Represents the quoted price of credit derivative contracts.

No definition available.

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Namespace Prefix:	ago_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Fair value before effect of credit spread as of the balance sheet date of credit risk derivative assets, net of credit risk derivative liabilities, which includes all such derivative instruments in hedging and nonhedging relationships that are recognized on the balance sheet.

+ References

No definition available.

+ Details

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

- Definition

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.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 815
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
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-Topic 815
-SubTopic 10
-Name Accounting Standards Codification
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-Paragraph 4K
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-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480434/815-10-50-4K

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-Topic 815
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
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-URI https://asc.fasb.org/1943274/2147480434/815-10-50-4L

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-Topic 815
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+ Details

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

- Definition

Fair value of credit risk derivative asset after deduction of credit risk derivative liability.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 815
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 4K
-Subparagraph (c)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480434/815-10-50-4K

+ Details

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

- Definition

Amount of credit risk valuation adjustment to derivative liabilities to properly reflect the credit quality of the entity.

+ References

No definition available.

+ Details

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

- Details

Name:	us-gaap_DerivativeInstrumentRiskAxis=ago_CreditRiskContract5YearSpreadMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	dei_LegalEntityAxis=ago_AssuredGuarantyIncMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DerivativeInstrumentRiskAxis=ago_CreditRiskContract1YearSpreadMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Contracts Accounted for as Credit Derivatives - CDS Spread and Components of Credit Derivative Assets (Liabilities) (Details) - USD ($) $ in Millions	Sep. 30, 2025	Dec. 31, 2024	Sep. 30, 2024	Dec. 31, 2023
Credit Derivatives
Fair value of credit derivatives before effect of AG credit spread	$ (59)	$ (64)
Plus: Effect of AG credit spread	33	35
Net fair value of credit derivatives	$ (26)	$ (29)
Five-year CDS spread \| AGC
Credit Derivatives
Quoted price of CDS contract (as a percent)	0.70%	0.75%	0.65%	0.66%
One-year CDS spread \| AGC
Credit Derivatives
Quoted price of CDS contract (as a percent)	0.24%	0.25%	0.26%	0.23%