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Condensed Consolidated Statements of Comprehensive Income (Loss) (Unaudited) (Parenthetical) - USD ($) $ in Millions | 3 Months Ended
--- | ---
Mar. 31, 2025 | Mar. 31, 2024
Statement of Comprehensive Income [Abstract] |  |
Investments with no credit impairment, tax | $ 6 | $ (5)
Investments with credit impairment, tax | $ 5 | $ 0

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- Definition Other Comprehensive Income (Loss), Available-for-sale Securities, With Credit Impairment, Tax + References No definition available. + Details
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- Definition Other Comprehensive Income (Loss), Available-for-sale Securities, Without Credit Impairment, Tax + 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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- References No definition available. + Details
Name: | us-gaap_StatementOfIncomeAndComprehensiveIncomeAbstract
Namespace Prefix: | us-gaap_
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Balance Type: | na
Period Type: | duration