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Contracts Accounted for as Insurance - Selected Information for Policies Paid In Installments (Details) - USD ($) $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2025	Dec. 31, 2024	Sep. 30, 2024	Dec. 31, 2023
Financial Guarantee Insurance Premiums [Line Items]
Premiums receivable, net of commissions payable	$ 1,583	$ 1,551	$ 1,513	$ 1,468
Financial Guarantee Policies Paid in Installments
Financial Guarantee Insurance Premiums [Line Items]
Premiums receivable, net of commissions payable	1,582	1,550
Deferred premium revenue	$ 1,855	$ 1,901
Weighted-average risk-free rate used to discount premiums	2.60%	2.50%
Weighted-average period of premiums receivable (in years)	12 years	12 years 3 months 18 days

X

- Definition

[Line Items] for Financial Guarantee Insurance Premiums [Table]

+ References

No definition available.

+ Details

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

X

- Definition

Amount of the premium receivable for financial guarantee insurance contracts reported in the statement of financial position. This element primarily would consist of those financial guarantee insurance contracts which premiums are to be collected over the contract period, but may, on occasion, include premiums which are to be collected in full at inception, but which straddle a financial reporting period.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 944
-SubTopic 310
-Name Accounting Standards Codification
-Section 50
-Paragraph 3
-Subparagraph (c)(1)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147477363/944-310-50-3

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 944
-SubTopic 310
-Name Accounting Standards Codification
-Section 50
-Paragraph 3
-Subparagraph (c)(5)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147477363/944-310-50-3

+ Details

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

X

- Definition

Weighted average period over which premium receivable for financial guarantee insurance contracts is collected where premium is received over the contract period, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 944
-SubTopic 310
-Name Accounting Standards Codification
-Section 50
-Paragraph 3
-Subparagraph (a)(5)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147477363/944-310-50-3

+ Details

Name:	us-gaap_FinancialGuaranteeInsuranceContractsPremiumReceivedOverContractPeriodPremiumReceivableWeightedAverageCollectionPeriod1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

X

- Definition

Weighted-average risk-free rate used to discount amount of premium expected to be collected for financial guarantee insurance contract where premium is to be received over contract period.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 944
-SubTopic 310
-Name Accounting Standards Codification
-Section 50
-Paragraph 3
-Subparagraph (a)(4)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147477363/944-310-50-3

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	duration

X

- Definition

Amount of unearned premium revenue in the statement of financial position for financial guarantee insurance contracts where the premium is received over the contract period.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 944
-SubTopic 310
-Name Accounting Standards Codification
-Section 50
-Paragraph 3
-Subparagraph (a)(2)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147477363/944-310-50-3

+ Details

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

X

- Details

Name:	srt_ProductOrServiceAxis=ago_FinancialGuaranteePoliciesPaidinInstallmentsMember
Namespace Prefix:
Data Type:	na
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Period Type: