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Contracts Accounted for as Insurance - Gross Premium Receivable Net of Commissions on Assumed Business Roll Forward (Details) - USD ($) $ in Millions | 9 Months Ended
--- | ---
Sep. 30, 2025 | Sep. 30, 2024
Gross Premium Receivable Net of Ceding Commissions [Roll Forward] |  |
Beginning of year | $ 1,551 | $ 1,468
Less: Specialty insurance premium receivable | 1 | 1
Financial guaranty insurance premiums receivable | 1,550 | 1,467
New business and supplemental premiums, net of commissions | 214 | 278
Gross premiums received, net of commissions | (265) | (267)
Adjustments: |  |
Changes in the expected term and debt service assumptions | (23) | (28)
Accretion of discount, net of commissions on assumed business | 26 | 19
Foreign exchange gain (loss) on remeasurement | 81 | 43
Change in allowance for credit losses | (1) | 0
Financial guaranty insurance premiums receivable | 1,582 | 1,512
Specialty insurance premium receivable | 1 | 1
Ending balance | $ 1,583 | $ 1,513

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- Definition Financial Guarantee Insurance Contracts, Premium Receivable, Change In Allowance For Credit Loss + References No definition available. + Details
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Namespace Prefix: | ago_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- Definition Financial Guarantee Insurance Contracts, Premium Receivable, Excluding Other Non Financial Guarantee + References No definition available. + Details
Name: | ago_FinancialGuaranteeInsuranceContractsPremiumReceivableExcludingOtherNonFinancialGuarantee
Namespace Prefix: | ago_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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- Definition Financial Guarantee Insurance Contracts, Premium Receivable, Specialty Insurance + References No definition available. + Details
Name: | ago_FinancialGuaranteeInsuranceContractsPremiumReceivableSpecialtyInsurance
Namespace Prefix: | ago_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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- Definition Represents gains or losses due to foreign exchange rate changes that relate to installment premium receivables on financial guaranty insurance contracts denominated in currencies other than U.S. dollar. + References No definition available. + Details
Name: | ago_FinancialGuaranteeInsuranceContractsPremiumsReceivableAdjustmentForeignExchangeTranslation
Namespace Prefix: | ago_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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

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- Definition Amount of new business written on financial guarantee insurance contracts. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Topic 944 -SubTopic 310 -Name Accounting Standards Codification -Section 50 -Paragraph 3 -Subparagraph (c)(3) -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: | duration

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- Definition Amount of cash inflow from premiums received for premium receivable on financial guarantee insurance contracts. + References Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Topic 230 -SubTopic 10 -Name Accounting Standards Codification -Section 45 -Paragraph 17 -Subparagraph (f) -Publisher FASB -URI https://asc.fasb.org/1943274/2147482740/230-10-45-17 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Topic 944 -SubTopic 310 -Name Accounting Standards Codification -Section 50 -Paragraph 3 -Subparagraph (c)(2) -Publisher FASB -URI https://asc.fasb.org/1943274/2147477363/944-310-50-3 + Details
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Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- Definition A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period. + References No definition available. + Details
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Period Type: | duration

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- Definition Amount of accretion of the discount for premiums receivable on financial guarantee insurance contracts. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Topic 944 -SubTopic 310 -Name Accounting Standards Codification -Section 50 -Paragraph 3 -Subparagraph (c)(4)(iii) -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: | duration

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- References No definition available. + Details
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- Definition Amount of increase (decrease) in the premium receivable on financial guarantee insurance contracts from changes in the period of the contract. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Topic 944 -SubTopic 310 -Name Accounting Standards Codification -Section 50 -Paragraph 3 -Subparagraph (c)(4)(i) -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: | debit
Period Type: | duration