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Outstanding Exposure - Narrative (Details) (USD $)
In Millions, unless otherwise specified
 3 Months Ended
Mar. 31, 2015
Dec. 31, 2014
Aug. 14, 2014
Schedule of Insured Financial Obligations [Line Items]      
Probability of paying more claims than being reimbursed (as a percent) 50.00%ago_InsuredFinancialObligationsProbabilityofPayingMoreClaimsthanReimbursedPercentage    
Net Debt Service Outstanding $ 584,659ago_InsuredFinancialObligationsInsuredContractualPaymentsOutstandingNet $ 609,622ago_InsuredFinancialObligationsInsuredContractualPaymentsOutstandingNet  
Indirect European exposure 10,000ago_InsuredFinancialObligationsIndirectEconomicExposureOutstandingPrincipalAmountNet    
Net par amount outstanding 389,099.0ago_InsuredFinancialObligationsOutstandingPrincipalAmountNet [1] 403,729.0ago_InsuredFinancialObligationsOutstandingPrincipalAmountNet [2]  
Pooled corporate obligations [Member]      
Schedule of Insured Financial Obligations [Line Items]      
Indirect European exposure 483ago_InsuredFinancialObligationsIndirectEconomicExposureOutstandingPrincipalAmountNet
/ us-gaap_UnderlyingAssetClassAxis
= ago_PooledCorporateObligationsMember		    
Structured Finance [Member]      
Schedule of Insured Financial Obligations [Line Items]      
Net Debt Service Outstanding 51,300ago_InsuredFinancialObligationsInsuredContractualPaymentsOutstandingNet
/ us-gaap_UnderlyingAssetClassAxis
= us-gaap_StructuredFinanceMember		 56,010ago_InsuredFinancialObligationsInsuredContractualPaymentsOutstandingNet
/ us-gaap_UnderlyingAssetClassAxis
= us-gaap_StructuredFinanceMember		  
Public Finance [Member]      
Schedule of Insured Financial Obligations [Line Items]      
Net Debt Service Outstanding 533,359ago_InsuredFinancialObligationsInsuredContractualPaymentsOutstandingNet
/ us-gaap_UnderlyingAssetClassAxis
= ago_PublicFinanceMember		 553,612ago_InsuredFinancialObligationsInsuredContractualPaymentsOutstandingNet
/ us-gaap_UnderlyingAssetClassAxis
= ago_PublicFinanceMember		  
BIG [Member]      
Schedule of Insured Financial Obligations [Line Items]      
Maximum period of liquidity claims (in years) 1 year    
Net par amount outstanding 17,860.0ago_InsuredFinancialObligationsOutstandingPrincipalAmountNet
/ us-gaap_InternalCreditAssessmentAxis
= ago_InternalNoninvestmentGradeMember		 18,247.0ago_InsuredFinancialObligationsOutstandingPrincipalAmountNet
/ us-gaap_InternalCreditAssessmentAxis
= ago_InternalNoninvestmentGradeMember		  
United Kingdom and Ireland [Member] | Mortgage Loans on Real Estate [Member]      
Schedule of Insured Financial Obligations [Line Items]      
Net Debt Service Outstanding 113ago_InsuredFinancialObligationsInsuredContractualPaymentsOutstandingNet
/ us-gaap_StatementGeographicalAxis
= country_IE
/ us-gaap_UnderlyingAssetClassAxis
= us-gaap_MortgageLoansOnRealEstateMember		 127ago_InsuredFinancialObligationsInsuredContractualPaymentsOutstandingNet
/ us-gaap_StatementGeographicalAxis
= country_IE
/ us-gaap_UnderlyingAssetClassAxis
= us-gaap_MortgageLoansOnRealEstateMember		  
Select European Countries [Member]      
Schedule of Insured Financial Obligations [Line Items]      
Indirect European exposure 355ago_InsuredFinancialObligationsIndirectEconomicExposureOutstandingPrincipalAmountNet
/ us-gaap_StatementGeographicalAxis
= ago_SelectEuropeanCountriesMember		    
Greece [Member] | Pooled corporate obligations [Member]      
Schedule of Insured Financial Obligations [Line Items]      
Indirect European exposure 11ago_InsuredFinancialObligationsIndirectEconomicExposureOutstandingPrincipalAmountNet
/ us-gaap_StatementGeographicalAxis
= country_GR
/ us-gaap_UnderlyingAssetClassAxis
= ago_PooledCorporateObligationsMember		    
Puerto Rico [Member]      
Schedule of Insured Financial Obligations [Line Items]      
Net par amount outstanding 4,940.0ago_InsuredFinancialObligationsOutstandingPrincipalAmountNet
/ us-gaap_StatementGeographicalAxis
= country_PR		 4,939.0ago_InsuredFinancialObligationsOutstandingPrincipalAmountNet
/ us-gaap_StatementGeographicalAxis
= country_PR		  
Percentage of outstanding bonds of PREPA     60.00%ago_RisksandUncertaintiesinEntitysBusinessGeographicExposureBondsHeldinGuaranteeofContestedDebtPercent
/ us-gaap_StatementGeographicalAxis
= country_PR
Puerto Rico [Member] | Public Finance [Member]      
Schedule of Insured Financial Obligations [Line Items]      
Net par amount outstanding 4,900.0ago_InsuredFinancialObligationsOutstandingPrincipalAmountNet
/ us-gaap_StatementGeographicalAxis
= country_PR
/ us-gaap_UnderlyingAssetClassAxis
= ago_PublicFinanceMember		    
Puerto Rico [Member] | BIG [Member]      
Schedule of Insured Financial Obligations [Line Items]      
Net Debt Service Outstanding 7,681ago_InsuredFinancialObligationsInsuredContractualPaymentsOutstandingNet
/ us-gaap_InternalCreditAssessmentAxis
= ago_InternalNoninvestmentGradeMember
/ us-gaap_StatementGeographicalAxis
= country_PR		    
Net par amount outstanding 4,671.0ago_InsuredFinancialObligationsOutstandingPrincipalAmountNet
/ us-gaap_InternalCreditAssessmentAxis
= ago_InternalNoninvestmentGradeMember
/ us-gaap_StatementGeographicalAxis
= country_PR		    
Puerto Rico [Member] | BIG [Member] | Public Finance [Member]      
Schedule of Insured Financial Obligations [Line Items]      
Net par amount outstanding 4,700.0ago_InsuredFinancialObligationsOutstandingPrincipalAmountNet
/ us-gaap_InternalCreditAssessmentAxis
= ago_InternalNoninvestmentGradeMember
/ us-gaap_StatementGeographicalAxis
= country_PR
/ us-gaap_UnderlyingAssetClassAxis
= ago_PublicFinanceMember		    
Commitment to Provide Guarantees [Member] | Structured Finance [Member]      
Schedule of Insured Financial Obligations [Line Items]      
Outstanding commitments to provide guaranties 38us-gaap_OtherCommitment
/ us-gaap_OtherCommitmentsAxis
= ago_CommitmenttoProvideGuaranteesMember
/ us-gaap_UnderlyingAssetClassAxis
= us-gaap_StructuredFinanceMember		    
Commitment to Provide Guarantees [Member] | Public Finance [Member]      
Schedule of Insured Financial Obligations [Line Items]      
Outstanding commitments to provide guaranties 410us-gaap_OtherCommitment
/ us-gaap_OtherCommitmentsAxis
= ago_CommitmenttoProvideGuaranteesMember
/ us-gaap_UnderlyingAssetClassAxis
= ago_PublicFinanceMember		    
Commitments due before expiration date 210ago_OtherCommitmentDuebeforeFilingDate
/ us-gaap_OtherCommitmentsAxis
= ago_CommitmenttoProvideGuaranteesMember
/ us-gaap_UnderlyingAssetClassAxis
= ago_PublicFinanceMember		    
Other commitments, future minimum payments, Remainder of Fiscal Year $ 75us-gaap_OtherCommitmentsFutureMinimumPaymentsRemainderOfFiscalYear
/ us-gaap_OtherCommitmentsAxis
= ago_CommitmenttoProvideGuaranteesMember
/ us-gaap_UnderlyingAssetClassAxis
= ago_PublicFinanceMember		    
Minimum [Member] | Public Finance [Member]      
Schedule of Insured Financial Obligations [Line Items]      
Expiration date for insured financial obligation commitments Apr. 09, 2015    
Minimum [Member] | BIG [Member]      
Schedule of Insured Financial Obligations [Line Items]      
Constant discount rate (as a percent) 4.50%ago_InsuredFinancialObligationsDiscountRate
/ us-gaap_InternalCreditAssessmentAxis
= ago_InternalNoninvestmentGradeMember
/ us-gaap_RangeAxis
= us-gaap_MinimumMember		    
Maximum [Member] | Public Finance [Member]      
Schedule of Insured Financial Obligations [Line Items]      
Expiration date for insured financial obligation commitments Feb. 25, 2017    
Maximum [Member] | BIG [Member]      
Schedule of Insured Financial Obligations [Line Items]      
Constant discount rate (as a percent) 5.00%ago_InsuredFinancialObligationsDiscountRate
/ us-gaap_InternalCreditAssessmentAxis
= ago_InternalNoninvestmentGradeMember
/ us-gaap_RangeAxis
= us-gaap_MaximumMember		    
[1] Excludes $1.3 billion of loss mitigation securities insured and held by the Company as of March 31, 2015, which are primarily in the BIG category.
[2] Excludes $1.3 billion of loss mitigation securities insured and held by the Company as of December 31, 2014, which are primarily in the BIG category.