
15,222ago_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationInsuredContractualPaymentsOutstandingPrincipalNet
/ us-gaap_InternalCreditAssessmentAxis
= ago_InternalNoninvestmentGradeMember
Interest    
Total 7,393ago_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationInsuredContractualPaymentsOutstandingInterestNet
/ us-gaap_InternalCreditAssessmentAxis
= ago_InternalNoninvestmentGradeMember
7,651ago_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationInsuredContractualPaymentsOutstandingInterestNet
/ us-gaap_InternalCreditAssessmentAxis
= ago_InternalNoninvestmentGradeMember
Total net outstanding exposure    
Total 22,330ago_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationInsuredContractualPaymentsOutstandingNet
/ us-gaap_InternalCreditAssessmentAxis
= ago_InternalNoninvestmentGradeMember
[2] 22,873ago_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationInsuredContractualPaymentsOutstandingNet
/ us-gaap_InternalCreditAssessmentAxis
= ago_InternalNoninvestmentGradeMember
[2]
Expected cash outflows (inflows)    
Total 3,001us-gaap_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationNetClaimLiability
/ us-gaap_InternalCreditAssessmentAxis
= ago_InternalNoninvestmentGradeMember
3,118us-gaap_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationNetClaimLiability
/ us-gaap_InternalCreditAssessmentAxis
= ago_InternalNoninvestmentGradeMember
Potential recoveries    
Total, Undiscounted R&W (165)ago_ScheduleofInsuredFinancialObligationswithCreditDeteriorationReductionsofGrossClaimLiabilityPotentialRecoveriesofRepresentationsandWarrantiesNetUndiscounted
/ us-gaap_InternalCreditAssessmentAxis
= ago_InternalNoninvestmentGradeMember
(239)ago_ScheduleofInsuredFinancialObligationswithCreditDeteriorationReductionsofGrossClaimLiabilityPotentialRecoveriesofRepresentationsandWarrantiesNetUndiscounted
/ us-gaap_InternalCreditAssessmentAxis
= ago_InternalNoninvestmentGradeMember
Total, Other (1,458)ago_ScheduleofInsuredFinancialObligationswithCreditDeteriorationReductionsofGrossClaimLiabilityPotentialRecoveriesNetOther
/ us-gaap_InternalCreditAssessmentAxis
= ago_InternalNoninvestmentGradeMember
[3] (1,477)ago_ScheduleofInsuredFinancialObligationswithCreditDeteriorationReductionsofGrossClaimLiabilityPotentialRecoveriesNetOther
/ us-gaap_InternalCreditAssessmentAxis
= ago_InternalNoninvestmentGradeMember
[3]
Total (1,623)ago_ScheduleofInsuredFinancialObligationswithCreditDeteriorationReductionsofGrossClaimLiabilityNetPotentialRecoveries
/ us-gaap_InternalCreditAssessmentAxis
= ago_InternalNoninvestmentGradeMember
(1,716)ago_ScheduleofInsuredFinancialObligationswithCreditDeteriorationReductionsofGrossClaimLiabilityNetPotentialRecoveries
/ us-gaap_InternalCreditAssessmentAxis
= ago_InternalNoninvestmentGradeMember
Subtotal    
Total 1,378ago_ScheduleofInsuredFinancialObligationswithCreditDeteriorationNetClaimLiabilityUndiscountedDirectAssumedNet
/ us-gaap_InternalCreditAssessmentAxis
= ago_InternalNoninvestmentGradeMember
1,402ago_ScheduleofInsuredFinancialObligationswithCreditDeteriorationNetClaimLiabilityUndiscountedDirectAssumedNet
/ us-gaap_InternalCreditAssessmentAxis
= ago_InternalNoninvestmentGradeMember
Discount    
Total (389)ago_ScheduleofInsuredFinancialObligationswithCreditDeteriorationReductionsofGrossClaimLiabilityNetofReinsuranceRecoverableDiscount
/ us-gaap_InternalCreditAssessmentAxis
= ago_InternalNoninvestmentGradeMember
(413)ago_ScheduleofInsuredFinancialObligationswithCreditDeteriorationReductionsofGrossClaimLiabilityNetofReinsuranceRecoverableDiscount
/ us-gaap_InternalCreditAssessmentAxis
= ago_InternalNoninvestmentGradeMember
Present value of expected cash flows    
Net expected loss to be paid 989ago_ScheduleofInsuredFinancialObligationswithCreditDeteriorationNetClaimLiabilityNetofReinsuranceRecoverable
/ us-gaap_InternalCreditAssessmentAxis
= ago_InternalNoninvestmentGradeMember
989ago_ScheduleofInsuredFinancialObligationswithCreditDeteriorationNetClaimLiabilityNetofReinsuranceRecoverable
/ us-gaap_InternalCreditAssessmentAxis
= ago_InternalNoninvestmentGradeMember
Deferred premium revenue    
Total 557ago_ScheduleofInsuredFinancialObligationswithCreditDeteriorationStatementofFinancialPositionUnearnedPremiumRevenueNetofReinsuranceRecoverable
/ us-gaap_InternalCreditAssessmentAxis
= ago_InternalNoninvestmentGradeMember
584ago_ScheduleofInsuredFinancialObligationswithCreditDeteriorationStatementofFinancialPositionUnearnedPremiumRevenueNetofReinsuranceRecoverable
/ us-gaap_InternalCreditAssessmentAxis
= ago_InternalNoninvestmentGradeMember
Reserves (salvage)    
Total 581ago_LiabilityforUnpaidClaimsandClaimsAdjustmentExpenseNetofReinsuranceandSalvageSubrogationandOtherRecoveries
/ us-gaap_InternalCreditAssessmentAxis
= ago_InternalNoninvestmentGradeMember
[4] 571ago_LiabilityforUnpaidClaimsandClaimsAdjustmentExpenseNetofReinsuranceandSalvageSubrogationandOtherRecoveries
/ us-gaap_InternalCreditAssessmentAxis
= ago_InternalNoninvestmentGradeMember
[4]
BIG [Member] | Consolidated Entity Excluding Consolidation of Variable Interest Entities (VIE) [Member]    
Number of risks    
Total (in contracts) 356us-gaap_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationNumberOfContractsIssuedAndOutstanding
/ us-gaap_InternalCreditAssessmentAxis
= ago_InternalNoninvestmentGradeMember
/ dei_LegalEntityAxis
= us-gaap_ConsolidatedEntityExcludingVariableInterestEntitiesVIEMember
[1] 358us-gaap_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationNumberOfContractsIssuedAndOutstanding
/ us-gaap_InternalCreditAssessmentAxis
= ago_InternalNoninvestmentGradeMember
/ dei_LegalEntityAxis
= us-gaap_ConsolidatedEntityExcludingVariableInterestEntitiesVIEMember
[1]
Remaining weighted average contract period    
Total (in years) 10 years 1 month 6 days 10 years 3 months 18 days
Principal    
Total 14,937ago_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationInsuredContractualPaymentsOutstandingPrincipalNet
/ us-gaap_InternalCreditAssessmentAxis
= ago_InternalNoninvestmentGradeMember
/ dei_LegalEntityAxis
= us-gaap_ConsolidatedEntityExcludingVariableInterestEntitiesVIEMember
15,222ago_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationInsuredContractualPaymentsOutstandingPrincipalNet
/ us-gaap_InternalCreditAssessmentAxis
= ago_InternalNoninvestmentGradeMember
/ dei_LegalEntityAxis
= us-gaap_ConsolidatedEntityExcludingVariableInterestEntitiesVIEMember
Interest    
Total 7,393ago_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationInsuredContractualPaymentsOutstandingInterestNet
/ us-gaap_InternalCreditAssessmentAxis
= ago_InternalNoninvestmentGradeMember
/ dei_LegalEntityAxis
= us-gaap_ConsolidatedEntityExcludingVariableInterestEntitiesVIEMember
7,651ago_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationInsuredContractualPaymentsOutstandingInterestNet
/ us-gaap_InternalCreditAssessmentAxis
= ago_InternalNoninvestmentGradeMember
/ dei_LegalEntityAxis
= us-gaap_ConsolidatedEntityExcludingVariableInterestEntitiesVIEMember
Total net outstanding exposure    
Total 22,330ago_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationInsuredContractualPaymentsOutstandingNet
/ us-gaap_InternalCreditAssessmentAxis
= ago_InternalNoninvestmentGradeMember
/ dei_LegalEntityAxis
= us-gaap_ConsolidatedEntityExcludingVariableInterestEntitiesVIEMember
[2] 22,873ago_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationInsuredContractualPaymentsOutstandingNet
/ us-gaap_InternalCreditAssessmentAxis
= ago_InternalNoninvestmentGradeMember
/ dei_LegalEntityAxis
= us-gaap_ConsolidatedEntityExcludingVariableInterestEntitiesVIEMember
[2]
Expected cash outflows (inflows)    
Total 3,344us-gaap_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationNetClaimLiability
/ us-gaap_InternalCreditAssessmentAxis
= ago_InternalNoninvestmentGradeMember
/ dei_LegalEntityAxis
= us-gaap_ConsolidatedEntityExcludingVariableInterestEntitiesVIEMember
3,463us-gaap_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationNetClaimLiability
/ us-gaap_InternalCreditAssessmentAxis
= ago_InternalNoninvestmentGradeMember
/ dei_LegalEntityAxis
= us-gaap_ConsolidatedEntityExcludingVariableInterestEntitiesVIEMember
Potential recoveries    
Total, Undiscounted R&W (173)ago_ScheduleofInsuredFinancialObligationswithCreditDeteriorationReductionsofGrossClaimLiabilityPotentialRecoveriesofRepresentationsandWarrantiesNetUndiscounted
/ us-gaap_InternalCreditAssessmentAxis
= ago_InternalNoninvestmentGradeMember
/ dei_LegalEntityAxis
= us-gaap_ConsolidatedEntityExcludingVariableInterestEntitiesVIEMember
(247)ago_ScheduleofInsuredFinancialObligationswithCreditDeteriorationReductionsofGrossClaimLiabilityPotentialRecoveriesofRepresentationsandWarrantiesNetUndiscounted
/ us-gaap_InternalCreditAssessmentAxis
= ago_InternalNoninvestmentGradeMember
/ dei_LegalEntityAxis
= us-gaap_ConsolidatedEntityExcludingVariableInterestEntitiesVIEMember
Total, Other (1,638)ago_ScheduleofInsuredFinancialObligationswithCreditDeteriorationReductionsofGrossClaimLiabilityPotentialRecoveriesNetOther
/ us-gaap_InternalCreditAssessmentAxis
= ago_InternalNoninvestmentGradeMember
/ dei_LegalEntityAxis
= us-gaap_ConsolidatedEntityExcludingVariableInterestEntitiesVIEMember
[3] (1,654)ago_ScheduleofInsuredFinancialObligationswithCreditDeteriorationReductionsofGrossClaimLiabilityPotentialRecoveriesNetOther
/ us-gaap_InternalCreditAssessmentAxis
= ago_InternalNoninvestmentGradeMember
/ dei_LegalEntityAxis
= us-gaap_ConsolidatedEntityExcludingVariableInterestEntitiesVIEMember
[3]
Total (1,811)ago_ScheduleofInsuredFinancialObligationswithCreditDeteriorationReductionsofGrossClaimLiabilityNetPotentialRecoveries
/ us-gaap_InternalCreditAssessmentAxis
= ago_InternalNoninvestmentGradeMember
/ dei_LegalEntityAxis
= us-gaap_ConsolidatedEntityExcludingVariableInterestEntitiesVIEMember
(1,901)ago_ScheduleofInsuredFinancialObligationswithCreditDeteriorationReductionsofGrossClaimLiabilityNetPotentialRecoveries
/ us-gaap_InternalCreditAssessmentAxis
= ago_InternalNoninvestmentGradeMember
/ dei_LegalEntityAxis
= us-gaap_ConsolidatedEntityExcludingVariableInterestEntitiesVIEMember
Subtotal    
Total 1,533ago_ScheduleofInsuredFinancialObligationswithCreditDeteriorationNetClaimLiabilityUndiscountedDirectAssumedNet
/ us-gaap_InternalCreditAssessmentAxis
= ago_InternalNoninvestmentGradeMember
/ dei_LegalEntityAxis
= us-gaap_ConsolidatedEntityExcludingVariableInterestEntitiesVIEMember
1,562ago_ScheduleofInsuredFinancialObligationswithCreditDeteriorationNetClaimLiabilityUndiscountedDirectAssumedNet
/ us-gaap_InternalCreditAssessmentAxis
= ago_InternalNoninvestmentGradeMember
/ dei_LegalEntityAxis
= us-gaap_ConsolidatedEntityExcludingVariableInterestEntitiesVIEMember
Discount    
Total (422)ago_ScheduleofInsuredFinancialObligationswithCreditDeteriorationReductionsofGrossClaimLiabilityNetofReinsuranceRecoverableDiscount
/ us-gaap_InternalCreditAssessmentAxis
= ago_InternalNoninvestmentGradeMember
/ dei_LegalEntityAxis
= us-gaap_ConsolidatedEntityExcludingVariableInterestEntitiesVIEMember
(447)ago_ScheduleofInsuredFinancialObligationswithCreditDeteriorationReductionsofGrossClaimLiabilityNetofReinsuranceRecoverableDiscount
/ us-gaap_InternalCreditAssessmentAxis
= ago_InternalNoninvestmentGradeMember
/ dei_LegalEntityAxis
= us-gaap_ConsolidatedEntityExcludingVariableInterestEntitiesVIEMember
Present value of expected cash flows    
Net expected loss to be paid 1,111ago_ScheduleofInsuredFinancialObligationswithCreditDeteriorationNetClaimLiabilityNetofReinsuranceRecoverable
/ us-gaap_InternalCreditAssessmentAxis
= ago_InternalNoninvestmentGradeMember
/ dei_LegalEntityAxis
= us-gaap_ConsolidatedEntityExcludingVariableInterestEntitiesVIEMember
1,115ago_ScheduleofInsuredFinancialObligationswithCreditDeteriorationNetClaimLiabilityNetofReinsuranceRecoverable
/ us-gaap_InternalCreditAssessmentAxis
= ago_InternalNoninvestmentGradeMember
/ dei_LegalEntityAxis
= us-gaap_ConsolidatedEntityExcludingVariableInterestEntitiesVIEMember
Deferred premium revenue    
Total 668ago_ScheduleofInsuredFinancialObligationswithCreditDeteriorationStatementofFinancialPositionUnearnedPremiumRevenueNetofReinsuranceRecoverable
/ us-gaap_InternalCreditAssessmentAxis
= ago_InternalNoninvestmentGradeMember
/ dei_LegalEntityAxis
= us-gaap_ConsolidatedEntityExcludingVariableInterestEntitiesVIEMember
700ago_ScheduleofInsuredFinancialObligationswithCreditDeteriorationStatementofFinancialPositionUnearnedPremiumRevenueNetofReinsuranceRecoverable
/ us-gaap_InternalCreditAssessmentAxis
= ago_InternalNoninvestmentGradeMember
/ dei_LegalEntityAxis
= us-gaap_ConsolidatedEntityExcludingVariableInterestEntitiesVIEMember
Reserves (salvage)    
Total 658ago_LiabilityforUnpaidClaimsandClaimsAdjustmentExpenseNetofReinsuranceandSalvageSubrogationandOtherRecoveries
/ us-gaap_InternalCreditAssessmentAxis
= ago_InternalNoninvestmentGradeMember
/ dei_LegalEntityAxis
= us-gaap_ConsolidatedEntityExcludingVariableInterestEntitiesVIEMember
[4] 650ago_LiabilityforUnpaidClaimsandClaimsAdjustmentExpenseNetofReinsuranceandSalvageSubrogationandOtherRecoveries
/ us-gaap_InternalCreditAssessmentAxis
= ago_InternalNoninvestmentGradeMember
/ dei_LegalEntityAxis
= us-gaap_ConsolidatedEntityExcludingVariableInterestEntitiesVIEMember
[4]
BIG [Member] | Consolidated Entity Excluding Consolidation of Variable Interest Entities (VIE) [Member] | BIG 1 [Member]    
Number of risks    
Ceded (in contracts) (61)ago_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationNumberOfContractsIssuedAndOutstandingCeded
/ us-gaap_InternalCreditAssessmentAxis
= ago_InternalNoninvestmentGradeMember
/ dei_LegalEntityAxis
= us-gaap_ConsolidatedEntityExcludingVariableInterestEntitiesVIEMember
/ us-gaap_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationBySurveillanceCategoryAxis
= ago_InternalNoninvestmentGradeLevel1Member
[1] (59)ago_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationNumberOfContractsIssuedAndOutstandingCeded
/ us-gaap_InternalCreditAssessmentAxis
= ago_InternalNoninvestmentGradeMember
/ dei_LegalEntityAxis
= us-gaap_ConsolidatedEntityExcludingVariableInterestEntitiesVIEMember
/ us-gaap_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationBySurveillanceCategoryAxis
= ago_InternalNoninvestmentGradeLevel1Member
[1]
Total (in contracts) 164us-gaap_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationNumberOfContractsIssuedAndOutstanding
/ us-gaap_InternalCreditAssessmentAxis
= ago_InternalNoninvestmentGradeMember
/ dei_LegalEntityAxis
= us-gaap_ConsolidatedEntityExcludingVariableInterestEntitiesVIEMember
/ us-gaap_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationBySurveillanceCategoryAxis
= ago_InternalNoninvestmentGradeLevel1Member
[1] 164us-gaap_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationNumberOfContractsIssuedAndOutstanding
/ us-gaap_InternalCreditAssessmentAxis
= ago_InternalNoninvestmentGradeMember
/ dei_LegalEntityAxis
= us-gaap_ConsolidatedEntityExcludingVariableInterestEntitiesVIEMember
/ us-gaap_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationBySurveillanceCategoryAxis
= ago_InternalNoninvestmentGradeLevel1Member
[1]
Remaining weighted average contract period    
Gross (in years) 9 years 9 months 12 days 9 years 10 months 24 days
Ceded (in years) 7 years 2 months 12 days 7 years 4 months 24 days
Principal    
Gross 11,993us-gaap_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationInsuredContractualPaymentsOutstandingPrincipal
/ us-gaap_InternalCreditAssessmentAxis
= ago_InternalNoninvestmentGradeMember
/ dei_LegalEntityAxis
= us-gaap_ConsolidatedEntityExcludingVariableInterestEntitiesVIEMember
/ us-gaap_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationBySurveillanceCategoryAxis
= ago_InternalNoninvestmentGradeLevel1Member
12,358us-gaap_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationInsuredContractualPaymentsOutstandingPrincipal
/ us-gaap_InternalCreditAssessmentAxis
= ago_InternalNoninvestmentGradeMember
/ dei_LegalEntityAxis
= us-gaap_ConsolidatedEntityExcludingVariableInterestEntitiesVIEMember
/ us-gaap_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationBySurveillanceCategoryAxis
= ago_InternalNoninvestmentGradeLevel1Member
Ceded (2,106)ago_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationInsuredContractualPaymentsOutstandingPrincipalCeded
/ us-gaap_InternalCreditAssessmentAxis
= ago_InternalNoninvestmentGradeMember
/ dei_LegalEntityAxis
= us-gaap_ConsolidatedEntityExcludingVariableInterestEntitiesVIEMember
/ us-gaap_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationBySurveillanceCategoryAxis
= ago_InternalNoninvestmentGradeLevel1Member
(2,163)ago_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationInsuredContractualPaymentsOutstandingPrincipalCeded
/ us-gaap_InternalCreditAssessmentAxis
= ago_InternalNoninvestmentGradeMember
/ dei_LegalEntityAxis
= us-gaap_ConsolidatedEntityExcludingVariableInterestEntitiesVIEMember
/ us-gaap_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationBySurveillanceCategoryAxis
= ago_InternalNoninvestmentGradeLevel1Member
Interest    
Gross 6,082us-gaap_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationInsuredContractualPaymentsOutstandingInterest
/ us-gaap_InternalCreditAssessmentAxis
= ago_InternalNoninvestmentGradeMember
/ dei_LegalEntityAxis
= us-gaap_ConsolidatedEntityExcludingVariableInterestEntitiesVIEMember
/ us-gaap_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationBySurveillanceCategoryAxis
= ago_InternalNoninvestmentGradeLevel1Member
6,350us-gaap_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationInsuredContractualPaymentsOutstandingInterest
/ us-gaap_InternalCreditAssessmentAxis
= ago_InternalNoninvestmentGradeMember
/ dei_LegalEntityAxis
= us-gaap_ConsolidatedEntityExcludingVariableInterestEntitiesVIEMember
/ us-gaap_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationBySurveillanceCategoryAxis
= ago_InternalNoninvestmentGradeLevel1Member
Ceded (792)ago_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationInsuredContractualPaymentsOutstandingInterestCeded
/ us-gaap_InternalCreditAssessmentAxis
= ago_InternalNoninvestmentGradeMember
/ dei_LegalEntityAxis
= us-gaap_ConsolidatedEntityExcludingVariableInterestEntitiesVIEMember
/ us-gaap_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationBySurveillanceCategoryAxis
= ago_InternalNoninvestmentGradeLevel1Member
(838)ago_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationInsuredContractualPaymentsOutstandingInterestCeded
/ us-gaap_InternalCreditAssessmentAxis
= ago_InternalNoninvestmentGradeMember
/ dei_LegalEntityAxis
= us-gaap_ConsolidatedEntityExcludingVariableInterestEntitiesVIEMember
/ us-gaap_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationBySurveillanceCategoryAxis
= ago_InternalNoninvestmentGradeLevel1Member
Total net outstanding exposure    
Gross 18,075us-gaap_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationInsuredContractualPaymentsOutstanding
/ us-gaap_InternalCreditAssessmentAxis
= ago_InternalNoninvestmentGradeMember
/ dei_LegalEntityAxis
= us-gaap_ConsolidatedEntityExcludingVariableInterestEntitiesVIEMember
/ us-gaap_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationBySurveillanceCategoryAxis
= ago_InternalNoninvestmentGradeLevel1Member
[2] 18,708us-gaap_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationInsuredContractualPaymentsOutstanding
/ us-gaap_InternalCreditAssessmentAxis
= ago_InternalNoninvestmentGradeMember
/ dei_LegalEntityAxis
= us-gaap_ConsolidatedEntityExcludingVariableInterestEntitiesVIEMember
/ us-gaap_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationBySurveillanceCategoryAxis
= ago_InternalNoninvestmentGradeLevel1Member
[2]
Ceded (2,898)ago_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationInsuredContractualPaymentsOutstandingCeded
/ us-gaap_InternalCreditAssessmentAxis
= ago_InternalNoninvestmentGradeMember
/ dei_LegalEntityAxis
= us-gaap_ConsolidatedEntityExcludingVariableInterestEntitiesVIEMember
/ us-gaap_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationBySurveillanceCategoryAxis
= ago_InternalNoninvestmentGradeLevel1Member
[2] (3,001)ago_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationInsuredContractualPaymentsOutstandingCeded
/ us-gaap_InternalCreditAssessmentAxis
= ago_InternalNoninvestmentGradeMember
/ dei_LegalEntityAxis
= us-gaap_ConsolidatedEntityExcludingVariableInterestEntitiesVIEMember
/ us-gaap_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationBySurveillanceCategoryAxis
= ago_InternalNoninvestmentGradeLevel1Member
[2]
Expected cash outflows (inflows)    
Gross 1,655us-gaap_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationGrossClaimLiability
/ us-gaap_InternalCreditAssessmentAxis
= ago_InternalNoninvestmentGradeMember
/ dei_LegalEntityAxis
= us-gaap_ConsolidatedEntityExcludingVariableInterestEntitiesVIEMember
/ us-gaap_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationBySurveillanceCategoryAxis
= ago_InternalNoninvestmentGradeLevel1Member
1,762us-gaap_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationGrossClaimLiability
/ us-gaap_InternalCreditAssessmentAxis
= ago_InternalNoninvestmentGradeMember
/ dei_LegalEntityAxis
= us-gaap_ConsolidatedEntityExcludingVariableInterestEntitiesVIEMember
/ us-gaap_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationBySurveillanceCategoryAxis
= ago_InternalNoninvestmentGradeLevel1Member
Ceded (616)ago_ScheduleofInsuredFinancialObligationswithCreditDeteriorationClaimLiabilityCeded
/ us-gaap_InternalCreditAssessmentAxis
= ago_InternalNoninvestmentGradeMember
/ dei_LegalEntityAxis
= us-gaap_ConsolidatedEntityExcludingVariableInterestEntitiesVIEMember
/ us-gaap_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationBySurveillanceCategoryAxis
= ago_InternalNoninvestmentGradeLevel1Member
(626)ago_ScheduleofInsuredFinancialObligationswithCreditDeteriorationClaimLiabilityCeded
/ us-gaap_InternalCreditAssessmentAxis
= ago_InternalNoninvestmentGradeMember
/ dei_LegalEntityAxis
= us-gaap_ConsolidatedEntityExcludingVariableInterestEntitiesVIEMember
/ us-gaap_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationBySurveillanceCategoryAxis
= ago_InternalNoninvestmentGradeLevel1Member
Total 64us-gaap_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationNetClaimLiability
/ us-gaap_InternalCreditAssessmentAxis
= ago_InternalNoninvestmentGradeMember
/ dei_LegalEntityAxis
= us-gaap_ConsolidatedEntityExcludingVariableInterestEntitiesVIEMember
/ us-gaap_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationBySurveillanceCategoryAxis
= ago_InternalNoninvestmentGradeLevel1Member
39us-gaap_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationNetClaimLiability
/ us-gaap_InternalCreditAssessmentAxis
= ago_InternalNoninvestmentGradeMember
/ dei_LegalEntityAxis
= us-gaap_ConsolidatedEntityExcludingVariableInterestEntitiesVIEMember
/ us-gaap_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationBySurveillanceCategoryAxis
= ago_InternalNoninvestmentGradeLevel1Member
Potential recoveries    
Gross, Undiscounted R&W (1)ago_ScheduleofInsuredFinancialObligationswithCreditDeteriorationGrossClaimLiabilityPotentialRecoveriesUndiscountedRepresentationsandWarrantiesGross
/ us-gaap_InternalCreditAssessmentAxis
= ago_InternalNoninvestmentGradeMember
/ dei_LegalEntityAxis
= us-gaap_ConsolidatedEntityExcludingVariableInterestEntitiesVIEMember
/ us-gaap_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationBySurveillanceCategoryAxis
= ago_InternalNoninvestmentGradeLevel1Member
(39)ago_ScheduleofInsuredFinancialObligationswithCreditDeteriorationGrossClaimLiabilityPotentialRecoveriesUndiscountedRepresentationsandWarrantiesGross
/ us-gaap_InternalCreditAssessmentAxis
= ago_InternalNoninvestmentGradeMember
/ dei_LegalEntityAxis
= us-gaap_ConsolidatedEntityExcludingVariableInterestEntitiesVIEMember
/ us-gaap_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationBySurveillanceCategoryAxis
= ago_InternalNoninvestmentGradeLevel1Member
Ceded, Undiscounted R&W (1)ago_ScheduleofInsuredFinancialObligationswithCreditDeteriorationGrossClaimLiabilityPotentialRecoveriesUndiscountedRepresentationsandWarrantiesCeded
/ us-gaap_InternalCreditAssessmentAxis
= ago_InternalNoninvestmentGradeMember
/ dei_LegalEntityAxis
= us-gaap_ConsolidatedEntityExcludingVariableInterestEntitiesVIEMember
/ us-gaap_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationBySurveillanceCategoryAxis
= ago_InternalNoninvestmentGradeLevel1Member
0ago_ScheduleofInsuredFinancialObligationswithCreditDeteriorationGrossClaimLiabilityPotentialRecoveriesUndiscountedRepresentationsandWarrantiesCeded
/ us-gaap_InternalCreditAssessmentAxis
= ago_InternalNoninvestmentGradeMember
/ dei_LegalEntityAxis
= us-gaap_ConsolidatedEntityExcludingVariableInterestEntitiesVIEMember
/ us-gaap_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationBySurveillanceCategoryAxis
= ago_InternalNoninvestmentGradeLevel1Member
Gross, Other (1,583)ago_ScheduleofInsuredFinancialObligationswithCreditDeteriorationGrossClaimLiabilityPotentialRecoveriesOther
/ us-gaap_InternalCreditAssessmentAxis
= ago_InternalNoninvestmentGradeMember
/ dei_LegalEntityAxis
= us-gaap_ConsolidatedEntityExcludingVariableInterestEntitiesVIEMember
/ us-gaap_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationBySurveillanceCategoryAxis
= ago_InternalNoninvestmentGradeLevel1Member
[3] (1,687)ago_ScheduleofInsuredFinancialObligationswithCreditDeteriorationGrossClaimLiabilityPotentialRecoveriesOther
/ us-gaap_InternalCreditAssessmentAxis
= ago_InternalNoninvestmentGradeMember
/ dei_LegalEntityAxis
= us-gaap_ConsolidatedEntityExcludingVariableInterestEntitiesVIEMember
/ us-gaap_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationBySurveillanceCategoryAxis
= ago_InternalNoninvestmentGradeLevel1Member
[3]
Ceded, Other 600ago_ScheduleofInsuredFinancialObligationswithCreditDeteriorationGrossClaimLiabilityPotentialRecoveriesCededOther
/ us-gaap_InternalCreditAssessmentAxis
= ago_InternalNoninvestmentGradeMember
/ dei_LegalEntityAxis
= us-gaap_ConsolidatedEntityExcludingVariableInterestEntitiesVIEMember
/ us-gaap_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationBySurveillanceCategoryAxis
= ago_InternalNoninvestmentGradeLevel1Member
[3] 608ago_ScheduleofInsuredFinancialObligationswithCreditDeteriorationGrossClaimLiabilityPotentialRecoveriesCededOther
/ us-gaap_InternalCreditAssessmentAxis
= ago_InternalNoninvestmentGradeMember
/ dei_LegalEntityAxis
= us-gaap_ConsolidatedEntityExcludingVariableInterestEntitiesVIEMember
/ us-gaap_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationBySurveillanceCategoryAxis
= ago_InternalNoninvestmentGradeLevel1Member
[3]
Ceded 599ago_ScheduleofInsuredFinancialObligationswithCreditDeteriorationReductionsofGrossClaimLiabilityPotentialRecoveriesCeded
/ us-gaap_InternalCreditAssessmentAxis
= ago_InternalNoninvestmentGradeMember
/ dei_LegalEntityAxis
= us-gaap_ConsolidatedEntityExcludingVariableInterestEntitiesVIEMember
/ us-gaap_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationBySurveillanceCategoryAxis
= ago_InternalNoninvestmentGradeLevel1Member
608ago_ScheduleofInsuredFinancialObligationswithCreditDeteriorationReductionsofGrossClaimLiabilityPotentialRecoveriesCeded
/ us-gaap_InternalCreditAssessmentAxis
= ago_InternalNoninvestmentGradeMember
/ dei_LegalEntityAxis
= us-gaap_ConsolidatedEntityExcludingVariableInterestEntitiesVIEMember
/ us-gaap_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationBySurveillanceCategoryAxis
= ago_InternalNoninvestmentGradeLevel1Member
Gross (1,584)us-gaap_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationReductionsOfGrossClaimLiabilityGrossPotentialRecoveries
/ us-gaap_InternalCreditAssessmentAxis
= ago_InternalNoninvestmentGradeMember
/ dei_LegalEntityAxis
= us-gaap_ConsolidatedEntityExcludingVariableInterestEntitiesVIEMember
/ us-gaap_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationBySurveillanceCategoryAxis
= ago_InternalNoninvestmentGradeLevel1Member
(1,726)us-gaap_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationReductionsOfGrossClaimLiabilityGrossPotentialRecoveries
/ us-gaap_InternalCreditAssessmentAxis
= ago_InternalNoninvestmentGradeMember
/ dei_LegalEntityAxis
= us-gaap_ConsolidatedEntityExcludingVariableInterestEntitiesVIEMember
/ us-gaap_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationBySurveillanceCategoryAxis
= ago_InternalNoninvestmentGradeLevel1Member
Subtotal    
Gross 71ago_ScheduleofInsuredFinancialObligationswithCreditDeteriorationNetClaimLiabilityUndiscountedDirectAssumed
/ us-gaap_InternalCreditAssessmentAxis
= ago_InternalNoninvestmentGradeMember
/ dei_LegalEntityAxis
= us-gaap_ConsolidatedEntityExcludingVariableInterestEntitiesVIEMember
/ us-gaap_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationBySurveillanceCategoryAxis
= ago_InternalNoninvestmentGradeLevel1Member
36ago_ScheduleofInsuredFinancialObligationswithCreditDeteriorationNetClaimLiabilityUndiscountedDirectAssumed
/ us-gaap_InternalCreditAssessmentAxis
= ago_InternalNoninvestmentGradeMember
/ dei_LegalEntityAxis
= us-gaap_ConsolidatedEntityExcludingVariableInterestEntitiesVIEMember
/ us-gaap_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationBySurveillanceCategoryAxis
= ago_InternalNoninvestmentGradeLevel1Member
Ceded (17)ago_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationGrossClaimLiabilityCeded
/ us-gaap_InternalCreditAssessmentAxis
= ago_InternalNoninvestmentGradeMember
/ dei_LegalEntityAxis
= us-gaap_ConsolidatedEntityExcludingVariableInterestEntitiesVIEMember
/ us-gaap_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationBySurveillanceCategoryAxis
= ago_InternalNoninvestmentGradeLevel1Member
(18)ago_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationGrossClaimLiabilityCeded
/ us-gaap_InternalCreditAssessmentAxis
= ago_InternalNoninvestmentGradeMember
/ dei_LegalEntityAxis
= us-gaap_ConsolidatedEntityExcludingVariableInterestEntitiesVIEMember
/ us-gaap_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationBySurveillanceCategoryAxis
= ago_InternalNoninvestmentGradeLevel1Member
Discount    
Gross (7)us-gaap_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationReductionsOfGrossClaimLiabilityNetDiscount
/ us-gaap_InternalCreditAssessmentAxis
= ago_InternalNoninvestmentGradeMember
/ dei_LegalEntityAxis
= us-gaap_ConsolidatedEntityExcludingVariableInterestEntitiesVIEMember
/ us-gaap_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationBySurveillanceCategoryAxis
= ago_InternalNoninvestmentGradeLevel1Member
3us-gaap_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationReductionsOfGrossClaimLiabilityNetDiscount
/ us-gaap_InternalCreditAssessmentAxis
= ago_InternalNoninvestmentGradeMember
/ dei_LegalEntityAxis
= us-gaap_ConsolidatedEntityExcludingVariableInterestEntitiesVIEMember
/ us-gaap_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationBySurveillanceCategoryAxis
= ago_InternalNoninvestmentGradeLevel1Member
Ceded 0ago_ScheduleofInsuredFinancialObligationswithCreditDeteriorationReductionsofGrossClaimLiabilityCededNetDiscount
/ us-gaap_InternalCreditAssessmentAxis
= ago_InternalNoninvestmentGradeMember
/ dei_LegalEntityAxis
= us-gaap_ConsolidatedEntityExcludingVariableInterestEntitiesVIEMember
/ us-gaap_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationBySurveillanceCategoryAxis
= ago_InternalNoninvestmentGradeLevel1Member
0ago_ScheduleofInsuredFinancialObligationswithCreditDeteriorationReductionsofGrossClaimLiabilityCededNetDiscount
/ us-gaap_InternalCreditAssessmentAxis
= ago_InternalNoninvestmentGradeMember
/ dei_LegalEntityAxis
= us-gaap_ConsolidatedEntityExcludingVariableInterestEntitiesVIEMember
/ us-gaap_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationBySurveillanceCategoryAxis
= ago_InternalNoninvestmentGradeLevel1Member
Present value of expected cash flows    
Ceded (17)ago_ScheduleofInsuredFinancialObligationswithCreditDeteriorationNetClaimLiabilityCeded
/ us-gaap_InternalCreditAssessmentAxis
= ago_InternalNoninvestmentGradeMember
/ dei_LegalEntityAxis
= us-gaap_ConsolidatedEntityExcludingVariableInterestEntitiesVIEMember
/ us-gaap_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationBySurveillanceCategoryAxis
= ago_InternalNoninvestmentGradeLevel1Member
(18)ago_ScheduleofInsuredFinancialObligationswithCreditDeteriorationNetClaimLiabilityCeded
/ us-gaap_InternalCreditAssessmentAxis
= ago_InternalNoninvestmentGradeMember
/ dei_LegalEntityAxis
= us-gaap_ConsolidatedEntityExcludingVariableInterestEntitiesVIEMember
/ us-gaap_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationBySurveillanceCategoryAxis
= ago_InternalNoninvestmentGradeLevel1Member
Deferred premium revenue    
Gross 402us-gaap_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationStatementOfFinancialPositionUnearnedPremiumRevenue
/ us-gaap_InternalCreditAssessmentAxis
= ago_InternalNoninvestmentGradeMember
/ dei_LegalEntityAxis
= us-gaap_ConsolidatedEntityExcludingVariableInterestEntitiesVIEMember
/ us-gaap_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationBySurveillanceCategoryAxis
= ago_InternalNoninvestmentGradeLevel1Member
378us-gaap_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationStatementOfFinancialPositionUnearnedPremiumRevenue
/ us-gaap_InternalCreditAssessmentAxis
= ago_InternalNoninvestmentGradeMember
/ dei_LegalEntityAxis
= us-gaap_ConsolidatedEntityExcludingVariableInterestEntitiesVIEMember
/ us-gaap_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationBySurveillanceCategoryAxis
= ago_InternalNoninvestmentGradeLevel1Member
Ceded (75)ago_ScheduleofInsuredFinancialObligationswithCreditDeteriorationStatementofFinancialPositionUnearnedPremiumRevenueCeded
/ us-gaap_InternalCreditAssessmentAxis
= ago_InternalNoninvestmentGradeMember
/ dei_LegalEntityAxis
= us-gaap_ConsolidatedEntityExcludingVariableInterestEntitiesVIEMember
/ us-gaap_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationBySurveillanceCategoryAxis
= ago_InternalNoninvestmentGradeLevel1Member
(70)ago_ScheduleofInsuredFinancialObligationswithCreditDeteriorationStatementofFinancialPositionUnearnedPremiumRevenueCeded
/ us-gaap_InternalCreditAssessmentAxis
= ago_InternalNoninvestmentGradeMember
/ dei_LegalEntityAxis
= us-gaap_ConsolidatedEntityExcludingVariableInterestEntitiesVIEMember
/ us-gaap_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationBySurveillanceCategoryAxis
= ago_InternalNoninvestmentGradeLevel1Member
Reserves (salvage)    
Gross (10)us-gaap_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationStatementOfFinancialPositionClaimLiability
/ us-gaap_InternalCreditAssessmentAxis
= ago_InternalNoninvestmentGradeMember
/ dei_LegalEntityAxis
= us-gaap_ConsolidatedEntityExcludingVariableInterestEntitiesVIEMember
/ us-gaap_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationBySurveillanceCategoryAxis
= ago_InternalNoninvestmentGradeLevel1Member
[4] (42)us-gaap_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationStatementOfFinancialPositionClaimLiability
/ us-gaap_InternalCreditAssessmentAxis
= ago_InternalNoninvestmentGradeMember
/ dei_LegalEntityAxis
= us-gaap_ConsolidatedEntityExcludingVariableInterestEntitiesVIEMember
/ us-gaap_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationBySurveillanceCategoryAxis
= ago_InternalNoninvestmentGradeLevel1Member
[4]
Ceded (7)ago_ScheduleofInsuredFinancialObligationswithCreditDeteriorationStatementofFinancialPositionClaimLiabilityCeded
/ us-gaap_InternalCreditAssessmentAxis
= ago_InternalNoninvestmentGradeMember
/ dei_LegalEntityAxis
= us-gaap_ConsolidatedEntityExcludingVariableInterestEntitiesVIEMember
/ us-gaap_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationBySurveillanceCategoryAxis
= ago_InternalNoninvestmentGradeLevel1Member
[4] (5)ago_ScheduleofInsuredFinancialObligationswithCreditDeteriorationStatementofFinancialPositionClaimLiabilityCeded
/ us-gaap_InternalCreditAssessmentAxis
= ago_InternalNoninvestmentGradeMember
/ dei_LegalEntityAxis
= us-gaap_ConsolidatedEntityExcludingVariableInterestEntitiesVIEMember
/ us-gaap_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationBySurveillanceCategoryAxis
= ago_InternalNoninvestmentGradeLevel1Member
[4]
BIG [Member] | Consolidated Entity Excluding Consolidation of Variable Interest Entities (VIE) [Member] | BIG 2 [Member]    
Number of risks    
Ceded (in contracts) (15)ago_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationNumberOfContractsIssuedAndOutstandingCeded
/ us-gaap_InternalCreditAssessmentAxis
= ago_InternalNoninvestmentGradeMember
/ dei_LegalEntityAxis
= us-gaap_ConsolidatedEntityExcludingVariableInterestEntitiesVIEMember
/ us-gaap_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationBySurveillanceCategoryAxis
= ago_InternalNoninvestmentGradeLevel2Member
[1] (15)ago_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationNumberOfContractsIssuedAndOutstandingCeded
/ us-gaap_InternalCreditAssessmentAxis
= ago_InternalNoninvestmentGradeMember
/ dei_LegalEntityAxis
= us-gaap_ConsolidatedEntityExcludingVariableInterestEntitiesVIEMember
/ us-gaap_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationBySurveillanceCategoryAxis
= ago_InternalNoninvestmentGradeLevel2Member
[1]
Total (in contracts) 73us-gaap_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationNumberOfContractsIssuedAndOutstanding
/ us-gaap_InternalCreditAssessmentAxis
= ago_InternalNoninvestmentGradeMember
/ dei_LegalEntityAxis
= us-gaap_ConsolidatedEntityExcludingVariableInterestEntitiesVIEMember
/ us-gaap_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationBySurveillanceCategoryAxis
= ago_InternalNoninvestmentGradeLevel2Member
[1] 75us-gaap_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationNumberOfContractsIssuedAndOutstanding
/ us-gaap_InternalCreditAssessmentAxis
= ago_InternalNoninvestmentGradeMember
/ dei_LegalEntityAxis
= us-gaap_ConsolidatedEntityExcludingVariableInterestEntitiesVIEMember
/ us-gaap_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationBySurveillanceCategoryAxis
= ago_InternalNoninvestmentGradeLevel2Member
[1]
Remaining weighted average contract period    
Gross (in years) 8 years 8 months 12 days 10 years 1 month 6 days
Ceded (in years) 7 years 2 months 12 days 8 years 10 months 24 days
Principal    
Gross 2,701us-gaap_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationInsuredContractualPaymentsOutstandingPrincipal
/ us-gaap_InternalCreditAssessmentAxis
= ago_InternalNoninvestmentGradeMember
/ dei_LegalEntityAxis
= us-gaap_ConsolidatedEntityExcludingVariableInterestEntitiesVIEMember
/ us-gaap_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationBySurveillanceCategoryAxis
= ago_InternalNoninvestmentGradeLevel2Member
2,421us-gaap_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationInsuredContractualPaymentsOutstandingPrincipal
/ us-gaap_InternalCreditAssessmentAxis
= ago_InternalNoninvestmentGradeMember
/ dei_LegalEntityAxis
= us-gaap_ConsolidatedEntityExcludingVariableInterestEntitiesVIEMember
/ us-gaap_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationBySurveillanceCategoryAxis
= ago_InternalNoninvestmentGradeLevel2Member
Ceded (356)ago_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationInsuredContractualPaymentsOutstandingPrincipalCeded
/ us-gaap_InternalCreditAssessmentAxis
= ago_InternalNoninvestmentGradeMember
/ dei_LegalEntityAxis
= us-gaap_ConsolidatedEntityExcludingVariableInterestEntitiesVIEMember
/ us-gaap_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationBySurveillanceCategoryAxis
= ago_InternalNoninvestmentGradeLevel2Member
(286)ago_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationInsuredContractualPaymentsOutstandingPrincipalCeded
/ us-gaap_InternalCreditAssessmentAxis
= ago_InternalNoninvestmentGradeMember
/ dei_LegalEntityAxis
= us-gaap_ConsolidatedEntityExcludingVariableInterestEntitiesVIEMember
/ us-gaap_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationBySurveillanceCategoryAxis
= ago_InternalNoninvestmentGradeLevel2Member
Interest    
Gross 1,262us-gaap_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationInsuredContractualPaymentsOutstandingInterest
/ us-gaap_InternalCreditAssessmentAxis
= ago_InternalNoninvestmentGradeMember
/ dei_LegalEntityAxis
= us-gaap_ConsolidatedEntityExcludingVariableInterestEntitiesVIEMember
/ us-gaap_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationBySurveillanceCategoryAxis
= ago_InternalNoninvestmentGradeLevel2Member
1,274us-gaap_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationInsuredContractualPaymentsOutstandingInterest
/ us-gaap_InternalCreditAssessmentAxis
= ago_InternalNoninvestmentGradeMember
/ dei_LegalEntityAxis
= us-gaap_ConsolidatedEntityExcludingVariableInterestEntitiesVIEMember
/ us-gaap_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationBySurveillanceCategoryAxis
= ago_InternalNoninvestmentGradeLevel2Member
Ceded (127)ago_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationInsuredContractualPaymentsOutstandingInterestCeded
/ us-gaap_InternalCreditAssessmentAxis
= ago_InternalNoninvestmentGradeMember
/ dei_LegalEntityAxis
= us-gaap_ConsolidatedEntityExcludingVariableInterestEntitiesVIEMember
/ us-gaap_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationBySurveillanceCategoryAxis
= ago_InternalNoninvestmentGradeLevel2Member
(121)ago_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationInsuredContractualPaymentsOutstandingInterestCeded
/ us-gaap_InternalCreditAssessmentAxis
= ago_InternalNoninvestmentGradeMember
/ dei_LegalEntityAxis
= us-gaap_ConsolidatedEntityExcludingVariableInterestEntitiesVIEMember
/ us-gaap_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationBySurveillanceCategoryAxis
= ago_InternalNoninvestmentGradeLevel2Member
Total net outstanding exposure    
Gross 3,963us-gaap_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationInsuredContractualPaymentsOutstanding
/ us-gaap_InternalCreditAssessmentAxis
= ago_InternalNoninvestmentGradeMember
/ dei_LegalEntityAxis
= us-gaap_ConsolidatedEntityExcludingVariableInterestEntitiesVIEMember
/ us-gaap_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationBySurveillanceCategoryAxis
= ago_InternalNoninvestmentGradeLevel2Member
[2] 3,695us-gaap_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationInsuredContractualPaymentsOutstanding
/ us-gaap_InternalCreditAssessmentAxis
= ago_InternalNoninvestmentGradeMember
/ dei_LegalEntityAxis
= us-gaap_ConsolidatedEntityExcludingVariableInterestEntitiesVIEMember
/ us-gaap_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationBySurveillanceCategoryAxis
= ago_InternalNoninvestmentGradeLevel2Member
[2]
Ceded (483)ago_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationInsuredContractualPaymentsOutstandingCeded
/ us-gaap_InternalCreditAssessmentAxis
= ago_InternalNoninvestmentGradeMember
/ dei_LegalEntityAxis
= us-gaap_ConsolidatedEntityExcludingVariableInterestEntitiesVIEMember
/ us-gaap_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationBySurveillanceCategoryAxis
= ago_InternalNoninvestmentGradeLevel2Member
[2] (407)ago_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationInsuredContractualPaymentsOutstandingCeded
/ us-gaap_InternalCreditAssessmentAxis
= ago_InternalNoninvestmentGradeMember
/ dei_LegalEntityAxis
= us-gaap_ConsolidatedEntityExcludingVariableInterestEntitiesVIEMember
/ us-gaap_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationBySurveillanceCategoryAxis
= ago_InternalNoninvestmentGradeLevel2Member
[2]
Expected cash outflows (inflows)    
Gross 759us-gaap_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationGrossClaimLiability
/ us-gaap_InternalCreditAssessmentAxis
= ago_InternalNoninvestmentGradeMember
/ dei_LegalEntityAxis
= us-gaap_ConsolidatedEntityExcludingVariableInterestEntitiesVIEMember
/ us-gaap_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationBySurveillanceCategoryAxis
= ago_InternalNoninvestmentGradeLevel2Member
763us-gaap_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationGrossClaimLiability
/ us-gaap_InternalCreditAssessmentAxis
= ago_InternalNoninvestmentGradeMember
/ dei_LegalEntityAxis
= us-gaap_ConsolidatedEntityExcludingVariableInterestEntitiesVIEMember
/ us-gaap_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationBySurveillanceCategoryAxis
= ago_InternalNoninvestmentGradeLevel2Member
Ceded (84)ago_ScheduleofInsuredFinancialObligationswithCreditDeteriorationClaimLiabilityCeded
/ us-gaap_InternalCreditAssessmentAxis
= ago_InternalNoninvestmentGradeMember
/ dei_LegalEntityAxis
= us-gaap_ConsolidatedEntityExcludingVariableInterestEntitiesVIEMember
/ us-gaap_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationBySurveillanceCategoryAxis
= ago_InternalNoninvestmentGradeLevel2Member
(77)ago_ScheduleofInsuredFinancialObligationswithCreditDeteriorationClaimLiabilityCeded
/ us-gaap_InternalCreditAssessmentAxis
= ago_InternalNoninvestmentGradeMember
/ dei_LegalEntityAxis
= us-gaap_ConsolidatedEntityExcludingVariableInterestEntitiesVIEMember
/ us-gaap_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationBySurveillanceCategoryAxis
= ago_InternalNoninvestmentGradeLevel2Member
Total 345us-gaap_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationNetClaimLiability
/ us-gaap_InternalCreditAssessmentAxis
= ago_InternalNoninvestmentGradeMember
/ dei_LegalEntityAxis
= us-gaap_ConsolidatedEntityExcludingVariableInterestEntitiesVIEMember
/ us-gaap_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationBySurveillanceCategoryAxis
= ago_InternalNoninvestmentGradeLevel2Member
392us-gaap_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationNetClaimLiability
/ us-gaap_InternalCreditAssessmentAxis
= ago_InternalNoninvestmentGradeMember
/ dei_LegalEntityAxis
= us-gaap_ConsolidatedEntityExcludingVariableInterestEntitiesVIEMember
/ us-gaap_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationBySurveillanceCategoryAxis
= ago_InternalNoninvestmentGradeLevel2Member
Potential recoveries    
Gross, Undiscounted R&W (47)ago_ScheduleofInsuredFinancialObligationswithCreditDeteriorationGrossClaimLiabilityPotentialRecoveriesUndiscountedRepresentationsandWarrantiesGross
/ us-gaap_InternalCreditAssessmentAxis
= ago_InternalNoninvestmentGradeMember
/ dei_LegalEntityAxis
= us-gaap_ConsolidatedEntityExcludingVariableInterestEntitiesVIEMember
/ us-gaap_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationBySurveillanceCategoryAxis
= ago_InternalNoninvestmentGradeLevel2Member
(48)ago_ScheduleofInsuredFinancialObligationswithCreditDeteriorationGrossClaimLiabilityPotentialRecoveriesUndiscountedRepresentationsandWarrantiesGross
/ us-gaap_InternalCreditAssessmentAxis
= ago_InternalNoninvestmentGradeMember
/ dei_LegalEntityAxis
= us-gaap_ConsolidatedEntityExcludingVariableInterestEntitiesVIEMember
/ us-gaap_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationBySurveillanceCategoryAxis
= ago_InternalNoninvestmentGradeLevel2Member
Ceded, Undiscounted R&W 2ago_ScheduleofInsuredFinancialObligationswithCreditDeteriorationGrossClaimLiabilityPotentialRecoveriesUndiscountedRepresentationsandWarrantiesCeded
/ us-gaap_InternalCreditAssessmentAxis
= ago_InternalNoninvestmentGradeMember
/ dei_LegalEntityAxis
= us-gaap_ConsolidatedEntityExcludingVariableInterestEntitiesVIEMember
/ us-gaap_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationBySurveillanceCategoryAxis
= ago_InternalNoninvestmentGradeLevel2Member
2ago_ScheduleofInsuredFinancialObligationswithCreditDeteriorationGrossClaimLiabilityPotentialRecoveriesUndiscountedRepresentationsandWarrantiesCeded
/ us-gaap_InternalCreditAssessmentAxis
= ago_InternalNoninvestmentGradeMember
/ dei_LegalEntityAxis
= us-gaap_ConsolidatedEntityExcludingVariableInterestEntitiesVIEMember
/ us-gaap_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationBySurveillanceCategoryAxis
= ago_InternalNoninvestmentGradeLevel2Member
Gross, Other (271)ago_ScheduleofInsuredFinancialObligationswithCreditDeteriorationGrossClaimLiabilityPotentialRecoveriesOther
/ us-gaap_InternalCreditAssessmentAxis
= ago_InternalNoninvestmentGradeMember
/ dei_LegalEntityAxis
= us-gaap_ConsolidatedEntityExcludingVariableInterestEntitiesVIEMember
/ us-gaap_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationBySurveillanceCategoryAxis
= ago_InternalNoninvestmentGradeLevel2Member
[3] (206)ago_ScheduleofInsuredFinancialObligationswithCreditDeteriorationGrossClaimLiabilityPotentialRecoveriesOther
/ us-gaap_InternalCreditAssessmentAxis
= ago_InternalNoninvestmentGradeMember
/ dei_LegalEntityAxis
= us-gaap_ConsolidatedEntityExcludingVariableInterestEntitiesVIEMember
/ us-gaap_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationBySurveillanceCategoryAxis
= ago_InternalNoninvestmentGradeLevel2Member
[3]
Ceded, Other 18ago_ScheduleofInsuredFinancialObligationswithCreditDeteriorationGrossClaimLiabilityPotentialRecoveriesCededOther
/ us-gaap_InternalCreditAssessmentAxis
= ago_InternalNoninvestmentGradeMember
/ dei_LegalEntityAxis
= us-gaap_ConsolidatedEntityExcludingVariableInterestEntitiesVIEMember
/ us-gaap_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationBySurveillanceCategoryAxis
= ago_InternalNoninvestmentGradeLevel2Member
[3] 5ago_ScheduleofInsuredFinancialObligationswithCreditDeteriorationGrossClaimLiabilityPotentialRecoveriesCededOther
/ us-gaap_InternalCreditAssessmentAxis
= ago_InternalNoninvestmentGradeMember
/ dei_LegalEntityAxis
= us-gaap_ConsolidatedEntityExcludingVariableInterestEntitiesVIEMember
/ us-gaap_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationBySurveillanceCategoryAxis
= ago_InternalNoninvestmentGradeLevel2Member
[3]
Ceded 20ago_ScheduleofInsuredFinancialObligationswithCreditDeteriorationReductionsofGrossClaimLiabilityPotentialRecoveriesCeded
/ us-gaap_InternalCreditAssessmentAxis
= ago_InternalNoninvestmentGradeMember
/ dei_LegalEntityAxis
= us-gaap_ConsolidatedEntityExcludingVariableInterestEntitiesVIEMember
/ us-gaap_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationBySurveillanceCategoryAxis
= ago_InternalNoninvestmentGradeLevel2Member
7ago_ScheduleofInsuredFinancialObligationswithCreditDeteriorationReductionsofGrossClaimLiabilityPotentialRecoveriesCeded
/ us-gaap_InternalCreditAssessmentAxis
= ago_InternalNoninvestmentGradeMember
/ dei_LegalEntityAxis
= us-gaap_ConsolidatedEntityExcludingVariableInterestEntitiesVIEMember
/ us-gaap_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationBySurveillanceCategoryAxis
= ago_InternalNoninvestmentGradeLevel2Member
Gross (318)us-gaap_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationReductionsOfGrossClaimLiabilityGrossPotentialRecoveries
/ us-gaap_InternalCreditAssessmentAxis
= ago_InternalNoninvestmentGradeMember
/ dei_LegalEntityAxis
= us-gaap_ConsolidatedEntityExcludingVariableInterestEntitiesVIEMember
/ us-gaap_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationBySurveillanceCategoryAxis
= ago_InternalNoninvestmentGradeLevel2Member
(254)us-gaap_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationReductionsOfGrossClaimLiabilityGrossPotentialRecoveries
/ us-gaap_InternalCreditAssessmentAxis
= ago_InternalNoninvestmentGradeMember
/ dei_LegalEntityAxis
= us-gaap_ConsolidatedEntityExcludingVariableInterestEntitiesVIEMember
/ us-gaap_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationBySurveillanceCategoryAxis
= ago_InternalNoninvestmentGradeLevel2Member
Subtotal    
Gross 441ago_ScheduleofInsuredFinancialObligationswithCreditDeteriorationNetClaimLiabilityUndiscountedDirectAssumed
/ us-gaap_InternalCreditAssessmentAxis
= ago_InternalNoninvestmentGradeMember
/ dei_LegalEntityAxis
= us-gaap_ConsolidatedEntityExcludingVariableInterestEntitiesVIEMember
/ us-gaap_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationBySurveillanceCategoryAxis
= ago_InternalNoninvestmentGradeLevel2Member
509ago_ScheduleofInsuredFinancialObligationswithCreditDeteriorationNetClaimLiabilityUndiscountedDirectAssumed
/ us-gaap_InternalCreditAssessmentAxis
= ago_InternalNoninvestmentGradeMember
/ dei_LegalEntityAxis
= us-gaap_ConsolidatedEntityExcludingVariableInterestEntitiesVIEMember
/ us-gaap_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationBySurveillanceCategoryAxis
= ago_InternalNoninvestmentGradeLevel2Member
Ceded (64)ago_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationGrossClaimLiabilityCeded
/ us-gaap_InternalCreditAssessmentAxis
= ago_InternalNoninvestmentGradeMember
/ dei_LegalEntityAxis
= us-gaap_ConsolidatedEntityExcludingVariableInterestEntitiesVIEMember
/ us-gaap_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationBySurveillanceCategoryAxis
= ago_InternalNoninvestmentGradeLevel2Member
(70)ago_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationGrossClaimLiabilityCeded
/ us-gaap_InternalCreditAssessmentAxis
= ago_InternalNoninvestmentGradeMember
/ dei_LegalEntityAxis
= us-gaap_ConsolidatedEntityExcludingVariableInterestEntitiesVIEMember
/ us-gaap_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationBySurveillanceCategoryAxis
= ago_InternalNoninvestmentGradeLevel2Member
Discount    
Gross (96)us-gaap_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationReductionsOfGrossClaimLiabilityNetDiscount
/ us-gaap_InternalCreditAssessmentAxis
= ago_InternalNoninvestmentGradeMember
/ dei_LegalEntityAxis
= us-gaap_ConsolidatedEntityExcludingVariableInterestEntitiesVIEMember
/ us-gaap_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationBySurveillanceCategoryAxis
= ago_InternalNoninvestmentGradeLevel2Member
(117)us-gaap_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationReductionsOfGrossClaimLiabilityNetDiscount
/ us-gaap_InternalCreditAssessmentAxis
= ago_InternalNoninvestmentGradeMember
/ dei_LegalEntityAxis
= us-gaap_ConsolidatedEntityExcludingVariableInterestEntitiesVIEMember
/ us-gaap_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationBySurveillanceCategoryAxis
= ago_InternalNoninvestmentGradeLevel2Member
Ceded 10ago_ScheduleofInsuredFinancialObligationswithCreditDeteriorationReductionsofGrossClaimLiabilityCededNetDiscount
/ us-gaap_InternalCreditAssessmentAxis
= ago_InternalNoninvestmentGradeMember
/ dei_LegalEntityAxis
= us-gaap_ConsolidatedEntityExcludingVariableInterestEntitiesVIEMember
/ us-gaap_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationBySurveillanceCategoryAxis
= ago_InternalNoninvestmentGradeLevel2Member
11ago_ScheduleofInsuredFinancialObligationswithCreditDeteriorationReductionsofGrossClaimLiabilityCededNetDiscount
/ us-gaap_InternalCreditAssessmentAxis
= ago_InternalNoninvestmentGradeMember
/ dei_LegalEntityAxis
= us-gaap_ConsolidatedEntityExcludingVariableInterestEntitiesVIEMember
/ us-gaap_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationBySurveillanceCategoryAxis
= ago_InternalNoninvestmentGradeLevel2Member
Present value of expected cash flows    
Ceded (54)ago_ScheduleofInsuredFinancialObligationswithCreditDeteriorationNetClaimLiabilityCeded
/ us-gaap_InternalCreditAssessmentAxis
= ago_InternalNoninvestmentGradeMember
/ dei_LegalEntityAxis
= us-gaap_ConsolidatedEntityExcludingVariableInterestEntitiesVIEMember
/ us-gaap_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationBySurveillanceCategoryAxis
= ago_InternalNoninvestmentGradeLevel2Member
(59)ago_ScheduleofInsuredFinancialObligationswithCreditDeteriorationNetClaimLiabilityCeded
/ us-gaap_InternalCreditAssessmentAxis
= ago_InternalNoninvestmentGradeMember
/ dei_LegalEntityAxis
= us-gaap_ConsolidatedEntityExcludingVariableInterestEntitiesVIEMember
/ us-gaap_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationBySurveillanceCategoryAxis
= ago_InternalNoninvestmentGradeLevel2Member
Deferred premium revenue    
Gross 104us-gaap_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationStatementOfFinancialPositionUnearnedPremiumRevenue
/ us-gaap_InternalCreditAssessmentAxis
= ago_InternalNoninvestmentGradeMember
/ dei_LegalEntityAxis
= us-gaap_ConsolidatedEntityExcludingVariableInterestEntitiesVIEMember
/ us-gaap_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationBySurveillanceCategoryAxis
= ago_InternalNoninvestmentGradeLevel2Member
119us-gaap_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationStatementOfFinancialPositionUnearnedPremiumRevenue
/ us-gaap_InternalCreditAssessmentAxis
= ago_InternalNoninvestmentGradeMember
/ dei_LegalEntityAxis
= us-gaap_ConsolidatedEntityExcludingVariableInterestEntitiesVIEMember
/ us-gaap_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationBySurveillanceCategoryAxis
= ago_InternalNoninvestmentGradeLevel2Member
Ceded (6)ago_ScheduleofInsuredFinancialObligationswithCreditDeteriorationStatementofFinancialPositionUnearnedPremiumRevenueCeded
/ us-gaap_InternalCreditAssessmentAxis
= ago_InternalNoninvestmentGradeMember
/ dei_LegalEntityAxis
= us-gaap_ConsolidatedEntityExcludingVariableInterestEntitiesVIEMember
/ us-gaap_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationBySurveillanceCategoryAxis
= ago_InternalNoninvestmentGradeLevel2Member
(6)ago_ScheduleofInsuredFinancialObligationswithCreditDeteriorationStatementofFinancialPositionUnearnedPremiumRevenueCeded
/ us-gaap_InternalCreditAssessmentAxis
= ago_InternalNoninvestmentGradeMember
/ dei_LegalEntityAxis
= us-gaap_ConsolidatedEntityExcludingVariableInterestEntitiesVIEMember
/ us-gaap_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationBySurveillanceCategoryAxis
= ago_InternalNoninvestmentGradeLevel2Member
Reserves (salvage)    
Gross 269us-gaap_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationStatementOfFinancialPositionClaimLiability
/ us-gaap_InternalCreditAssessmentAxis
= ago_InternalNoninvestmentGradeMember
/ dei_LegalEntityAxis
= us-gaap_ConsolidatedEntityExcludingVariableInterestEntitiesVIEMember
/ us-gaap_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationBySurveillanceCategoryAxis
= ago_InternalNoninvestmentGradeLevel2Member
[4] 278us-gaap_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationStatementOfFinancialPositionClaimLiability
/ us-gaap_InternalCreditAssessmentAxis
= ago_InternalNoninvestmentGradeMember
/ dei_LegalEntityAxis
= us-gaap_ConsolidatedEntityExcludingVariableInterestEntitiesVIEMember
/ us-gaap_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationBySurveillanceCategoryAxis
= ago_InternalNoninvestmentGradeLevel2Member
[4]
Ceded (52)ago_ScheduleofInsuredFinancialObligationswithCreditDeteriorationStatementofFinancialPositionClaimLiabilityCeded
/ us-gaap_InternalCreditAssessmentAxis
= ago_InternalNoninvestmentGradeMember
/ dei_LegalEntityAxis
= us-gaap_ConsolidatedEntityExcludingVariableInterestEntitiesVIEMember
/ us-gaap_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationBySurveillanceCategoryAxis
= ago_InternalNoninvestmentGradeLevel2Member
[4] (53)ago_ScheduleofInsuredFinancialObligationswithCreditDeteriorationStatementofFinancialPositionClaimLiabilityCeded
/ us-gaap_InternalCreditAssessmentAxis
= ago_InternalNoninvestmentGradeMember
/ dei_LegalEntityAxis
= us-gaap_ConsolidatedEntityExcludingVariableInterestEntitiesVIEMember
/ us-gaap_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationBySurveillanceCategoryAxis
= ago_InternalNoninvestmentGradeLevel2Member
[4]
BIG [Member] | Consolidated Entity Excluding Consolidation of Variable Interest Entities (VIE) [Member] | BIG 3 [Member]    
Number of risks    
Ceded (in contracts) (38)ago_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationNumberOfContractsIssuedAndOutstandingCeded
/ us-gaap_InternalCreditAssessmentAxis
= ago_InternalNoninvestmentGradeMember
/ dei_LegalEntityAxis
= us-gaap_ConsolidatedEntityExcludingVariableInterestEntitiesVIEMember
/ us-gaap_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationBySurveillanceCategoryAxis
= ago_InternalNoninvestmentGradeLevel3Member
[1] (38)ago_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationNumberOfContractsIssuedAndOutstandingCeded
/ us-gaap_InternalCreditAssessmentAxis
= ago_InternalNoninvestmentGradeMember
/ dei_LegalEntityAxis
= us-gaap_ConsolidatedEntityExcludingVariableInterestEntitiesVIEMember
/ us-gaap_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationBySurveillanceCategoryAxis
= ago_InternalNoninvestmentGradeLevel3Member
[1]
Total (in contracts) 119us-gaap_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationNumberOfContractsIssuedAndOutstanding
/ us-gaap_InternalCreditAssessmentAxis
= ago_InternalNoninvestmentGradeMember
/ dei_LegalEntityAxis
= us-gaap_ConsolidatedEntityExcludingVariableInterestEntitiesVIEMember
/ us-gaap_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationBySurveillanceCategoryAxis
= ago_InternalNoninvestmentGradeLevel3Member
[1] 119us-gaap_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationNumberOfContractsIssuedAndOutstanding
/ us-gaap_InternalCreditAssessmentAxis
= ago_InternalNoninvestmentGradeMember
/ dei_LegalEntityAxis
= us-gaap_ConsolidatedEntityExcludingVariableInterestEntitiesVIEMember
/ us-gaap_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationBySurveillanceCategoryAxis
= ago_InternalNoninvestmentGradeLevel3Member
[1]
Remaining weighted average contract period    
Gross (in years) 9 years 10 months 24 days 9 years 7 months 6 days
Ceded (in years) 7 years 3 months 18 days 6 years 10 months 24 days
Principal    
Gross 2,847us-gaap_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationInsuredContractualPaymentsOutstandingPrincipal
/ us-gaap_InternalCreditAssessmentAxis
= ago_InternalNoninvestmentGradeMember
/ dei_LegalEntityAxis
= us-gaap_ConsolidatedEntityExcludingVariableInterestEntitiesVIEMember
/ us-gaap_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationBySurveillanceCategoryAxis
= ago_InternalNoninvestmentGradeLevel3Member
3,067us-gaap_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationInsuredContractualPaymentsOutstandingPrincipal
/ us-gaap_InternalCreditAssessmentAxis
= ago_InternalNoninvestmentGradeMember
/ dei_LegalEntityAxis
= us-gaap_ConsolidatedEntityExcludingVariableInterestEntitiesVIEMember
/ us-gaap_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationBySurveillanceCategoryAxis
= ago_InternalNoninvestmentGradeLevel3Member
Ceded (142)ago_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationInsuredContractualPaymentsOutstandingPrincipalCeded
/ us-gaap_InternalCreditAssessmentAxis
= ago_InternalNoninvestmentGradeMember
/ dei_LegalEntityAxis
= us-gaap_ConsolidatedEntityExcludingVariableInterestEntitiesVIEMember
/ us-gaap_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationBySurveillanceCategoryAxis
= ago_InternalNoninvestmentGradeLevel3Member
(175)ago_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationInsuredContractualPaymentsOutstandingPrincipalCeded
/ us-gaap_InternalCreditAssessmentAxis
= ago_InternalNoninvestmentGradeMember
/ dei_LegalEntityAxis
= us-gaap_ConsolidatedEntityExcludingVariableInterestEntitiesVIEMember
/ us-gaap_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationBySurveillanceCategoryAxis
= ago_InternalNoninvestmentGradeLevel3Member
Interest    
Gross 1,013us-gaap_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationInsuredContractualPaymentsOutstandingInterest
/ us-gaap_InternalCreditAssessmentAxis
= ago_InternalNoninvestmentGradeMember
/ dei_LegalEntityAxis
= us-gaap_ConsolidatedEntityExcludingVariableInterestEntitiesVIEMember
/ us-gaap_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationBySurveillanceCategoryAxis
= ago_InternalNoninvestmentGradeLevel3Member
1,034us-gaap_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationInsuredContractualPaymentsOutstandingInterest
/ us-gaap_InternalCreditAssessmentAxis
= ago_InternalNoninvestmentGradeMember
/ dei_LegalEntityAxis
= us-gaap_ConsolidatedEntityExcludingVariableInterestEntitiesVIEMember
/ us-gaap_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationBySurveillanceCategoryAxis
= ago_InternalNoninvestmentGradeLevel3Member
Ceded (45)ago_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationInsuredContractualPaymentsOutstandingInterestCeded
/ us-gaap_InternalCreditAssessmentAxis
= ago_InternalNoninvestmentGradeMember
/ dei_LegalEntityAxis
= us-gaap_ConsolidatedEntityExcludingVariableInterestEntitiesVIEMember
/ us-gaap_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationBySurveillanceCategoryAxis
= ago_InternalNoninvestmentGradeLevel3Member
(48)ago_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationInsuredContractualPaymentsOutstandingInterestCeded
/ us-gaap_InternalCreditAssessmentAxis
= ago_InternalNoninvestmentGradeMember
/ dei_LegalEntityAxis
= us-gaap_ConsolidatedEntityExcludingVariableInterestEntitiesVIEMember
/ us-gaap_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationBySurveillanceCategoryAxis
= ago_InternalNoninvestmentGradeLevel3Member
Total net outstanding exposure    
Gross 3,860us-gaap_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationInsuredContractualPaymentsOutstanding
/ us-gaap_InternalCreditAssessmentAxis
= ago_InternalNoninvestmentGradeMember
/ dei_LegalEntityAxis
= us-gaap_ConsolidatedEntityExcludingVariableInterestEntitiesVIEMember
/ us-gaap_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationBySurveillanceCategoryAxis
= ago_InternalNoninvestmentGradeLevel3Member
[2] 4,101us-gaap_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationInsuredContractualPaymentsOutstanding
/ us-gaap_InternalCreditAssessmentAxis
= ago_InternalNoninvestmentGradeMember
/ dei_LegalEntityAxis
= us-gaap_ConsolidatedEntityExcludingVariableInterestEntitiesVIEMember
/ us-gaap_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationBySurveillanceCategoryAxis
= ago_InternalNoninvestmentGradeLevel3Member
[2]
Ceded (187)ago_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationInsuredContractualPaymentsOutstandingCeded
/ us-gaap_InternalCreditAssessmentAxis
= ago_InternalNoninvestmentGradeMember
/ dei_LegalEntityAxis
= us-gaap_ConsolidatedEntityExcludingVariableInterestEntitiesVIEMember
/ us-gaap_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationBySurveillanceCategoryAxis
= ago_InternalNoninvestmentGradeLevel3Member
[2] (223)ago_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationInsuredContractualPaymentsOutstandingCeded
/ us-gaap_InternalCreditAssessmentAxis
= ago_InternalNoninvestmentGradeMember
/ dei_LegalEntityAxis
= us-gaap_ConsolidatedEntityExcludingVariableInterestEntitiesVIEMember
/ us-gaap_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationBySurveillanceCategoryAxis
= ago_InternalNoninvestmentGradeLevel3Member
[2]
Expected cash outflows (inflows)    
Gross 1,721us-gaap_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationGrossClaimLiability
/ us-gaap_InternalCreditAssessmentAxis
= ago_InternalNoninvestmentGradeMember
/ dei_LegalEntityAxis
= us-gaap_ConsolidatedEntityExcludingVariableInterestEntitiesVIEMember
/ us-gaap_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationBySurveillanceCategoryAxis
= ago_InternalNoninvestmentGradeLevel3Member
1,716us-gaap_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationGrossClaimLiability
/ us-gaap_InternalCreditAssessmentAxis
= ago_InternalNoninvestmentGradeMember
/ dei_LegalEntityAxis
= us-gaap_ConsolidatedEntityExcludingVariableInterestEntitiesVIEMember
/ us-gaap_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationBySurveillanceCategoryAxis
= ago_InternalNoninvestmentGradeLevel3Member
Ceded (91)ago_ScheduleofInsuredFinancialObligationswithCreditDeteriorationClaimLiabilityCeded
/ us-gaap_InternalCreditAssessmentAxis
= ago_InternalNoninvestmentGradeMember
/ dei_LegalEntityAxis
= us-gaap_ConsolidatedEntityExcludingVariableInterestEntitiesVIEMember
/ us-gaap_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationBySurveillanceCategoryAxis
= ago_InternalNoninvestmentGradeLevel3Member
(75)ago_ScheduleofInsuredFinancialObligationswithCreditDeteriorationClaimLiabilityCeded
/ us-gaap_InternalCreditAssessmentAxis
= ago_InternalNoninvestmentGradeMember
/ dei_LegalEntityAxis
= us-gaap_ConsolidatedEntityExcludingVariableInterestEntitiesVIEMember
/ us-gaap_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationBySurveillanceCategoryAxis
= ago_InternalNoninvestmentGradeLevel3Member
Total 803us-gaap_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationNetClaimLiability
/ us-gaap_InternalCreditAssessmentAxis
= ago_InternalNoninvestmentGradeMember
/ dei_LegalEntityAxis
= us-gaap_ConsolidatedEntityExcludingVariableInterestEntitiesVIEMember
/ us-gaap_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationBySurveillanceCategoryAxis
= ago_InternalNoninvestmentGradeLevel3Member
788us-gaap_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationNetClaimLiability
/ us-gaap_InternalCreditAssessmentAxis
= ago_InternalNoninvestmentGradeMember
/ dei_LegalEntityAxis
= us-gaap_ConsolidatedEntityExcludingVariableInterestEntitiesVIEMember
/ us-gaap_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationBySurveillanceCategoryAxis
= ago_InternalNoninvestmentGradeLevel3Member
Potential recoveries    
Gross, Undiscounted R&W (133)ago_ScheduleofInsuredFinancialObligationswithCreditDeteriorationGrossClaimLiabilityPotentialRecoveriesUndiscountedRepresentationsandWarrantiesGross
/ us-gaap_InternalCreditAssessmentAxis
= ago_InternalNoninvestmentGradeMember
/ dei_LegalEntityAxis
= us-gaap_ConsolidatedEntityExcludingVariableInterestEntitiesVIEMember
/ us-gaap_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationBySurveillanceCategoryAxis
= ago_InternalNoninvestmentGradeLevel3Member
(171)ago_ScheduleofInsuredFinancialObligationswithCreditDeteriorationGrossClaimLiabilityPotentialRecoveriesUndiscountedRepresentationsandWarrantiesGross
/ us-gaap_InternalCreditAssessmentAxis
= ago_InternalNoninvestmentGradeMember
/ dei_LegalEntityAxis
= us-gaap_ConsolidatedEntityExcludingVariableInterestEntitiesVIEMember
/ us-gaap_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationBySurveillanceCategoryAxis
= ago_InternalNoninvestmentGradeLevel3Member
Ceded, Undiscounted R&W 7ago_ScheduleofInsuredFinancialObligationswithCreditDeteriorationGrossClaimLiabilityPotentialRecoveriesUndiscountedRepresentationsandWarrantiesCeded
/ us-gaap_InternalCreditAssessmentAxis
= ago_InternalNoninvestmentGradeMember
/ dei_LegalEntityAxis
= us-gaap_ConsolidatedEntityExcludingVariableInterestEntitiesVIEMember
/ us-gaap_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationBySurveillanceCategoryAxis
= ago_InternalNoninvestmentGradeLevel3Member
9ago_ScheduleofInsuredFinancialObligationswithCreditDeteriorationGrossClaimLiabilityPotentialRecoveriesUndiscountedRepresentationsandWarrantiesCeded
/ us-gaap_InternalCreditAssessmentAxis
= ago_InternalNoninvestmentGradeMember
/ dei_LegalEntityAxis
= us-gaap_ConsolidatedEntityExcludingVariableInterestEntitiesVIEMember
/ us-gaap_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationBySurveillanceCategoryAxis
= ago_InternalNoninvestmentGradeLevel3Member
Gross, Other (449)ago_ScheduleofInsuredFinancialObligationswithCreditDeteriorationGrossClaimLiabilityPotentialRecoveriesOther
/ us-gaap_InternalCreditAssessmentAxis
= ago_InternalNoninvestmentGradeMember
/ dei_LegalEntityAxis
= us-gaap_ConsolidatedEntityExcludingVariableInterestEntitiesVIEMember
/ us-gaap_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationBySurveillanceCategoryAxis
= ago_InternalNoninvestmentGradeLevel3Member
[3] (404)ago_ScheduleofInsuredFinancialObligationswithCreditDeteriorationGrossClaimLiabilityPotentialRecoveriesOther
/ us-gaap_InternalCreditAssessmentAxis
= ago_InternalNoninvestmentGradeMember
/ dei_LegalEntityAxis
= us-gaap_ConsolidatedEntityExcludingVariableInterestEntitiesVIEMember
/ us-gaap_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationBySurveillanceCategoryAxis
= ago_InternalNoninvestmentGradeLevel3Member
[3]
Ceded, Other 47ago_ScheduleofInsuredFinancialObligationswithCreditDeteriorationGrossClaimLiabilityPotentialRecoveriesCededOther
/ us-gaap_InternalCreditAssessmentAxis
= ago_InternalNoninvestmentGradeMember
/ dei_LegalEntityAxis
= us-gaap_ConsolidatedEntityExcludingVariableInterestEntitiesVIEMember
/ us-gaap_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationBySurveillanceCategoryAxis
= ago_InternalNoninvestmentGradeLevel3Member
[3] 30ago_ScheduleofInsuredFinancialObligationswithCreditDeteriorationGrossClaimLiabilityPotentialRecoveriesCededOther
/ us-gaap_InternalCreditAssessmentAxis
= ago_InternalNoninvestmentGradeMember
/ dei_LegalEntityAxis
= us-gaap_ConsolidatedEntityExcludingVariableInterestEntitiesVIEMember
/ us-gaap_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationBySurveillanceCategoryAxis
= ago_InternalNoninvestmentGradeLevel3Member
[3]
Ceded 54ago_ScheduleofInsuredFinancialObligationswithCreditDeteriorationReductionsofGrossClaimLiabilityPotentialRecoveriesCeded
/ us-gaap_InternalCreditAssessmentAxis
= ago_InternalNoninvestmentGradeMember
/ dei_LegalEntityAxis
= us-gaap_ConsolidatedEntityExcludingVariableInterestEntitiesVIEMember
/ us-gaap_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationBySurveillanceCategoryAxis
= ago_InternalNoninvestmentGradeLevel3Member
39ago_ScheduleofInsuredFinancialObligationswithCreditDeteriorationReductionsofGrossClaimLiabilityPotentialRecoveriesCeded
/ us-gaap_InternalCreditAssessmentAxis
= ago_InternalNoninvestmentGradeMember
/ dei_LegalEntityAxis
= us-gaap_ConsolidatedEntityExcludingVariableInterestEntitiesVIEMember
/ us-gaap_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationBySurveillanceCategoryAxis
= ago_InternalNoninvestmentGradeLevel3Member
Gross (582)us-gaap_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationReductionsOfGrossClaimLiabilityGrossPotentialRecoveries
/ us-gaap_InternalCreditAssessmentAxis
= ago_InternalNoninvestmentGradeMember
/ dei_LegalEntityAxis
= us-gaap_ConsolidatedEntityExcludingVariableInterestEntitiesVIEMember
/ us-gaap_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationBySurveillanceCategoryAxis
= ago_InternalNoninvestmentGradeLevel3Member
(575)us-gaap_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationReductionsOfGrossClaimLiabilityGrossPotentialRecoveries
/ us-gaap_InternalCreditAssessmentAxis
= ago_InternalNoninvestmentGradeMember
/ dei_LegalEntityAxis
= us-gaap_ConsolidatedEntityExcludingVariableInterestEntitiesVIEMember
/ us-gaap_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationBySurveillanceCategoryAxis
= ago_InternalNoninvestmentGradeLevel3Member
Subtotal    
Gross 1,139ago_ScheduleofInsuredFinancialObligationswithCreditDeteriorationNetClaimLiabilityUndiscountedDirectAssumed
/ us-gaap_InternalCreditAssessmentAxis
= ago_InternalNoninvestmentGradeMember
/ dei_LegalEntityAxis
= us-gaap_ConsolidatedEntityExcludingVariableInterestEntitiesVIEMember
/ us-gaap_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationBySurveillanceCategoryAxis
= ago_InternalNoninvestmentGradeLevel3Member
1,141ago_ScheduleofInsuredFinancialObligationswithCreditDeteriorationNetClaimLiabilityUndiscountedDirectAssumed
/ us-gaap_InternalCreditAssessmentAxis
= ago_InternalNoninvestmentGradeMember
/ dei_LegalEntityAxis
= us-gaap_ConsolidatedEntityExcludingVariableInterestEntitiesVIEMember
/ us-gaap_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationBySurveillanceCategoryAxis
= ago_InternalNoninvestmentGradeLevel3Member
Ceded (37)ago_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationGrossClaimLiabilityCeded
/ us-gaap_InternalCreditAssessmentAxis
= ago_InternalNoninvestmentGradeMember
/ dei_LegalEntityAxis
= us-gaap_ConsolidatedEntityExcludingVariableInterestEntitiesVIEMember
/ us-gaap_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationBySurveillanceCategoryAxis
= ago_InternalNoninvestmentGradeLevel3Member
(36)ago_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationGrossClaimLiabilityCeded
/ us-gaap_InternalCreditAssessmentAxis
= ago_InternalNoninvestmentGradeMember
/ dei_LegalEntityAxis
= us-gaap_ConsolidatedEntityExcludingVariableInterestEntitiesVIEMember
/ us-gaap_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationBySurveillanceCategoryAxis
= ago_InternalNoninvestmentGradeLevel3Member
Discount    
Gross (336)us-gaap_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationReductionsOfGrossClaimLiabilityNetDiscount
/ us-gaap_InternalCreditAssessmentAxis
= ago_InternalNoninvestmentGradeMember
/ dei_LegalEntityAxis
= us-gaap_ConsolidatedEntityExcludingVariableInterestEntitiesVIEMember
/ us-gaap_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationBySurveillanceCategoryAxis
= ago_InternalNoninvestmentGradeLevel3Member
(353)us-gaap_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationReductionsOfGrossClaimLiabilityNetDiscount
/ us-gaap_InternalCreditAssessmentAxis
= ago_InternalNoninvestmentGradeMember
/ dei_LegalEntityAxis
= us-gaap_ConsolidatedEntityExcludingVariableInterestEntitiesVIEMember
/ us-gaap_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationBySurveillanceCategoryAxis
= ago_InternalNoninvestmentGradeLevel3Member
Ceded 7ago_ScheduleofInsuredFinancialObligationswithCreditDeteriorationReductionsofGrossClaimLiabilityCededNetDiscount
/ us-gaap_InternalCreditAssessmentAxis
= ago_InternalNoninvestmentGradeMember
/ dei_LegalEntityAxis
= us-gaap_ConsolidatedEntityExcludingVariableInterestEntitiesVIEMember
/ us-gaap_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationBySurveillanceCategoryAxis
= ago_InternalNoninvestmentGradeLevel3Member
9ago_ScheduleofInsuredFinancialObligationswithCreditDeteriorationReductionsofGrossClaimLiabilityCededNetDiscount
/ us-gaap_InternalCreditAssessmentAxis
= ago_InternalNoninvestmentGradeMember
/ dei_LegalEntityAxis
= us-gaap_ConsolidatedEntityExcludingVariableInterestEntitiesVIEMember
/ us-gaap_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationBySurveillanceCategoryAxis
= ago_InternalNoninvestmentGradeLevel3Member
Present value of expected cash flows    
Ceded (30)ago_ScheduleofInsuredFinancialObligationswithCreditDeteriorationNetClaimLiabilityCeded
/ us-gaap_InternalCreditAssessmentAxis
= ago_InternalNoninvestmentGradeMember
/ dei_LegalEntityAxis
= us-gaap_ConsolidatedEntityExcludingVariableInterestEntitiesVIEMember
/ us-gaap_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationBySurveillanceCategoryAxis
= ago_InternalNoninvestmentGradeLevel3Member
(27)ago_ScheduleofInsuredFinancialObligationswithCreditDeteriorationNetClaimLiabilityCeded
/ us-gaap_InternalCreditAssessmentAxis
= ago_InternalNoninvestmentGradeMember
/ dei_LegalEntityAxis
= us-gaap_ConsolidatedEntityExcludingVariableInterestEntitiesVIEMember
/ us-gaap_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationBySurveillanceCategoryAxis
= ago_InternalNoninvestmentGradeLevel3Member
Deferred premium revenue    
Gross 268us-gaap_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationStatementOfFinancialPositionUnearnedPremiumRevenue
/ us-gaap_InternalCreditAssessmentAxis
= ago_InternalNoninvestmentGradeMember
/ dei_LegalEntityAxis
= us-gaap_ConsolidatedEntityExcludingVariableInterestEntitiesVIEMember
/ us-gaap_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationBySurveillanceCategoryAxis
= ago_InternalNoninvestmentGradeLevel3Member
312us-gaap_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationStatementOfFinancialPositionUnearnedPremiumRevenue
/ us-gaap_InternalCreditAssessmentAxis
= ago_InternalNoninvestmentGradeMember
/ dei_LegalEntityAxis
= us-gaap_ConsolidatedEntityExcludingVariableInterestEntitiesVIEMember
/ us-gaap_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationBySurveillanceCategoryAxis
= ago_InternalNoninvestmentGradeLevel3Member
Ceded (25)ago_ScheduleofInsuredFinancialObligationswithCreditDeteriorationStatementofFinancialPositionUnearnedPremiumRevenueCeded
/ us-gaap_InternalCreditAssessmentAxis
= ago_InternalNoninvestmentGradeMember
/ dei_LegalEntityAxis
= us-gaap_ConsolidatedEntityExcludingVariableInterestEntitiesVIEMember
/ us-gaap_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationBySurveillanceCategoryAxis
= ago_InternalNoninvestmentGradeLevel3Member
(33)ago_ScheduleofInsuredFinancialObligationswithCreditDeteriorationStatementofFinancialPositionUnearnedPremiumRevenueCeded
/ us-gaap_InternalCreditAssessmentAxis
= ago_InternalNoninvestmentGradeMember
/ dei_LegalEntityAxis
= us-gaap_ConsolidatedEntityExcludingVariableInterestEntitiesVIEMember
/ us-gaap_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationBySurveillanceCategoryAxis
= ago_InternalNoninvestmentGradeLevel3Member
Reserves (salvage)    
Gross 468us-gaap_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationStatementOfFinancialPositionClaimLiability
/ us-gaap_InternalCreditAssessmentAxis
= ago_InternalNoninvestmentGradeMember
/ dei_LegalEntityAxis
= us-gaap_ConsolidatedEntityExcludingVariableInterestEntitiesVIEMember
/ us-gaap_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationBySurveillanceCategoryAxis
= ago_InternalNoninvestmentGradeLevel3Member
[4] 482us-gaap_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationStatementOfFinancialPositionClaimLiability
/ us-gaap_InternalCreditAssessmentAxis
= ago_InternalNoninvestmentGradeMember
/ dei_LegalEntityAxis
= us-gaap_ConsolidatedEntityExcludingVariableInterestEntitiesVIEMember
/ us-gaap_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationBySurveillanceCategoryAxis
= ago_InternalNoninvestmentGradeLevel3Member
[4]
Ceded (10)ago_ScheduleofInsuredFinancialObligationswithCreditDeteriorationStatementofFinancialPositionClaimLiabilityCeded
/ us-gaap_InternalCreditAssessmentAxis
= ago_InternalNoninvestmentGradeMember
/ dei_LegalEntityAxis
= us-gaap_ConsolidatedEntityExcludingVariableInterestEntitiesVIEMember
/ us-gaap_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationBySurveillanceCategoryAxis
= ago_InternalNoninvestmentGradeLevel3Member
[4] (10)ago_ScheduleofInsuredFinancialObligationswithCreditDeteriorationStatementofFinancialPositionClaimLiabilityCeded
/ us-gaap_InternalCreditAssessmentAxis
= ago_InternalNoninvestmentGradeMember
/ dei_LegalEntityAxis
= us-gaap_ConsolidatedEntityExcludingVariableInterestEntitiesVIEMember
/ us-gaap_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationBySurveillanceCategoryAxis
= ago_InternalNoninvestmentGradeLevel3Member
[4]
BIG [Member] | Variable Interest Entity, Primary Beneficiary [Member]    
Expected cash outflows (inflows)    
Total (343)us-gaap_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationNetClaimLiability
/ us-gaap_InternalCreditAssessmentAxis
= ago_InternalNoninvestmentGradeMember
/ dei_LegalEntityAxis
= us-gaap_VariableInterestEntityPrimaryBeneficiaryAggregatedDisclosureMember
(345)us-gaap_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationNetClaimLiability
/ us-gaap_InternalCreditAssessmentAxis
= ago_InternalNoninvestmentGradeMember
/ dei_LegalEntityAxis
= us-gaap_VariableInterestEntityPrimaryBeneficiaryAggregatedDisclosureMember
Potential recoveries    
Total, Undiscounted R&W 8ago_ScheduleofInsuredFinancialObligationswithCreditDeteriorationReductionsofGrossClaimLiabilityPotentialRecoveriesofRepresentationsandWarrantiesNetUndiscounted
/ us-gaap_InternalCreditAssessmentAxis
= ago_InternalNoninvestmentGradeMember
/ dei_LegalEntityAxis
= us-gaap_VariableInterestEntityPrimaryBeneficiaryAggregatedDisclosureMember
8ago_ScheduleofInsuredFinancialObligationswithCreditDeteriorationReductionsofGrossClaimLiabilityPotentialRecoveriesofRepresentationsandWarrantiesNetUndiscounted
/ us-gaap_InternalCreditAssessmentAxis
= ago_InternalNoninvestmentGradeMember
/ dei_LegalEntityAxis
= us-gaap_VariableInterestEntityPrimaryBeneficiaryAggregatedDisclosureMember
Total, Other 180ago_ScheduleofInsuredFinancialObligationswithCreditDeteriorationReductionsofGrossClaimLiabilityPotentialRecoveriesNetOther
/ us-gaap_InternalCreditAssessmentAxis
= ago_InternalNoninvestmentGradeMember
/ dei_LegalEntityAxis
= us-gaap_VariableInterestEntityPrimaryBeneficiaryAggregatedDisclosureMember
[3] 177ago_ScheduleofInsuredFinancialObligationswithCreditDeteriorationReductionsofGrossClaimLiabilityPotentialRecoveriesNetOther
/ us-gaap_InternalCreditAssessmentAxis
= ago_InternalNoninvestmentGradeMember
/ dei_LegalEntityAxis
= us-gaap_VariableInterestEntityPrimaryBeneficiaryAggregatedDisclosureMember
[3]
Total 188ago_ScheduleofInsuredFinancialObligationswithCreditDeteriorationReductionsofGrossClaimLiabilityNetPotentialRecoveries
/ us-gaap_InternalCreditAssessmentAxis
= ago_InternalNoninvestmentGradeMember
/ dei_LegalEntityAxis
= us-gaap_VariableInterestEntityPrimaryBeneficiaryAggregatedDisclosureMember
185ago_ScheduleofInsuredFinancialObligationswithCreditDeteriorationReductionsofGrossClaimLiabilityNetPotentialRecoveries
/ us-gaap_InternalCreditAssessmentAxis
= ago_InternalNoninvestmentGradeMember
/ dei_LegalEntityAxis
= us-gaap_VariableInterestEntityPrimaryBeneficiaryAggregatedDisclosureMember
Subtotal    
Total (155)ago_ScheduleofInsuredFinancialObligationswithCreditDeteriorationNetClaimLiabilityUndiscountedDirectAssumedNet
/ us-gaap_InternalCreditAssessmentAxis
= ago_InternalNoninvestmentGradeMember
/ dei_LegalEntityAxis
= us-gaap_VariableInterestEntityPrimaryBeneficiaryAggregatedDisclosureMember
(160)ago_ScheduleofInsuredFinancialObligationswithCreditDeteriorationNetClaimLiabilityUndiscountedDirectAssumedNet
/ us-gaap_InternalCreditAssessmentAxis
= ago_InternalNoninvestmentGradeMember
/ dei_LegalEntityAxis
= us-gaap_VariableInterestEntityPrimaryBeneficiaryAggregatedDisclosureMember
Discount    
Total 33ago_ScheduleofInsuredFinancialObligationswithCreditDeteriorationReductionsofGrossClaimLiabilityNetofReinsuranceRecoverableDiscount
/ us-gaap_InternalCreditAssessmentAxis
= ago_InternalNoninvestmentGradeMember
/ dei_LegalEntityAxis
= us-gaap_VariableInterestEntityPrimaryBeneficiaryAggregatedDisclosureMember
34ago_ScheduleofInsuredFinancialObligationswithCreditDeteriorationReductionsofGrossClaimLiabilityNetofReinsuranceRecoverableDiscount
/ us-gaap_InternalCreditAssessmentAxis
= ago_InternalNoninvestmentGradeMember
/ dei_LegalEntityAxis
= us-gaap_VariableInterestEntityPrimaryBeneficiaryAggregatedDisclosureMember
Present value of expected cash flows    
Net expected loss to be paid (122)ago_ScheduleofInsuredFinancialObligationswithCreditDeteriorationNetClaimLiabilityNetofReinsuranceRecoverable
/ us-gaap_InternalCreditAssessmentAxis
= ago_InternalNoninvestmentGradeMember
/ dei_LegalEntityAxis
= us-gaap_VariableInterestEntityPrimaryBeneficiaryAggregatedDisclosureMember
(126)ago_ScheduleofInsuredFinancialObligationswithCreditDeteriorationNetClaimLiabilityNetofReinsuranceRecoverable
/ us-gaap_InternalCreditAssessmentAxis
= ago_InternalNoninvestmentGradeMember
/ dei_LegalEntityAxis
= us-gaap_VariableInterestEntityPrimaryBeneficiaryAggregatedDisclosureMember
Deferred premium revenue    
Total (111)ago_ScheduleofInsuredFinancialObligationswithCreditDeteriorationStatementofFinancialPositionUnearnedPremiumRevenueNetofReinsuranceRecoverable
/ us-gaap_InternalCreditAssessmentAxis
= ago_InternalNoninvestmentGradeMember
/ dei_LegalEntityAxis
= us-gaap_VariableInterestEntityPrimaryBeneficiaryAggregatedDisclosureMember
(116)ago_ScheduleofInsuredFinancialObligationswithCreditDeteriorationStatementofFinancialPositionUnearnedPremiumRevenueNetofReinsuranceRecoverable
/ us-gaap_InternalCreditAssessmentAxis
= ago_InternalNoninvestmentGradeMember
/ dei_LegalEntityAxis
= us-gaap_VariableInterestEntityPrimaryBeneficiaryAggregatedDisclosureMember
Reserves (salvage)    
Total $ (77)ago_LiabilityforUnpaidClaimsandClaimsAdjustmentExpenseNetofReinsuranceandSalvageSubrogationandOtherRecoveries
/ us-gaap_InternalCreditAssessmentAxis
= ago_InternalNoninvestmentGradeMember
/ dei_LegalEntityAxis
= us-gaap_VariableInterestEntityPrimaryBeneficiaryAggregatedDisclosureMember
[4] $ (79)ago_LiabilityforUnpaidClaimsandClaimsAdjustmentExpenseNetofReinsuranceandSalvageSubrogationandOtherRecoveries
/ us-gaap_InternalCreditAssessmentAxis
= ago_InternalNoninvestmentGradeMember
/ dei_LegalEntityAxis
= us-gaap_VariableInterestEntityPrimaryBeneficiaryAggregatedDisclosureMember
[4]
[1] A risk represents the aggregate of the financial guaranty policies that share the same revenue source for purposes of making Debt Service payments. The ceded number of risks represents the number of risks for which the Company ceded a portion of its exposure
[2] Includes BIG amounts related to FG VIEs.
[3] Includes excess spread and draws on HELOCs.
[4] See table “Components of net reserves (salvage).”