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Outstanding Exposure - BIG Net Par Outstanding (Details) $ in Millions	Sep. 30, 2025 USD ($) risk	Dec. 31, 2024 USD ($) risk
Schedule of Insured Financial Obligations [Line Items]
Net par outstanding, credit derivative	$ 3,759	$ 4,212
Net	274,875	261,552
BIG
Schedule of Insured Financial Obligations [Line Items]
Net par outstanding, financial guaranty insurance	11,349	10,150
Net par outstanding, credit derivative	29	32
Net	$ 11,378	$ 10,182
Number of risks, financial guaranty insurance | risk	197	207
Number of risks, credit derivative | risk	7	7
Number of risks | risk	204	214
BIG | BIG 1
Schedule of Insured Financial Obligations [Line Items]
Net par outstanding, financial guaranty insurance	$ 7,226	$ 8,074
Net par outstanding, credit derivative	25	28
Net	$ 7,251	$ 8,102
Number of risks, financial guaranty insurance | risk	92	98
Number of risks, credit derivative | risk	3	3
Number of risks | risk	95	101
BIG | BIG 2
Schedule of Insured Financial Obligations [Line Items]
Net par outstanding, financial guaranty insurance	$ 2,893	$ 702
Net par outstanding, credit derivative	4	4
Net	$ 2,897	$ 706
Number of risks, financial guaranty insurance | risk	10	12
Number of risks, credit derivative | risk	1	1
Number of risks | risk	11	13
BIG | BIG 3
Schedule of Insured Financial Obligations [Line Items]
Net par outstanding, financial guaranty insurance	$ 1,230	$ 1,374
Net par outstanding, credit derivative	0	0
Net	$ 1,230	$ 1,374
Number of risks, financial guaranty insurance | risk	95	97
Number of risks, credit derivative | risk	3	3
Number of risks | risk	98	100

X

- Definition

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

+ References

No definition available.

+ Details

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Namespace Prefix:	ago_
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X

- 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

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X

- Definition

Represents the number of unique credits in the insured portfolio.

+ References

No definition available.

+ Details

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X

- 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

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Namespace Prefix:	ago_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
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X

- 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

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- Definition

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

+ References

No definition available.

+ Details

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X

- 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

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