Guarantor Obligations, Net Expected Loss Development After Recoveries for Representations and Warranties

No definition available.

Name:	ago_GuarantorObligationsNetExpectedLossDevelopmentAfterRecoveriesforRepresentationsandWarranties
Namespace Prefix:	ago_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Guarantor Obligations, Net Expected Loss Development After Recoveries for Representations and Warranties, Changes In Discount Rates

+ References

No definition available.

+ Details

Name:	ago_GuarantorObligationsNetExpectedLossDevelopmentAfterRecoveriesforRepresentationsandWarrantiesChangesInDiscountRates
Namespace Prefix:	ago_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Guarantor Obligations, Net Expected Loss to be Paid After Recoveries for Representations and Warranties, Discount Rate

+ References

No definition available.

+ Details

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

- Definition

Guarantor Obligations, Net Expected Loss to be Paid After Recoveries for Representations and Warranties, Weighted Average Discount Rate

+ References

No definition available.

+ Details

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

- Definition

Guarantor Obligations, Net Expected Loss to be Paid After Recoveries for Representations and Warranties

+ References

No definition available.

+ Details

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

- Definition

Guarantor Obligations, Projected Loss Assumption, Deferred Loan Balances to be Recovered, Percent

+ References

No definition available.

+ Details

Name:	ago_GuarantorObligationsProjectedLossAssumptionDeferredLoanBalancesToBeRecoveredPercent
Namespace Prefix:	ago_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

[Line Items] for Schedule of Expected Losses to be Paid [Table]

+ References

No definition available.

+ Details

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

- Details

Name:	us-gaap_UnderlyingAssetClassAxis=us-gaap_ResidentialMortgageBackedSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_StatementGeographicalAxis=country_US
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_RangeAxis=srt_MinimumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_RangeAxis=srt_MaximumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_MortgageLoansOnRealEstateDescriptionLoanCategoryAxis=us-gaap_FirstMortgageMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_RangeAxis=srt_WeightedAverageMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	ago_DeferredLoanRecoveryScenarioAxis=ago_BaseScenarioMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_MortgageLoansOnRealEstateDescriptionLoanCategoryAxis=us-gaap_SecondMortgageMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Expected Loss to be Paid (Recovered) - Net Economic Loss Development (Benefit) (Details) - USD ($) $ in Millions	3 Months Ended		12 Months Ended
	Mar. 31, 2026	Mar. 31, 2025	Dec. 31, 2025	Dec. 31, 2024
Schedule of Expected Losses to be Paid [Line Items]
Net Expected Loss to be Paid (Recovered)	$ 141	$ 150	$ 101	$ 106
RMBS losses	44	(15)
Changes in discount rates	(4)	5
RMBS \| United States
Schedule of Expected Losses to be Paid [Line Items]
Net Expected Loss to be Paid (Recovered)	(48)	(37)	$ (54)	$ (43)
RMBS losses	(2)	(3)
Changes in discount rates	$ 0	1
Weighted average discount rate	4.00%		3.84%
RMBS \| United States \| Minimum
Schedule of Expected Losses to be Paid [Line Items]
Range of discount rates	3.63%		3.45%
RMBS \| United States \| Maximum
Schedule of Expected Losses to be Paid [Line Items]
Range of discount rates	5.06%		4.97%
RMBS \| United States \| First Lien
Schedule of Expected Losses to be Paid [Line Items]
Net Expected Loss to be Paid (Recovered)	$ 92		$ 90
RMBS losses	$ 0	2
RMBS \| United States \| First Lien \| Weighted Average \| Base Scenario
Schedule of Expected Losses to be Paid [Line Items]
Percent of deferred loan balances to be recovered	50.00%		50.00%
RMBS \| United States \| Second Lien
Schedule of Expected Losses to be Paid [Line Items]
Net Expected Loss to be Paid (Recovered)	$ (140)		$ (144)
RMBS losses	$ (2)	$ (5)
Percent of deferred loan balances to be recovered	2.00%
RMBS \| United States \| Second Lien \| Weighted Average \| Base Scenario
Schedule of Expected Losses to be Paid [Line Items]
Percent of deferred loan balances to be recovered	60.00%		60.00%