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

-- None. No documentation exists for this element. --

No definition available.

Name:	ago_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationExpectedCashFlowsAbstract
Namespace Prefix:	ago_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

Schedule of Insured Financial Obligations with Credit Deterioration, Gross Claim Liability, Net of Reinsurance

No definition available.

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

-- None. No documentation exists for this element. --

No definition available.

Name:	ago_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationGrossClaimLiabilitySubtotalAbstract
Namespace Prefix:	ago_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

-- None. No documentation exists for this element. --

No definition available.

Name:	ago_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationInsuredContractualPaymentsInterestAbstract
Namespace Prefix:	ago_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

This element represents the amount of insured contractual payments outstanding attributable to interest accrued on the underlying insured obligations for financial guaranty insurance contracts presented in the context of each surveillance grouping or category used by management to monitor and track potential deterioration in the credit worthiness of the insured party's ability to meet its financial obligations related to the contracts underlying financial guaranty insurance contracts, before adjusting for the effect of consolidating VIEs.

No definition available.

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

This element represents the amount of insured contractual payments outstanding on the underlying insured obligations for financial guaranty insurance contracts presented in the context of each surveillance grouping or category used by management to monitor and track potential deterioration in the credit worthiness of the insured party's ability to meet its financial obligations related to the contracts underlying financial guaranty insurance contracts, before adjusting for the effect of consolidating VIEs.

No definition available.

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

-- None. No documentation exists for this element. --

No definition available.

Name:	ago_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationInsuredContractualPaymentsOutstandingPrincipalAbstract
Namespace Prefix:	ago_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

This element represents the amount of insured contractual payments outstanding attributable to the principal of the underlying insured obligations for financial guaranty insurance contracts presented in the context of each surveillance grouping or category used by management to monitor and track potential deterioration in the credit worthiness of the insured party's ability to meet its financial obligations related to the contracts underlying financial guaranty insurance contracts, before adjusting for the effect of consolidating VIEs.

No definition available.

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

-- None. No documentation exists for this element. --

No definition available.

Name:	ago_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationNumberOfRisksAbstract
Namespace Prefix:	ago_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

-- None. No documentation exists for this element. --

No definition available.

Name:	ago_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationReductionsOfGrossClaimLiabilityGrossPotentialRecoveriesAbstract
Namespace Prefix:	ago_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

-- None. No documentation exists for this element. --

No definition available.

Name:	ago_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationReductionsOfGrossClaimLiabilityNetDiscountAbstract
Namespace Prefix:	ago_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

Schedule of Insured Financial Obligations with Credit Deterioration, Reductions of Gross Claim Liability, Net of Reinsurance Recoverable, Discount (Accretion)

No definition available.

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

-- None. No documentation exists for this element. --

No definition available.

Name:	ago_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationRemainingWeightedAverageContractPeriodAbstract
Namespace Prefix:	ago_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

-- None. No documentation exists for this element. --

No definition available.

Name:	ago_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationReservesSalvageAbstract
Namespace Prefix:	ago_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

-- None. No documentation exists for this element. --

No definition available.

Name:	ago_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationStatementOfFinancialPositionUnearnedPremiumRevenueAbstract
Namespace Prefix:	ago_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

Schedule of Insured Financial Obligations with Credit Deterioration, Net Claim Liability, Net of Reinsurance Recoverable

No definition available.

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

Schedule of Insured Financial Obligations with Credit Deterioration, Net Claim Liability, Undiscounted, Direct Assumed

No definition available.

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

Schedule of Insured Financial Obligations with Credit Deterioration, Net Claim Liability, Undiscounted, Direct Assumed, Net

No definition available.

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

Schedule of Insured Financial Obligations with Credit Deterioration, Reductions of Gross Claim Liability, Net Potential Recoveries

No definition available.

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

This element represents the remaining weighted average contract period of financial guaranty insurance contracts for each surveillance grouping or category used by management to monitor and track potential deterioration in the credit worthiness of the insured party's ability to meet its financial obligations related to the contracts underlying financial guaranty insurance contracts, before adjusting for the effect of consolidating VIEs.

No definition available.

Name:	ago_ScheduleofInsuredFinancialObligationswithCreditDeteriorationRemainingWeightedAverageContractPeriodNet
Namespace Prefix:	ago_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

Schedule of Insured Financial Obligations with Credit Deterioration, Statement of Financial Position, Unearned Premium Revenue, Net of Reinsurance Recoverable

No definition available.

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

Remaining weighted average contract period of financial guarantee insurance contracts, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

Reference 1: http://www.xbrl.org/2003/role/exampleRef
-Topic 944
-SubTopic 40
-Name Accounting Standards Codification
-Section 55
-Paragraph 34
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480046/944-40-55-34

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 944
-SubTopic 40
-Name Accounting Standards Codification
-Section 50
-Paragraph 9
-Subparagraph (a)(5)(ii)
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-URI https://asc.fasb.org/1943274/2147480081/944-40-50-9

Name:	us-gaap_InsuredFinancialObligationsWithCreditDeteriorationRemainingWeightedAverageContractPeriod
Namespace Prefix:	us-gaap_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

This element represents the gross claim liability for financial guarantee insurance contracts presented in the context of each surveillance grouping or category used by management to monitor and track potential deterioration in the credit worthiness of the insured party's ability to meet its financial obligations related to the contracts underlying financial guarantee insurance contracts the entity has issued.

Reference 1: http://www.xbrl.org/2003/role/exampleRef
-Topic 944
-SubTopic 40
-Name Accounting Standards Codification
-Section 55
-Paragraph 34
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480046/944-40-55-34

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-Topic 944
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Name:	us-gaap_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationGrossClaimLiability
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

This element represents the amount of insured contractual payments outstanding on the underlying insured obligations for financial guarantee insurance contracts presented in the context of each surveillance grouping or category used by management to monitor and track potential deterioration in the credit worthiness of the insured party's ability to meet its financial obligations related to the contracts underlying financial guarantee insurance contracts the entity has issued.

Reference 1: http://www.xbrl.org/2003/role/exampleRef
-Topic 944
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-Name Accounting Standards Codification
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-Topic 944
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Name:	us-gaap_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationInsuredContractualPaymentsOutstanding
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- References

No definition available.

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

This element represents the amount of insured contractual payments outstanding attributable to interest accrued on the underlying insured obligations for financial guarantee insurance contracts presented in the context of each surveillance grouping or category used by management to monitor and track potential deterioration in the credit worthiness of the insured party's ability to meet its financial obligations related to the contracts underlying financial guarantee insurance contracts the entity has issued.

Reference 1: http://www.xbrl.org/2003/role/exampleRef
-Topic 944
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-Name Accounting Standards Codification
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-Paragraph 34
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Name:	us-gaap_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationInsuredContractualPaymentsOutstandingInterest
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

This element represents the amount of insured contractual payments outstanding attributable to the principal of the underlying insured obligations for financial guarantee insurance contracts presented in the context of each surveillance grouping or category used by management to monitor and track potential deterioration in the credit worthiness of the insured party's ability to meet its financial obligations related to the contracts underlying financial guarantee insurance contracts the entity has issued.

Reference 1: http://www.xbrl.org/2003/role/exampleRef
-Topic 944
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Name:	us-gaap_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationInsuredContractualPaymentsOutstandingPrincipal
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

This element represents the net claim liability related to financial guarantee insurance contracts presented in the context of each surveillance grouping or category used by management to monitor and track potential deterioration in the credit worthiness of the insured party's ability to meet its financial obligations related to the contracts underlying financial guarantee insurance contracts the entity has issued.

Reference 1: http://www.xbrl.org/2003/role/exampleRef
-Topic 944
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-Topic 944
-SubTopic 40
-Name Accounting Standards Codification
-Section 50
-Paragraph 9
-Subparagraph (a)(5)(vii)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480081/944-40-50-9

Name:	us-gaap_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationNetClaimLiability
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- References

No definition available.

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

This element represents the number of issued and outstanding contracts for each surveillance grouping or category used by management to monitor and track potential deterioration in the credit worthiness of the insured party's ability to meet its financial obligations related to the contracts underlying financial guarantee insurance contracts the entity has issued.

Reference 1: http://www.xbrl.org/2003/role/exampleRef
-Topic 944
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Name:	us-gaap_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationNumberOfContractsIssuedAndOutstanding
Namespace Prefix:	us-gaap_
Data Type:	xbrli:decimalItemType
Balance Type:	na
Period Type:	instant

This element represents the gross potential recoveries against the gross claim liability for financial guarantee insurance contracts presented in the context of each surveillance grouping or category used by management to monitor and track potential deterioration in the credit worthiness of the insured party's ability to meet its financial obligations related to the contracts underlying financial guarantee insurance contracts the entity has issued.

Reference 1: http://www.xbrl.org/2003/role/exampleRef
-Topic 944
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-Paragraph 34
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-Topic 944
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-Section 50
-Paragraph 9
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-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480081/944-40-50-9

Name:	us-gaap_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationReductionsOfGrossClaimLiabilityGrossPotentialRecoveries
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

This element represents the net discount consisting of the difference between the discount related to the claim liability for and potential recoveries thereof related to financial guarantee insurance contracts presented in the context of each surveillance grouping or category used by management to monitor and track potential deterioration in the credit worthiness of the insured party's ability to meet its financial obligations related to the contracts underlying financial guarantee insurance contracts the entity has issued.

Reference 1: http://www.xbrl.org/2003/role/exampleRef
-Topic 944
-SubTopic 40
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-Section 55
-Paragraph 34
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Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 944
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-Section 50
-Paragraph 9
-Subparagraph (a)(5)(vi)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480081/944-40-50-9

Name:	us-gaap_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationReductionsOfGrossClaimLiabilityNetDiscount
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

This element represents the claim liability, reported on or included in the statement of financial position, related to financial guarantee insurance contracts and presented in the schedule of insured financial obligations with credit deterioration in the context of each surveillance grouping or category used by management to monitor and track potential deterioration in the credit worthiness of the insured party's ability to meet its financial obligations related to the contracts underlying financial guarantee insurance contracts the entity has issued.

Name:	us-gaap_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationStatementOfFinancialPositionClaimLiability
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

This element represents the amount of unearned premium revenue, reported on or included in the statement of financial position, related to financial guarantee insurance contracts and presented in the schedule of insured financial obligations with credit deterioration in the context of each surveillance grouping or category used by management to monitor and track potential deterioration in the credit worthiness of the insured party's ability to meet its financial obligations related to the contracts underlying financial guarantee insurance contracts the entity has issued.

Reference 1: http://www.xbrl.org/2003/role/exampleRef
-Topic 944
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-Name Accounting Standards Codification
-Section 55
-Paragraph 34
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480046/944-40-55-34

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-Section 50
-Paragraph 9
-Subparagraph (a)(5)(ix)
-Publisher FASB
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Name:	us-gaap_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationStatementOfFinancialPositionUnearnedPremiumRevenue
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

Name:	us-gaap_InternalCreditAssessmentAxis=us-gaap_InternalNoninvestmentGradeMember
Namespace Prefix:
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Namespace Prefix:
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Period Type:

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Namespace Prefix:
Data Type:	na
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