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Long-Term Debt and Credit Facilities - Principal and Carrying Amounts of Debt (Details) (USD $)
Mar. 31, 2015
Dec. 31, 2014
Debt Instrument [Line Items]    
Principal $ 1,595,000,000us-gaap_DebtInstrumentFaceAmount $ 1,596,000,000us-gaap_DebtInstrumentFaceAmount
Carrying Value 1,304,000,000us-gaap_LongTermDebt 1,303,000,000us-gaap_LongTermDebt
AGUS [Member]    
Debt Instrument [Line Items]    
Principal 850,000,000us-gaap_DebtInstrumentFaceAmount
/ dei_LegalEntityAxis
= ago_AssuredGuarantyUSHoldingIncMember		 850,000,000us-gaap_DebtInstrumentFaceAmount
/ dei_LegalEntityAxis
= ago_AssuredGuarantyUSHoldingIncMember
Carrying Value 847,000,000us-gaap_LongTermDebt
/ dei_LegalEntityAxis
= ago_AssuredGuarantyUSHoldingIncMember		 847,000,000us-gaap_LongTermDebt
/ dei_LegalEntityAxis
= ago_AssuredGuarantyUSHoldingIncMember
AGMH [Member]    
Debt Instrument [Line Items]    
Principal 730,000,000us-gaap_DebtInstrumentFaceAmount
/ dei_LegalEntityAxis
= ago_AssuredGuarantyMunicipalHoldingsIncMember		 730,000,000us-gaap_DebtInstrumentFaceAmount
/ dei_LegalEntityAxis
= ago_AssuredGuarantyMunicipalHoldingsIncMember
Carrying Value 439,000,000us-gaap_LongTermDebt
/ dei_LegalEntityAxis
= ago_AssuredGuarantyMunicipalHoldingsIncMember		 437,000,000us-gaap_LongTermDebt
/ dei_LegalEntityAxis
= ago_AssuredGuarantyMunicipalHoldingsIncMember
AGM [Member]    
Debt Instrument [Line Items]    
Principal 15,000,000us-gaap_DebtInstrumentFaceAmount
/ dei_LegalEntityAxis
= ago_AssuredGuarantyMunicipalCorpMember		 16,000,000us-gaap_DebtInstrumentFaceAmount
/ dei_LegalEntityAxis
= ago_AssuredGuarantyMunicipalCorpMember
Carrying Value 18,000,000us-gaap_LongTermDebt
/ dei_LegalEntityAxis
= ago_AssuredGuarantyMunicipalCorpMember		 19,000,000us-gaap_LongTermDebt
/ dei_LegalEntityAxis
= ago_AssuredGuarantyMunicipalCorpMember
Senior Notes [Member] | AGUS [Member] | Senior Notes 7.0 Percent [Member]    
Debt Instrument [Line Items]    
Interest rate of debt (as a percent) 7.00%us-gaap_DebtInstrumentInterestRateStatedPercentage
/ us-gaap_DebtInstrumentAxis
= ago_SeniorNotes70PercentMember
/ dei_LegalEntityAxis
= ago_AssuredGuarantyUSHoldingIncMember
/ us-gaap_LongtermDebtTypeAxis
= us-gaap_SeniorNotesMember		 7.00%us-gaap_DebtInstrumentInterestRateStatedPercentage
/ us-gaap_DebtInstrumentAxis
= ago_SeniorNotes70PercentMember
/ dei_LegalEntityAxis
= ago_AssuredGuarantyUSHoldingIncMember
/ us-gaap_LongtermDebtTypeAxis
= us-gaap_SeniorNotesMember
Principal 200,000,000us-gaap_DebtInstrumentFaceAmount
/ us-gaap_DebtInstrumentAxis
= ago_SeniorNotes70PercentMember
/ dei_LegalEntityAxis
= ago_AssuredGuarantyUSHoldingIncMember
/ us-gaap_LongtermDebtTypeAxis
= us-gaap_SeniorNotesMember		 200,000,000us-gaap_DebtInstrumentFaceAmount
/ us-gaap_DebtInstrumentAxis
= ago_SeniorNotes70PercentMember
/ dei_LegalEntityAxis
= ago_AssuredGuarantyUSHoldingIncMember
/ us-gaap_LongtermDebtTypeAxis
= us-gaap_SeniorNotesMember
Carrying Value 198,000,000us-gaap_LongTermDebt
/ us-gaap_DebtInstrumentAxis
= ago_SeniorNotes70PercentMember
/ dei_LegalEntityAxis
= ago_AssuredGuarantyUSHoldingIncMember
/ us-gaap_LongtermDebtTypeAxis
= us-gaap_SeniorNotesMember		 198,000,000us-gaap_LongTermDebt
/ us-gaap_DebtInstrumentAxis
= ago_SeniorNotes70PercentMember
/ dei_LegalEntityAxis
= ago_AssuredGuarantyUSHoldingIncMember
/ us-gaap_LongtermDebtTypeAxis
= us-gaap_SeniorNotesMember
Senior Notes [Member] | AGUS [Member] | Senior Notes 5.0 Percent [Member]    
Debt Instrument [Line Items]    
Interest rate of debt (as a percent) 5.00%us-gaap_DebtInstrumentInterestRateStatedPercentage
/ us-gaap_DebtInstrumentAxis
= ago_SeniorNotes5.0PercentMember
/ dei_LegalEntityAxis
= ago_AssuredGuarantyUSHoldingIncMember
/ us-gaap_LongtermDebtTypeAxis
= us-gaap_SeniorNotesMember		 5.00%us-gaap_DebtInstrumentInterestRateStatedPercentage
/ us-gaap_DebtInstrumentAxis
= ago_SeniorNotes5.0PercentMember
/ dei_LegalEntityAxis
= ago_AssuredGuarantyUSHoldingIncMember
/ us-gaap_LongtermDebtTypeAxis
= us-gaap_SeniorNotesMember
Principal 500,000,000us-gaap_DebtInstrumentFaceAmount
/ us-gaap_DebtInstrumentAxis
= ago_SeniorNotes5.0PercentMember
/ dei_LegalEntityAxis
= ago_AssuredGuarantyUSHoldingIncMember
/ us-gaap_LongtermDebtTypeAxis
= us-gaap_SeniorNotesMember		 500,000,000us-gaap_DebtInstrumentFaceAmount
/ us-gaap_DebtInstrumentAxis
= ago_SeniorNotes5.0PercentMember
/ dei_LegalEntityAxis
= ago_AssuredGuarantyUSHoldingIncMember
/ us-gaap_LongtermDebtTypeAxis
= us-gaap_SeniorNotesMember
Carrying Value 499,000,000us-gaap_LongTermDebt
/ us-gaap_DebtInstrumentAxis
= ago_SeniorNotes5.0PercentMember
/ dei_LegalEntityAxis
= ago_AssuredGuarantyUSHoldingIncMember
/ us-gaap_LongtermDebtTypeAxis
= us-gaap_SeniorNotesMember		 499,000,000us-gaap_LongTermDebt
/ us-gaap_DebtInstrumentAxis
= ago_SeniorNotes5.0PercentMember
/ dei_LegalEntityAxis
= ago_AssuredGuarantyUSHoldingIncMember
/ us-gaap_LongtermDebtTypeAxis
= us-gaap_SeniorNotesMember
Enhanced Junior Subordinated Debentures [Member] | AGUS [Member]    
Debt Instrument [Line Items]    
Principal 150,000,000us-gaap_DebtInstrumentFaceAmount
/ dei_LegalEntityAxis
= ago_AssuredGuarantyUSHoldingIncMember
/ us-gaap_LongtermDebtTypeAxis
= ago_EnhancedJuniorSubordinatedDebenturesMember		 150,000,000us-gaap_DebtInstrumentFaceAmount
/ dei_LegalEntityAxis
= ago_AssuredGuarantyUSHoldingIncMember
/ us-gaap_LongtermDebtTypeAxis
= ago_EnhancedJuniorSubordinatedDebenturesMember
Carrying Value 150,000,000us-gaap_LongTermDebt
/ dei_LegalEntityAxis
= ago_AssuredGuarantyUSHoldingIncMember
/ us-gaap_LongtermDebtTypeAxis
= ago_EnhancedJuniorSubordinatedDebenturesMember		 150,000,000us-gaap_LongTermDebt
/ dei_LegalEntityAxis
= ago_AssuredGuarantyUSHoldingIncMember
/ us-gaap_LongtermDebtTypeAxis
= ago_EnhancedJuniorSubordinatedDebenturesMember
Corporate securities [Member] | AGMH [Member] | QUIBS 6.875 Percent [Member]    
Debt Instrument [Line Items]    
Interest rate of debt (as a percent) 6.875%us-gaap_DebtInstrumentInterestRateStatedPercentage
/ us-gaap_DebtInstrumentAxis
= ago_Quibs6875PercentMember
/ dei_LegalEntityAxis
= ago_AssuredGuarantyMunicipalHoldingsIncMember
/ us-gaap_LongtermDebtTypeAxis
= us-gaap_CorporateDebtSecuritiesMember		 6.875%us-gaap_DebtInstrumentInterestRateStatedPercentage
/ us-gaap_DebtInstrumentAxis
= ago_Quibs6875PercentMember
/ dei_LegalEntityAxis
= ago_AssuredGuarantyMunicipalHoldingsIncMember
/ us-gaap_LongtermDebtTypeAxis
= us-gaap_CorporateDebtSecuritiesMember
Principal 100,000,000us-gaap_DebtInstrumentFaceAmount
/ us-gaap_DebtInstrumentAxis
= ago_Quibs6875PercentMember
/ dei_LegalEntityAxis
= ago_AssuredGuarantyMunicipalHoldingsIncMember
/ us-gaap_LongtermDebtTypeAxis
= us-gaap_CorporateDebtSecuritiesMember		 100,000,000us-gaap_DebtInstrumentFaceAmount
/ us-gaap_DebtInstrumentAxis
= ago_Quibs6875PercentMember
/ dei_LegalEntityAxis
= ago_AssuredGuarantyMunicipalHoldingsIncMember
/ us-gaap_LongtermDebtTypeAxis
= us-gaap_CorporateDebtSecuritiesMember
Carrying Value 68,000,000us-gaap_LongTermDebt
/ us-gaap_DebtInstrumentAxis
= ago_Quibs6875PercentMember
/ dei_LegalEntityAxis
= ago_AssuredGuarantyMunicipalHoldingsIncMember
/ us-gaap_LongtermDebtTypeAxis
= us-gaap_CorporateDebtSecuritiesMember		 68,000,000us-gaap_LongTermDebt
/ us-gaap_DebtInstrumentAxis
= ago_Quibs6875PercentMember
/ dei_LegalEntityAxis
= ago_AssuredGuarantyMunicipalHoldingsIncMember
/ us-gaap_LongtermDebtTypeAxis
= us-gaap_CorporateDebtSecuritiesMember
Notes Payable, Other Payables [Member] | AGMH [Member] | 6.25% Notes [Member]    
Debt Instrument [Line Items]    
Interest rate of debt (as a percent) 6.25%us-gaap_DebtInstrumentInterestRateStatedPercentage
/ us-gaap_DebtInstrumentAxis
= ago_NotesPayable625PercentMember
/ dei_LegalEntityAxis
= ago_AssuredGuarantyMunicipalHoldingsIncMember
/ us-gaap_LongtermDebtTypeAxis
= us-gaap_NotesPayableOtherPayablesMember		 6.25%us-gaap_DebtInstrumentInterestRateStatedPercentage
/ us-gaap_DebtInstrumentAxis
= ago_NotesPayable625PercentMember
/ dei_LegalEntityAxis
= ago_AssuredGuarantyMunicipalHoldingsIncMember
/ us-gaap_LongtermDebtTypeAxis
= us-gaap_NotesPayableOtherPayablesMember
Principal 230,000,000us-gaap_DebtInstrumentFaceAmount
/ us-gaap_DebtInstrumentAxis
= ago_NotesPayable625PercentMember
/ dei_LegalEntityAxis
= ago_AssuredGuarantyMunicipalHoldingsIncMember
/ us-gaap_LongtermDebtTypeAxis
= us-gaap_NotesPayableOtherPayablesMember		 230,000,000us-gaap_DebtInstrumentFaceAmount
/ us-gaap_DebtInstrumentAxis
= ago_NotesPayable625PercentMember
/ dei_LegalEntityAxis
= ago_AssuredGuarantyMunicipalHoldingsIncMember
/ us-gaap_LongtermDebtTypeAxis
= us-gaap_NotesPayableOtherPayablesMember
Carrying Value 139,000,000us-gaap_LongTermDebt
/ us-gaap_DebtInstrumentAxis
= ago_NotesPayable625PercentMember
/ dei_LegalEntityAxis
= ago_AssuredGuarantyMunicipalHoldingsIncMember
/ us-gaap_LongtermDebtTypeAxis
= us-gaap_NotesPayableOtherPayablesMember		 139,000,000us-gaap_LongTermDebt
/ us-gaap_DebtInstrumentAxis
= ago_NotesPayable625PercentMember
/ dei_LegalEntityAxis
= ago_AssuredGuarantyMunicipalHoldingsIncMember
/ us-gaap_LongtermDebtTypeAxis
= us-gaap_NotesPayableOtherPayablesMember
Notes Payable, Other Payables [Member] | AGMH [Member] | Notes Payable 5.60 Percent [Member]    
Debt Instrument [Line Items]    
Interest rate of debt (as a percent) 5.60%us-gaap_DebtInstrumentInterestRateStatedPercentage
/ us-gaap_DebtInstrumentAxis
= ago_NotesPayable560PercentMember
/ dei_LegalEntityAxis
= ago_AssuredGuarantyMunicipalHoldingsIncMember
/ us-gaap_LongtermDebtTypeAxis
= us-gaap_NotesPayableOtherPayablesMember		 5.60%us-gaap_DebtInstrumentInterestRateStatedPercentage
/ us-gaap_DebtInstrumentAxis
= ago_NotesPayable560PercentMember
/ dei_LegalEntityAxis
= ago_AssuredGuarantyMunicipalHoldingsIncMember
/ us-gaap_LongtermDebtTypeAxis
= us-gaap_NotesPayableOtherPayablesMember
Principal 100,000,000us-gaap_DebtInstrumentFaceAmount
/ us-gaap_DebtInstrumentAxis
= ago_NotesPayable560PercentMember
/ dei_LegalEntityAxis
= ago_AssuredGuarantyMunicipalHoldingsIncMember
/ us-gaap_LongtermDebtTypeAxis
= us-gaap_NotesPayableOtherPayablesMember		 100,000,000us-gaap_DebtInstrumentFaceAmount
/ us-gaap_DebtInstrumentAxis
= ago_NotesPayable560PercentMember
/ dei_LegalEntityAxis
= ago_AssuredGuarantyMunicipalHoldingsIncMember
/ us-gaap_LongtermDebtTypeAxis
= us-gaap_NotesPayableOtherPayablesMember
Carrying Value 55,000,000us-gaap_LongTermDebt
/ us-gaap_DebtInstrumentAxis
= ago_NotesPayable560PercentMember
/ dei_LegalEntityAxis
= ago_AssuredGuarantyMunicipalHoldingsIncMember
/ us-gaap_LongtermDebtTypeAxis
= us-gaap_NotesPayableOtherPayablesMember		 55,000,000us-gaap_LongTermDebt
/ us-gaap_DebtInstrumentAxis
= ago_NotesPayable560PercentMember
/ dei_LegalEntityAxis
= ago_AssuredGuarantyMunicipalHoldingsIncMember
/ us-gaap_LongtermDebtTypeAxis
= us-gaap_NotesPayableOtherPayablesMember
Notes Payable, Other Payables [Member] | AGM [Member]    
Debt Instrument [Line Items]    
Principal 15,000,000us-gaap_DebtInstrumentFaceAmount
/ dei_LegalEntityAxis
= ago_AssuredGuarantyMunicipalCorpMember
/ us-gaap_LongtermDebtTypeAxis
= us-gaap_NotesPayableOtherPayablesMember		 16,000,000us-gaap_DebtInstrumentFaceAmount
/ dei_LegalEntityAxis
= ago_AssuredGuarantyMunicipalCorpMember
/ us-gaap_LongtermDebtTypeAxis
= us-gaap_NotesPayableOtherPayablesMember
Carrying Value 18,000,000us-gaap_LongTermDebt
/ dei_LegalEntityAxis
= ago_AssuredGuarantyMunicipalCorpMember
/ us-gaap_LongtermDebtTypeAxis
= us-gaap_NotesPayableOtherPayablesMember		 19,000,000us-gaap_LongTermDebt
/ dei_LegalEntityAxis
= ago_AssuredGuarantyMunicipalCorpMember
/ us-gaap_LongtermDebtTypeAxis
= us-gaap_NotesPayableOtherPayablesMember
Junior Subordinated Debt [Member] | AGMH [Member]    
Debt Instrument [Line Items]    
Principal 300,000,000us-gaap_DebtInstrumentFaceAmount
/ dei_LegalEntityAxis
= ago_AssuredGuarantyMunicipalHoldingsIncMember
/ us-gaap_LongtermDebtTypeAxis
= us-gaap_JuniorSubordinatedDebtMember		 300,000,000us-gaap_DebtInstrumentFaceAmount
/ dei_LegalEntityAxis
= ago_AssuredGuarantyMunicipalHoldingsIncMember
/ us-gaap_LongtermDebtTypeAxis
= us-gaap_JuniorSubordinatedDebtMember
Carrying Value $ 177,000,000us-gaap_LongTermDebt
/ dei_LegalEntityAxis
= ago_AssuredGuarantyMunicipalHoldingsIncMember
/ us-gaap_LongtermDebtTypeAxis
= us-gaap_JuniorSubordinatedDebtMember		 $ 175,000,000us-gaap_LongTermDebt
/ dei_LegalEntityAxis
= ago_AssuredGuarantyMunicipalHoldingsIncMember
/ us-gaap_LongtermDebtTypeAxis
= us-gaap_JuniorSubordinatedDebtMember