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Shareholders' Equity - Deferred Compensation (Details) - AGC - Supplemental Employee Retirement Plan | 12 Months Ended |
--- | --- | ---
Dec. 31, 2023 Unit shares | Dec. 31, 2022 Unit
Defined Benefit Plan Disclosure [Line Items] |  |
Number of AGL common shares represented by each unit in employer stock fund (in shares) | shares | 1 |
Number of units in AGC SERP (in units) | Unit | 74,309 | 74,309

X
- Definition Represents the number of units which the management officer elected to invest under the plan. + References No definition available. + Details
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Balance Type: | na
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X
- Definition Number of Shares of Common Stock Represented by Each Unit in Employer Stock Fund + References No definition available. + Details
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Namespace Prefix: | ago_
Data Type: | xbrli:sharesItemType
Balance Type: | na
Period Type: | duration

X
- Definition Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. + References No definition available. + Details
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- Details
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Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
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Namespace Prefix: |
Data Type: | na
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