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Leases - ROU Assets and Liabilities (Details) - USD ($) $ in Millions | 12 Months Ended
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Dec. 31, 2024 | Dec. 31, 2023
Leases [Abstract] |  |
ROU assets | $ 60 | $ 71
Lease liabilities | $ 84 | $ 97
Weighted average discount rate | 2.65% | 2.60%
Weighted average remaining lease term | 7 years 1 month 6 days | 7 years 9 months 18 days

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- Definition Finance and Operating Lease, Weighted Average Discount Rate, Percent + References No definition available. + Details
Name: | ago_FinanceAndOperatingLeaseWeightedAverageDiscountRatePercent
Namespace Prefix: | ago_
Data Type: | dtr-types1:percentItemType
Balance Type: | na
Period Type: | instant

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- Definition Finance and Operating Lease, Weighted Average Remaining Lease Term + References No definition available. + Details
Name: | ago_FinanceAndOperatingLeaseWeightedAverageRemainingLeaseTerm
Namespace Prefix: | ago_
Data Type: | xbrli:durationItemType
Balance Type: | na
Period Type: | duration

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- Definition Finance and Operating Lease, Liability + References No definition available. + Details
Name: | ago_FinanceandOperatingLeaseLiability
Namespace Prefix: | ago_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- Definition Finance and Operating Lease, Right-of-Use Asset + References No definition available. + Details
Name: | ago_FinanceandOperatingLeaseRightofUseAsset
Namespace Prefix: | ago_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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- References No definition available. + Details
Name: | us-gaap_LeasesAbstract
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration