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Outstanding Exposure - Narrative (Details) (USD $)
 12 Months Ended 0 Months Ended
Dec. 31, 2014
Jul. 17, 2014
Dec. 31, 2013
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 $ 609,622,000,000ago_InsuredFinancialObligationsInsuredContractualPaymentsOutstandingNet   $ 690,535,000,000ago_InsuredFinancialObligationsInsuredContractualPaymentsOutstandingNet  
Indirect European exposure 11,600,000,000ago_InsuredFinancialObligationsIndirectEconomicExposureOutstandingPrincipalAmountNet      
Net par amount outstanding 403,729,000,000ago_InsuredFinancialObligationsOutstandingPrincipalAmountNet [1]   459,107,000,000ago_InsuredFinancialObligationsOutstandingPrincipalAmountNet [2]  
RMBS [Member]        
Schedule of Insured Financial Obligations [Line Items]        
Minimum principal amount assumed for internal rating 1,000,000ago_InsuredFinancialObligationsMinimumPrincipleAssumedforInternalRating
/ us-gaap_UnderlyingAssetClassAxis
= us-gaap_ResidentialMortgageBackedSecuritiesMember		      
Pooled corporate obligations [Member]        
Schedule of Insured Financial Obligations [Line Items]        
Indirect European exposure 864,000,000ago_InsuredFinancialObligationsIndirectEconomicExposureOutstandingPrincipalAmountNet
/ us-gaap_UnderlyingAssetClassAxis
= ago_PooledCorporateObligationsMember		      
Structured Finance [Member]        
Schedule of Insured Financial Obligations [Line Items]        
Net Debt Service Outstanding 56,010,000,000ago_InsuredFinancialObligationsInsuredContractualPaymentsOutstandingNet
/ us-gaap_UnderlyingAssetClassAxis
= us-gaap_StructuredFinanceMember		   80,524,000,000ago_InsuredFinancialObligationsInsuredContractualPaymentsOutstandingNet
/ us-gaap_UnderlyingAssetClassAxis
= us-gaap_StructuredFinanceMember		  
Net par amount outstanding 50,247,000,000ago_InsuredFinancialObligationsOutstandingPrincipalAmountNet
/ us-gaap_UnderlyingAssetClassAxis
= us-gaap_StructuredFinanceMember		   72,928,000,000ago_InsuredFinancialObligationsOutstandingPrincipalAmountNet
/ us-gaap_UnderlyingAssetClassAxis
= us-gaap_StructuredFinanceMember		  
Public Finance [Member]        
Schedule of Insured Financial Obligations [Line Items]        
Net Debt Service Outstanding 553,612,000,000ago_InsuredFinancialObligationsInsuredContractualPaymentsOutstandingNet
/ us-gaap_UnderlyingAssetClassAxis
= ago_PublicFinanceMember		   610,011,000,000ago_InsuredFinancialObligationsInsuredContractualPaymentsOutstandingNet
/ us-gaap_UnderlyingAssetClassAxis
= ago_PublicFinanceMember		  
Net par amount outstanding 353,482,000,000ago_InsuredFinancialObligationsOutstandingPrincipalAmountNet
/ us-gaap_UnderlyingAssetClassAxis
= ago_PublicFinanceMember		   386,179,000,000ago_InsuredFinancialObligationsOutstandingPrincipalAmountNet
/ 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 18,247,000,000ago_InsuredFinancialObligationsOutstandingPrincipalAmountNet
/ us-gaap_InternalCreditAssessmentAxis
= ago_InternalNoninvestmentGradeMember		   22,538,000,000ago_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 127,000,000ago_InsuredFinancialObligationsInsuredContractualPaymentsOutstandingNet
/ us-gaap_StatementGeographicalAxis
= country_IE
/ us-gaap_UnderlyingAssetClassAxis
= us-gaap_MortgageLoansOnRealEstateMember		      
United Kingdom and Ireland [Member] | Mortgage Loans on Real Estate [Member]        
Schedule of Insured Financial Obligations [Line Items]        
Net Debt Service Outstanding     152,000,000ago_InsuredFinancialObligationsInsuredContractualPaymentsOutstandingNet
/ us-gaap_StatementGeographicalAxis
= ago_UnitedKingdomandIrelandMember
/ us-gaap_UnderlyingAssetClassAxis
= us-gaap_MortgageLoansOnRealEstateMember		  
Select European Countries [Member]        
Schedule of Insured Financial Obligations [Line Items]        
Indirect European exposure 418,000,000ago_InsuredFinancialObligationsIndirectEconomicExposureOutstandingPrincipalAmountNet
/ us-gaap_StatementGeographicalAxis
= ago_SelectEuropeanCountriesMember		      
Greece [Member] | Pooled corporate obligations [Member]        
Schedule of Insured Financial Obligations [Line Items]        
Indirect European exposure 12,000,000ago_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 4,939,000,000ago_InsuredFinancialObligationsOutstandingPrincipalAmountNet
/ us-gaap_StatementGeographicalAxis
= country_PR		   5,441,000,000ago_InsuredFinancialObligationsOutstandingPrincipalAmountNet
/ us-gaap_StatementGeographicalAxis
= country_PR		  
Deposit in reserve account utilized to pay debt service due on bonds   42,000,000ago_RisksandUncertaintiesinEntitysBusinessGeographicExposureContestedDebtPaymentAmount
/ 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 4,900,000,000ago_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 7,797,000,000ago_InsuredFinancialObligationsInsuredContractualPaymentsOutstandingNet
/ us-gaap_InternalCreditAssessmentAxis
= ago_InternalNoninvestmentGradeMember
/ us-gaap_StatementGeographicalAxis
= country_PR		      
Net par amount outstanding 4,670,000,000ago_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 4,700,000,000ago_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 276,000,000us-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 248,000,000us-gaap_OtherCommitment
/ us-gaap_OtherCommitmentsAxis
= ago_CommitmenttoProvideGuaranteesMember
/ us-gaap_UnderlyingAssetClassAxis
= ago_PublicFinanceMember		      
Outstanding commitments to provide guaranties, amount expiring prior to December 31, 2014 $ 124,000,000us-gaap_OtherCommitmentDueInNextTwelveMonths
/ us-gaap_OtherCommitmentsAxis
= ago_CommitmenttoProvideGuaranteesMember
/ us-gaap_UnderlyingAssetClassAxis
= ago_PublicFinanceMember		      
Minimum [Member]        
Schedule of Insured Financial Obligations [Line Items]        
Constant discount rate (as a percent) 4.50%ago_InsuredFinancialObligationsDiscountRate
/ us-gaap_RangeAxis
= us-gaap_MinimumMember		      
Minimum [Member] | Public Finance [Member]        
Schedule of Insured Financial Obligations [Line Items]        
Expiration date for insured financial obligation commitments Jan. 15, 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]        
Schedule of Insured Financial Obligations [Line Items]        
Constant discount rate (as a percent) 5.00%ago_InsuredFinancialObligationsDiscountRate
/ us-gaap_RangeAxis
= us-gaap_MaximumMember		      
Maximum [Member] | Public Finance [Member]        
Schedule of Insured Financial Obligations [Line Items]        
Expiration date for insured financial obligation commitments Feb. 17, 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 December 31, 2014, which are primarily in the BIG category.
[2] Excludes $1.2 billion of loss mitigation securities insured and held by the Company as of December 31, 2013, which are primarily in the BIG category.