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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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Namespace Prefix:	ago_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

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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Balance Type:	na
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X

- Details

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Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

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Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: