Name:	ago_AssumedExpectedLossAndLossAdjustmentExpensePayable
Namespace Prefix:	ago_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

The carrying amount as of the balance sheet date of financial guaranty premiums due the entity for all insurance assumed from other insurers.

No definition available.

Name:	ago_AssumedPremiumReceivable
Namespace Prefix:	ago_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

Total expected amount of future recoveries as of the balance sheet date from reinsurers relating to ceded financial guaranty business.

No definition available.

Name:	ago_CededExpectedLossAndLossAdjustmentExpense
Namespace Prefix:	ago_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

The consideration due to the assuming enterprise, net of commissions as of the balance sheet date under the terms of the reinsurance contract in exchange for the right to receive reimbursement from such entity for covered claims and expenses.

No definition available.

Name:	ago_CededPremiumPayableNetofCommissions
Namespace Prefix:	ago_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

No definition available.

Name:	us-gaap_CededCreditRiskLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

Name:	us-gaap_CededCreditRiskAxis=ago_AmericanOverseasReinsuranceCompanyLimitedMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Name:	us-gaap_CededCreditRiskAxis=ago_TokioMarineAndNichidoFireInsuranceCoLtdMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Name:	us-gaap_CededCreditRiskAxis=ago_SyncoraGuaranteeIncMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Name:	us-gaap_CededCreditRiskAxis=ago_MitsuiSumitomoInsuranceCoLtdMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Name:	us-gaap_CededCreditRiskAxis=ago_SwissReinsuranceCoMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Name:	us-gaap_CededCreditRiskAxis=ago_AmbacAssuranceCorporationMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Name:	us-gaap_CededCreditRiskAxis=ago_AmbacAssuranceCorp.SegregatedaccountMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Name:	us-gaap_CededCreditRiskAxis=ago_CifgAssuranceNorthAmericaIncMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Name:	us-gaap_CededCreditRiskAxis=ago_MbiaInsuranceCorporationMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Name:	us-gaap_CededCreditRiskAxis=ago_NationalPublicFinanceGuaranteeCorporationMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Name:	us-gaap_CededCreditRiskAxis=ago_FinancialGuarantyInsuranceCoMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: