Outstanding Exposure - BIG Net Par Outstanding (Details) $ in Millions	Sep. 30, 2026 USD ($) risk	Dec. 31, 2025 USD ($) risk
Schedule of Insured Financial Obligations [Line Items]
Net par outstanding, credit derivative	$ 3,500	$ 3,800
Net	281,100	277,100
BIG
Schedule of Insured Financial Obligations [Line Items]
Net par outstanding, financial guaranty insurance	7,990	8,730
Net par outstanding, credit derivative	30	30
Net	$ 8,020	$ 8,760
Number of risks, financial guaranty insurance \| risk	186	195
Number of risks, credit derivative \| risk	7	7
Number of risks \| risk	193	202
BIG \| BIG 1
Schedule of Insured Financial Obligations [Line Items]
Net par outstanding, financial guaranty insurance	$ 3,110	$ 3,710
Net par outstanding, credit derivative	30	30
Net	$ 3,140	$ 3,740
Number of risks, financial guaranty insurance \| risk	80	87
Number of risks, credit derivative \| risk	3	3
Number of risks \| risk	83	90
BIG \| BIG 2
Schedule of Insured Financial Obligations [Line Items]
Net par outstanding, financial guaranty insurance	$ 3,820	$ 3,810
Net par outstanding, credit derivative	0	0
Net	$ 3,820	$ 3,810
Number of risks, financial guaranty insurance \| risk	14	13
Number of risks, credit derivative \| risk	1	1
Number of risks \| risk	15	14
BIG \| BIG 3
Schedule of Insured Financial Obligations [Line Items]
Net par outstanding, financial guaranty insurance	$ 1,060	$ 1,210
Net par outstanding, credit derivative	0	0
Net	$ 1,060	$ 1,210
Number of risks, financial guaranty insurance \| risk	92	95
Number of risks, credit derivative \| risk	3	3
Number of risks \| risk	95	98

Represents the number of unique credits in the insured portfolio accounted for as credit derivative.

No definition available.

Name:	ago_FinancialGuarantyContractsAccountedForAsCreditDerivativesNumberOfContracts
Namespace Prefix:	ago_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

- Definition

Represents the outstanding principal amounts of the underlying insured obligations that are accounted for as credit derivatives, net of protection purchased.

+ References

No definition available.

+ Details

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

- Definition

Represents the number of unique credits in the insured portfolio.

+ References

No definition available.

+ Details

Name:	ago_InsuredFinancialObligationsNumberofContracts
Namespace Prefix:	ago_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

- Definition

Represents the outstanding principal amounts of the underlying insured obligations for financial guaranty contracts, after netting out ceded contracts.

+ References

No definition available.

+ Details

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

- Definition

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.

+ References

No definition available.

+ Details

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

- Definition

[Line Items] for Schedule of Insured Financial Obligations [Table]

+ References

No definition available.

+ Details

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

- Definition

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.

+ References

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)(i)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480081/944-40-50-9

+ Details

Name:	us-gaap_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationNumberOfContractsIssuedAndOutstanding
Namespace Prefix:	us-gaap_
Data Type:	xbrli:decimalItemType
Balance Type:	na
Period Type:	instant

- Details

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Namespace Prefix:
Data Type:	na
Balance Type:
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- Details

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Namespace Prefix:
Data Type:	na
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- Details

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Namespace Prefix:
Data Type:	na
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- Details

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