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Fair Value Measurement - Fair Value Level 3 Rollforward Recurring Basis (Details) (USD $)
In Millions, unless otherwise specified
 3 Months Ended
Mar. 31, 2015
Mar. 31, 2014
FG VIEs' liabilities with recourse, at fair value [Member]    
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]    
Fair value at beginning of period $ (1,277)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationsRecurringBasisLiabilityValue
/ us-gaap_FairValueByLiabilityClassAxis
= ago_FinancialGuarantyVariableInterestLiabilitiesWithRecourseMember		 $ (1,790)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationsRecurringBasisLiabilityValue
/ us-gaap_FairValueByLiabilityClassAxis
= ago_FinancialGuarantyVariableInterestLiabilitiesWithRecourseMember
Net income (loss) 93us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityGainLossIncludedInEarnings
/ us-gaap_FairValueByLiabilityClassAxis
= ago_FinancialGuarantyVariableInterestLiabilitiesWithRecourseMember		 [1],[2] (72)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityGainLossIncludedInEarnings
/ us-gaap_FairValueByLiabilityClassAxis
= ago_FinancialGuarantyVariableInterestLiabilitiesWithRecourseMember		 [1],[2]
Other comprehensive income (loss) 0us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityGainLossIncludedInOtherComprehensiveIncome
/ us-gaap_FairValueByLiabilityClassAxis
= ago_FinancialGuarantyVariableInterestLiabilitiesWithRecourseMember		 [1] 0us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityGainLossIncludedInOtherComprehensiveIncome
/ us-gaap_FairValueByLiabilityClassAxis
= ago_FinancialGuarantyVariableInterestLiabilitiesWithRecourseMember		 [1]
Purchases 0us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityPurchases
/ us-gaap_FairValueByLiabilityClassAxis
= ago_FinancialGuarantyVariableInterestLiabilitiesWithRecourseMember		 0us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityPurchases
/ us-gaap_FairValueByLiabilityClassAxis
= ago_FinancialGuarantyVariableInterestLiabilitiesWithRecourseMember
Settlements 37us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilitySettlements
/ us-gaap_FairValueByLiabilityClassAxis
= ago_FinancialGuarantyVariableInterestLiabilitiesWithRecourseMember		 269us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilitySettlements
/ us-gaap_FairValueByLiabilityClassAxis
= ago_FinancialGuarantyVariableInterestLiabilitiesWithRecourseMember
FG VIE consolidations (131)ago_FairValueMeasuredwithUnobservableInputsReconciliationRecurringBasisLiabilityConsolidation
/ us-gaap_FairValueByLiabilityClassAxis
= ago_FinancialGuarantyVariableInterestLiabilitiesWithRecourseMember		 0ago_FairValueMeasuredwithUnobservableInputsReconciliationRecurringBasisLiabilityConsolidation
/ us-gaap_FairValueByLiabilityClassAxis
= ago_FinancialGuarantyVariableInterestLiabilitiesWithRecourseMember
FG VIE deconsolidations 0ago_FairValueMeasurementwithUnobservableInputsReconciliationRecurringBasisLiabilityDeconsolidation
/ us-gaap_FairValueByLiabilityClassAxis
= ago_FinancialGuarantyVariableInterestLiabilitiesWithRecourseMember		 247ago_FairValueMeasurementwithUnobservableInputsReconciliationRecurringBasisLiabilityDeconsolidation
/ us-gaap_FairValueByLiabilityClassAxis
= ago_FinancialGuarantyVariableInterestLiabilitiesWithRecourseMember
Fair value at end of period (1,278)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationsRecurringBasisLiabilityValue
/ us-gaap_FairValueByLiabilityClassAxis
= ago_FinancialGuarantyVariableInterestLiabilitiesWithRecourseMember		 (1,346)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationsRecurringBasisLiabilityValue
/ us-gaap_FairValueByLiabilityClassAxis
= ago_FinancialGuarantyVariableInterestLiabilitiesWithRecourseMember
Change in unrealized gains/(losses) related to financial instruments held (6)us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisChangeInUnrealizedGainLoss
/ us-gaap_FairValueByLiabilityClassAxis
= ago_FinancialGuarantyVariableInterestLiabilitiesWithRecourseMember		 (28)us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisChangeInUnrealizedGainLoss
/ us-gaap_FairValueByLiabilityClassAxis
= ago_FinancialGuarantyVariableInterestLiabilitiesWithRecourseMember
FG VIEs' liabilities without recourse, at fair value [Member]    
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]    
Fair value at beginning of period (142)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationsRecurringBasisLiabilityValue
/ us-gaap_FairValueByLiabilityClassAxis
= ago_FinancialGuarantyVariableInterestLiabilitiesWithoutRecourseMember		 (1,081)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationsRecurringBasisLiabilityValue
/ us-gaap_FairValueByLiabilityClassAxis
= ago_FinancialGuarantyVariableInterestLiabilitiesWithoutRecourseMember
Net income (loss) (5)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityGainLossIncludedInEarnings
/ us-gaap_FairValueByLiabilityClassAxis
= ago_FinancialGuarantyVariableInterestLiabilitiesWithoutRecourseMember		 [1],[2] (9)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityGainLossIncludedInEarnings
/ us-gaap_FairValueByLiabilityClassAxis
= ago_FinancialGuarantyVariableInterestLiabilitiesWithoutRecourseMember		 [1],[2]
Other comprehensive income (loss) 0us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityGainLossIncludedInOtherComprehensiveIncome
/ us-gaap_FairValueByLiabilityClassAxis
= ago_FinancialGuarantyVariableInterestLiabilitiesWithoutRecourseMember		 [1] 0us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityGainLossIncludedInOtherComprehensiveIncome
/ us-gaap_FairValueByLiabilityClassAxis
= ago_FinancialGuarantyVariableInterestLiabilitiesWithoutRecourseMember		 [1]
Purchases 0us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityPurchases
/ us-gaap_FairValueByLiabilityClassAxis
= ago_FinancialGuarantyVariableInterestLiabilitiesWithoutRecourseMember		 0us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityPurchases
/ us-gaap_FairValueByLiabilityClassAxis
= ago_FinancialGuarantyVariableInterestLiabilitiesWithoutRecourseMember
Settlements 2us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilitySettlements
/ us-gaap_FairValueByLiabilityClassAxis
= ago_FinancialGuarantyVariableInterestLiabilitiesWithoutRecourseMember		 12us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilitySettlements
/ us-gaap_FairValueByLiabilityClassAxis
= ago_FinancialGuarantyVariableInterestLiabilitiesWithoutRecourseMember
FG VIE consolidations 0ago_FairValueMeasuredwithUnobservableInputsReconciliationRecurringBasisLiabilityConsolidation
/ us-gaap_FairValueByLiabilityClassAxis
= ago_FinancialGuarantyVariableInterestLiabilitiesWithoutRecourseMember		 0ago_FairValueMeasuredwithUnobservableInputsReconciliationRecurringBasisLiabilityConsolidation
/ us-gaap_FairValueByLiabilityClassAxis
= ago_FinancialGuarantyVariableInterestLiabilitiesWithoutRecourseMember
FG VIE deconsolidations 0ago_FairValueMeasurementwithUnobservableInputsReconciliationRecurringBasisLiabilityDeconsolidation
/ us-gaap_FairValueByLiabilityClassAxis
= ago_FinancialGuarantyVariableInterestLiabilitiesWithoutRecourseMember		 977ago_FairValueMeasurementwithUnobservableInputsReconciliationRecurringBasisLiabilityDeconsolidation
/ us-gaap_FairValueByLiabilityClassAxis
= ago_FinancialGuarantyVariableInterestLiabilitiesWithoutRecourseMember
Fair value at end of period (145)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationsRecurringBasisLiabilityValue
/ us-gaap_FairValueByLiabilityClassAxis
= ago_FinancialGuarantyVariableInterestLiabilitiesWithoutRecourseMember		 (101)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationsRecurringBasisLiabilityValue
/ us-gaap_FairValueByLiabilityClassAxis
= ago_FinancialGuarantyVariableInterestLiabilitiesWithoutRecourseMember
Change in unrealized gains/(losses) related to financial instruments held (4)us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisChangeInUnrealizedGainLoss
/ us-gaap_FairValueByLiabilityClassAxis
= ago_FinancialGuarantyVariableInterestLiabilitiesWithoutRecourseMember		 (10)us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisChangeInUnrealizedGainLoss
/ us-gaap_FairValueByLiabilityClassAxis
= ago_FinancialGuarantyVariableInterestLiabilitiesWithoutRecourseMember
Credit Risk Contract [Member]    
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]    
Fair value at beginning of period (895)us-gaap_FairValueNetDerivativeAssetLiabilityMeasuredOnRecurringBasisWithUnobservableInputs
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_CreditRiskContractMember		 (1,693)us-gaap_FairValueNetDerivativeAssetLiabilityMeasuredOnRecurringBasisWithUnobservableInputs
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_CreditRiskContractMember
Net income (loss) 124us-gaap_FairValueNetDerivativeAssetLiabilityMeasuredOnRecurringBasisUnobservableInputsReconciliationGainLossIncludedInEarnings
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_CreditRiskContractMember		 [1],[3] (211)us-gaap_FairValueNetDerivativeAssetLiabilityMeasuredOnRecurringBasisUnobservableInputsReconciliationGainLossIncludedInEarnings
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_CreditRiskContractMember		 [1],[3]
Other comprehensive income (loss) 0us-gaap_FairValueNetDerivativeAssetLiabilityMeasuredOnRecurringBasisUnobservableInputsReconciliationGainLossIncludedInOtherComprehensiveIncomeLoss
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_CreditRiskContractMember		 [1] 0us-gaap_FairValueNetDerivativeAssetLiabilityMeasuredOnRecurringBasisUnobservableInputsReconciliationGainLossIncludedInOtherComprehensiveIncomeLoss
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_CreditRiskContractMember		 [1]
Purchases 0us-gaap_FairValueNetDerivativeAssetLiabilityMeasuredOnRecurringBasisUnobservableInputsReconciliationPurchases
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_CreditRiskContractMember		 0us-gaap_FairValueNetDerivativeAssetLiabilityMeasuredOnRecurringBasisUnobservableInputsReconciliationPurchases
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_CreditRiskContractMember
Settlements (11)us-gaap_FairValueNetDerivativeAssetLiabilityMeasuredOnRecurringBasisUnobservableInputsReconciliationSettlements
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_CreditRiskContractMember		 (19)us-gaap_FairValueNetDerivativeAssetLiabilityMeasuredOnRecurringBasisUnobservableInputsReconciliationSettlements
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_CreditRiskContractMember
FG VIE consolidations 0ago_FairValueNetDerivativeAssetLiabilityMeasuredonRecurringBasisUnobservableInputsReconciliationConsolidations
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_CreditRiskContractMember		 0ago_FairValueNetDerivativeAssetLiabilityMeasuredonRecurringBasisUnobservableInputsReconciliationConsolidations
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_CreditRiskContractMember
FG VIE deconsolidations 0ago_FairValueNetDerivativeAssetLiabilityMeasuredonRecurringBasisUnobservableInputsReconciliationDeconsolidations
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_CreditRiskContractMember		 0ago_FairValueNetDerivativeAssetLiabilityMeasuredonRecurringBasisUnobservableInputsReconciliationDeconsolidations
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_CreditRiskContractMember
Fair value at end of period (782)us-gaap_FairValueNetDerivativeAssetLiabilityMeasuredOnRecurringBasisWithUnobservableInputs
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_CreditRiskContractMember		 (1,923)us-gaap_FairValueNetDerivativeAssetLiabilityMeasuredOnRecurringBasisWithUnobservableInputs
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_CreditRiskContractMember
Change in unrealized gains/(losses) related to financial instruments held 103ago_FairValueNetDerivativeAssetLiabilityMeasuredonRecurringBasisChangeinUnrealizedGainLoss
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_CreditRiskContractMember		 (232)ago_FairValueNetDerivativeAssetLiabilityMeasuredonRecurringBasisChangeinUnrealizedGainLoss
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_CreditRiskContractMember
Obligations of state and political subdivisions [Member]    
Fair Value Level 3 Rollforward    
Fair value at beginning of period 38us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue
/ us-gaap_FairValueByAssetClassAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember		 36us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue
/ us-gaap_FairValueByAssetClassAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
Net Income (loss) 3us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInEarnings1
/ us-gaap_FairValueByAssetClassAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember		 [1],[4] 1us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInEarnings1
/ us-gaap_FairValueByAssetClassAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember		 [1],[4]
Other comprehensive income (loss) (2)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInOtherComprehensiveIncomeLoss
/ us-gaap_FairValueByAssetClassAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember		 [1] 1us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInOtherComprehensiveIncomeLoss
/ us-gaap_FairValueByAssetClassAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember		 [1]
Purchases 0us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetPurchases
/ us-gaap_FairValueByAssetClassAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember		 0us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetPurchases
/ us-gaap_FairValueByAssetClassAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
Settlements (31)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSettlements
/ us-gaap_FairValueByAssetClassAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember		 [5] 0us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSettlements
/ us-gaap_FairValueByAssetClassAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
FG VIE Consolidations 0ago_FairValueMeasurementwithUnobservableInputsReconciliationRecurringBasisAssetConsolidations
/ us-gaap_FairValueByAssetClassAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember		 0ago_FairValueMeasurementwithUnobservableInputsReconciliationRecurringBasisAssetConsolidations
/ us-gaap_FairValueByAssetClassAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
FG VIE deconsolidations 0ago_FairValueMeasurementwithUnobservableInputsReconciliationRecurringBasisAssetDeconsolidations
/ us-gaap_FairValueByAssetClassAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember		 0ago_FairValueMeasurementwithUnobservableInputsReconciliationRecurringBasisAssetDeconsolidations
/ us-gaap_FairValueByAssetClassAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
Fair value at end of period 8us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue
/ us-gaap_FairValueByAssetClassAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember		 38us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue
/ us-gaap_FairValueByAssetClassAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
Change in unrealized gains/(losses) related to financial instruments held 0us-gaap_FairValueAssetsMeasuredOnRecurringBasisChangeInUnrealizedGainLoss
/ us-gaap_FairValueByAssetClassAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember		 1us-gaap_FairValueAssetsMeasuredOnRecurringBasisChangeInUnrealizedGainLoss
/ us-gaap_FairValueByAssetClassAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
Corporate securities [Member]    
Fair Value Level 3 Rollforward    
Fair value at beginning of period 79us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue
/ us-gaap_FairValueByAssetClassAxis
= us-gaap_CorporateDebtSecuritiesMember		 136us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue
/ us-gaap_FairValueByAssetClassAxis
= us-gaap_CorporateDebtSecuritiesMember
Net Income (loss) 2us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInEarnings1
/ us-gaap_FairValueByAssetClassAxis
= us-gaap_CorporateDebtSecuritiesMember		 [1],[4] 3us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInEarnings1
/ us-gaap_FairValueByAssetClassAxis
= us-gaap_CorporateDebtSecuritiesMember		 [1],[4]
Other comprehensive income (loss) (2)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInOtherComprehensiveIncomeLoss
/ us-gaap_FairValueByAssetClassAxis
= us-gaap_CorporateDebtSecuritiesMember		 [1] 4us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInOtherComprehensiveIncomeLoss
/ us-gaap_FairValueByAssetClassAxis
= us-gaap_CorporateDebtSecuritiesMember		 [1]
Purchases 0us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetPurchases
/ us-gaap_FairValueByAssetClassAxis
= us-gaap_CorporateDebtSecuritiesMember		 0us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetPurchases
/ us-gaap_FairValueByAssetClassAxis
= us-gaap_CorporateDebtSecuritiesMember
Settlements 0us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSettlements
/ us-gaap_FairValueByAssetClassAxis
= us-gaap_CorporateDebtSecuritiesMember		 (5)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSettlements
/ us-gaap_FairValueByAssetClassAxis
= us-gaap_CorporateDebtSecuritiesMember
FG VIE Consolidations 0ago_FairValueMeasurementwithUnobservableInputsReconciliationRecurringBasisAssetConsolidations
/ us-gaap_FairValueByAssetClassAxis
= us-gaap_CorporateDebtSecuritiesMember		 0ago_FairValueMeasurementwithUnobservableInputsReconciliationRecurringBasisAssetConsolidations
/ us-gaap_FairValueByAssetClassAxis
= us-gaap_CorporateDebtSecuritiesMember
FG VIE deconsolidations 0ago_FairValueMeasurementwithUnobservableInputsReconciliationRecurringBasisAssetDeconsolidations
/ us-gaap_FairValueByAssetClassAxis
= us-gaap_CorporateDebtSecuritiesMember		 0ago_FairValueMeasurementwithUnobservableInputsReconciliationRecurringBasisAssetDeconsolidations
/ us-gaap_FairValueByAssetClassAxis
= us-gaap_CorporateDebtSecuritiesMember
Fair value at end of period 79us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue
/ us-gaap_FairValueByAssetClassAxis
= us-gaap_CorporateDebtSecuritiesMember		 138us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue
/ us-gaap_FairValueByAssetClassAxis
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Change in unrealized gains/(losses) related to financial instruments held (2)us-gaap_FairValueAssetsMeasuredOnRecurringBasisChangeInUnrealizedGainLoss
/ us-gaap_FairValueByAssetClassAxis
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/ us-gaap_FairValueByAssetClassAxis
= us-gaap_CorporateDebtSecuritiesMember
RMBS [Member]    
Fair Value Level 3 Rollforward    
Fair value at beginning of period 425us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue
/ us-gaap_FairValueByAssetClassAxis
= us-gaap_ResidentialMortgageBackedSecuritiesMember		 290us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue
/ us-gaap_FairValueByAssetClassAxis
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Net Income (loss) 9us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInEarnings1
/ us-gaap_FairValueByAssetClassAxis
= us-gaap_ResidentialMortgageBackedSecuritiesMember		 [1],[4] 4us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInEarnings1
/ us-gaap_FairValueByAssetClassAxis
= us-gaap_ResidentialMortgageBackedSecuritiesMember		 [1],[4]
Other comprehensive income (loss) 5us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInOtherComprehensiveIncomeLoss
/ us-gaap_FairValueByAssetClassAxis
= us-gaap_ResidentialMortgageBackedSecuritiesMember		 [1] 14us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInOtherComprehensiveIncomeLoss
/ us-gaap_FairValueByAssetClassAxis
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Purchases 9us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetPurchases
/ us-gaap_FairValueByAssetClassAxis
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/ us-gaap_FairValueByAssetClassAxis
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Settlements (65)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSettlements
/ us-gaap_FairValueByAssetClassAxis
= us-gaap_ResidentialMortgageBackedSecuritiesMember		 (15)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSettlements
/ us-gaap_FairValueByAssetClassAxis
= us-gaap_ResidentialMortgageBackedSecuritiesMember
FG VIE Consolidations 0ago_FairValueMeasurementwithUnobservableInputsReconciliationRecurringBasisAssetConsolidations
/ us-gaap_FairValueByAssetClassAxis
= us-gaap_ResidentialMortgageBackedSecuritiesMember		 0ago_FairValueMeasurementwithUnobservableInputsReconciliationRecurringBasisAssetConsolidations
/ us-gaap_FairValueByAssetClassAxis
= us-gaap_ResidentialMortgageBackedSecuritiesMember
FG VIE deconsolidations 0ago_FairValueMeasurementwithUnobservableInputsReconciliationRecurringBasisAssetDeconsolidations
/ us-gaap_FairValueByAssetClassAxis
= us-gaap_ResidentialMortgageBackedSecuritiesMember		 13ago_FairValueMeasurementwithUnobservableInputsReconciliationRecurringBasisAssetDeconsolidations
/ us-gaap_FairValueByAssetClassAxis
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Fair value at end of period 383us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue
/ us-gaap_FairValueByAssetClassAxis
= us-gaap_ResidentialMortgageBackedSecuritiesMember		 359us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue
/ us-gaap_FairValueByAssetClassAxis
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Change in unrealized gains/(losses) related to financial instruments held 7us-gaap_FairValueAssetsMeasuredOnRecurringBasisChangeInUnrealizedGainLoss
/ us-gaap_FairValueByAssetClassAxis
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Asset-backed Securities [Member]    
Fair Value Level 3 Rollforward    
Fair value at beginning of period 228us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue
/ us-gaap_FairValueByAssetClassAxis
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/ us-gaap_FairValueByAssetClassAxis
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Net Income (loss) (2)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInEarnings1
/ us-gaap_FairValueByAssetClassAxis
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/ us-gaap_FairValueByAssetClassAxis
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Other comprehensive income (loss) 1us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInOtherComprehensiveIncomeLoss
/ us-gaap_FairValueByAssetClassAxis
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/ us-gaap_FairValueByAssetClassAxis
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Purchases 0us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetPurchases
/ us-gaap_FairValueByAssetClassAxis
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/ us-gaap_FairValueByAssetClassAxis
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Settlements (1)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSettlements
/ us-gaap_FairValueByAssetClassAxis
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/ us-gaap_FairValueByAssetClassAxis
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FG VIE Consolidations 0ago_FairValueMeasurementwithUnobservableInputsReconciliationRecurringBasisAssetConsolidations
/ us-gaap_FairValueByAssetClassAxis
= us-gaap_AssetBackedSecuritiesMember		 0ago_FairValueMeasurementwithUnobservableInputsReconciliationRecurringBasisAssetConsolidations
/ us-gaap_FairValueByAssetClassAxis
= us-gaap_AssetBackedSecuritiesMember
FG VIE deconsolidations 0ago_FairValueMeasurementwithUnobservableInputsReconciliationRecurringBasisAssetDeconsolidations
/ us-gaap_FairValueByAssetClassAxis
= us-gaap_AssetBackedSecuritiesMember		 0ago_FairValueMeasurementwithUnobservableInputsReconciliationRecurringBasisAssetDeconsolidations
/ us-gaap_FairValueByAssetClassAxis
= us-gaap_AssetBackedSecuritiesMember
Fair value at end of period 226us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue
/ us-gaap_FairValueByAssetClassAxis
= us-gaap_AssetBackedSecuritiesMember		 252us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue
/ us-gaap_FairValueByAssetClassAxis
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/ us-gaap_FairValueByAssetClassAxis
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/ us-gaap_FairValueByAssetClassAxis
= us-gaap_AssetBackedSecuritiesMember
Other invested assets [Member]    
Fair Value Level 3 Rollforward    
Fair value at beginning of period 78us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue
/ us-gaap_FairValueByAssetClassAxis
= ago_OtherInvestedAssetsMember		 2us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue
/ us-gaap_FairValueByAssetClassAxis
= ago_OtherInvestedAssetsMember
Net Income (loss) (4)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInEarnings1
/ us-gaap_FairValueByAssetClassAxis
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/ us-gaap_FairValueByAssetClassAxis
= ago_OtherInvestedAssetsMember		 [1]
Other comprehensive income (loss) 2us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInOtherComprehensiveIncomeLoss
/ us-gaap_FairValueByAssetClassAxis
= ago_OtherInvestedAssetsMember		 [1] 1us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInOtherComprehensiveIncomeLoss
/ us-gaap_FairValueByAssetClassAxis
= ago_OtherInvestedAssetsMember		 [1]
Purchases 0us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetPurchases
/ us-gaap_FairValueByAssetClassAxis
= ago_OtherInvestedAssetsMember		 45us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetPurchases
/ us-gaap_FairValueByAssetClassAxis
= ago_OtherInvestedAssetsMember		 [5]
Settlements 0us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSettlements
/ us-gaap_FairValueByAssetClassAxis
= ago_OtherInvestedAssetsMember		 0us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSettlements
/ us-gaap_FairValueByAssetClassAxis
= ago_OtherInvestedAssetsMember
FG VIE Consolidations 0ago_FairValueMeasurementwithUnobservableInputsReconciliationRecurringBasisAssetConsolidations
/ us-gaap_FairValueByAssetClassAxis
= ago_OtherInvestedAssetsMember		 0ago_FairValueMeasurementwithUnobservableInputsReconciliationRecurringBasisAssetConsolidations
/ us-gaap_FairValueByAssetClassAxis
= ago_OtherInvestedAssetsMember
FG VIE deconsolidations 0ago_FairValueMeasurementwithUnobservableInputsReconciliationRecurringBasisAssetDeconsolidations
/ us-gaap_FairValueByAssetClassAxis
= ago_OtherInvestedAssetsMember		 0ago_FairValueMeasurementwithUnobservableInputsReconciliationRecurringBasisAssetDeconsolidations
/ us-gaap_FairValueByAssetClassAxis
= ago_OtherInvestedAssetsMember
Fair value at end of period 76us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue
/ us-gaap_FairValueByAssetClassAxis
= ago_OtherInvestedAssetsMember		 48us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue
/ us-gaap_FairValueByAssetClassAxis
= ago_OtherInvestedAssetsMember
Change in unrealized gains/(losses) related to financial instruments held 2us-gaap_FairValueAssetsMeasuredOnRecurringBasisChangeInUnrealizedGainLoss
/ us-gaap_FairValueByAssetClassAxis
= ago_OtherInvestedAssetsMember		 1us-gaap_FairValueAssetsMeasuredOnRecurringBasisChangeInUnrealizedGainLoss
/ us-gaap_FairValueByAssetClassAxis
= ago_OtherInvestedAssetsMember
FG VIEs' assets, at fair value [Member]    
Fair Value Level 3 Rollforward    
Fair value at beginning of period 1,398us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue
/ us-gaap_FairValueByAssetClassAxis
= ago_VariableInterestEntityAssetsMember		 2,565us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue
/ us-gaap_FairValueByAssetClassAxis
= ago_VariableInterestEntityAssetsMember
Net Income (loss) 23us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInEarnings1
/ us-gaap_FairValueByAssetClassAxis
= ago_VariableInterestEntityAssetsMember		 [1],[2] 82us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInEarnings1
/ us-gaap_FairValueByAssetClassAxis
= ago_VariableInterestEntityAssetsMember		 [1],[2]
Other comprehensive income (loss) 0us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInOtherComprehensiveIncomeLoss
/ us-gaap_FairValueByAssetClassAxis
= ago_VariableInterestEntityAssetsMember		 [1] 0us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInOtherComprehensiveIncomeLoss
/ us-gaap_FairValueByAssetClassAxis
= ago_VariableInterestEntityAssetsMember		 [1]
Purchases 0us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetPurchases
/ us-gaap_FairValueByAssetClassAxis
= ago_VariableInterestEntityAssetsMember		 0us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetPurchases
/ us-gaap_FairValueByAssetClassAxis
= ago_VariableInterestEntityAssetsMember
Settlements (30)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSettlements
/ us-gaap_FairValueByAssetClassAxis
= ago_VariableInterestEntityAssetsMember		 (286)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSettlements
/ us-gaap_FairValueByAssetClassAxis
= ago_VariableInterestEntityAssetsMember
FG VIE Consolidations 104ago_FairValueMeasurementwithUnobservableInputsReconciliationRecurringBasisAssetConsolidations
/ us-gaap_FairValueByAssetClassAxis
= ago_VariableInterestEntityAssetsMember		 0ago_FairValueMeasurementwithUnobservableInputsReconciliationRecurringBasisAssetConsolidations
/ us-gaap_FairValueByAssetClassAxis
= ago_VariableInterestEntityAssetsMember
FG VIE deconsolidations 0ago_FairValueMeasurementwithUnobservableInputsReconciliationRecurringBasisAssetDeconsolidations
/ us-gaap_FairValueByAssetClassAxis
= ago_VariableInterestEntityAssetsMember		 (1,104)ago_FairValueMeasurementwithUnobservableInputsReconciliationRecurringBasisAssetDeconsolidations
/ us-gaap_FairValueByAssetClassAxis
= ago_VariableInterestEntityAssetsMember
Fair value at end of period 1,495us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue
/ us-gaap_FairValueByAssetClassAxis
= ago_VariableInterestEntityAssetsMember		 1,257us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue
/ us-gaap_FairValueByAssetClassAxis
= ago_VariableInterestEntityAssetsMember
Change in unrealized gains/(losses) related to financial instruments held 34us-gaap_FairValueAssetsMeasuredOnRecurringBasisChangeInUnrealizedGainLoss
/ us-gaap_FairValueByAssetClassAxis
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/ us-gaap_FairValueByAssetClassAxis
= ago_VariableInterestEntityAssetsMember
Other assets [Member]    
Fair Value Level 3 Rollforward    
Fair value at beginning of period 35us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue
/ us-gaap_FairValueByAssetClassAxis
= ago_OtherAssetBackedSecuritiesMember		 46us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue
/ us-gaap_FairValueByAssetClassAxis
= ago_OtherAssetBackedSecuritiesMember
Net Income (loss) 2us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInEarnings1
/ us-gaap_FairValueByAssetClassAxis
= ago_OtherAssetBackedSecuritiesMember		 [1],[6] (9)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInEarnings1
/ us-gaap_FairValueByAssetClassAxis
= ago_OtherAssetBackedSecuritiesMember		 [1],[6]
Other comprehensive income (loss) 0us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInOtherComprehensiveIncomeLoss
/ us-gaap_FairValueByAssetClassAxis
= ago_OtherAssetBackedSecuritiesMember		 [1] 0us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInOtherComprehensiveIncomeLoss
/ us-gaap_FairValueByAssetClassAxis
= ago_OtherAssetBackedSecuritiesMember		 [1]
Purchases 0us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetPurchases
/ us-gaap_FairValueByAssetClassAxis
= ago_OtherAssetBackedSecuritiesMember		 0us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetPurchases
/ us-gaap_FairValueByAssetClassAxis
= ago_OtherAssetBackedSecuritiesMember
Settlements 0us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSettlements
/ us-gaap_FairValueByAssetClassAxis
= ago_OtherAssetBackedSecuritiesMember		 0us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSettlements
/ us-gaap_FairValueByAssetClassAxis
= ago_OtherAssetBackedSecuritiesMember
FG VIE Consolidations 0ago_FairValueMeasurementwithUnobservableInputsReconciliationRecurringBasisAssetConsolidations
/ us-gaap_FairValueByAssetClassAxis
= ago_OtherAssetBackedSecuritiesMember		 0ago_FairValueMeasurementwithUnobservableInputsReconciliationRecurringBasisAssetConsolidations
/ us-gaap_FairValueByAssetClassAxis
= ago_OtherAssetBackedSecuritiesMember
FG VIE deconsolidations 0ago_FairValueMeasurementwithUnobservableInputsReconciliationRecurringBasisAssetDeconsolidations
/ us-gaap_FairValueByAssetClassAxis
= ago_OtherAssetBackedSecuritiesMember		 0ago_FairValueMeasurementwithUnobservableInputsReconciliationRecurringBasisAssetDeconsolidations
/ us-gaap_FairValueByAssetClassAxis
= ago_OtherAssetBackedSecuritiesMember
Fair value at end of period 37us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue
/ us-gaap_FairValueByAssetClassAxis
= ago_OtherAssetBackedSecuritiesMember		 37us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue
/ us-gaap_FairValueByAssetClassAxis
= ago_OtherAssetBackedSecuritiesMember
Change in unrealized gains/(losses) related to financial instruments held $ 2us-gaap_FairValueAssetsMeasuredOnRecurringBasisChangeInUnrealizedGainLoss
/ us-gaap_FairValueByAssetClassAxis
= ago_OtherAssetBackedSecuritiesMember		 $ (9)us-gaap_FairValueAssetsMeasuredOnRecurringBasisChangeInUnrealizedGainLoss
/ us-gaap_FairValueByAssetClassAxis
= ago_OtherAssetBackedSecuritiesMember
[1] Realized and unrealized gains (losses) from changes in values of Level 3 financial instruments represent gains (losses) from changes in values of those financial instruments only for the periods in which the instruments were classified as Level 3.
[2] Included in fair value gains (losses) on FG VIEs.
[3] Reported in net change in fair value of credit derivatives.
[4] Included in net realized investment gains (losses) and net investment income.
[5] on-cash transaction.
[6] Recorded in fair value gains (losses) on CCS.