Represents the quoted price of credit derivative contracts.

No definition available.

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Namespace Prefix:	ago_
Data Type:	num:percentItemType
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Period Type:	instant

- Definition

Fair value before effect of credit spread as of the balance sheet date of credit risk derivative assets, net of credit risk derivative liabilities, which includes all such derivative instruments in hedging and nonhedging relationships that are recognized on the balance sheet.

+ References

No definition available.

+ Details

Name:	ago_CreditRiskDerivativesAtFairValueBeforeEffectOfCreditSpreadNet
Namespace Prefix:	ago_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- 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

Name:	us-gaap_CreditDerivativesLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Fair value of credit risk derivative asset after deduction of credit risk derivative liability.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 4K
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=121590274&loc=SL5708775-113959

+ Details

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

- Definition

Amount of credit risk valuation adjustment to derivative liabilities to properly reflect the credit quality of the entity.

+ References

No definition available.

+ Details

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

- Details

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Namespace Prefix:
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Balance Type:
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- Details

Name:	dei_LegalEntityAxis=ago_AssuredGuarantyCorpMember
Namespace Prefix:
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- Details

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Contracts Accounted for as Credit Derivatives - CDS Spread and Components of Credit Derivative Assets (Liabilities) (Details) - USD ($) $ in Millions	Sep. 30, 2020	Jun. 30, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Dec. 31, 2018
Credit Derivatives
Fair value of credit derivatives before effect of AGC credit spread	$ (356)		$ (261)
Plus: Effect of AGC credit spread	197		76
Net fair value of credit derivatives	$ (159)		$ (185)
Five-year CDS spread \| AGC
Credit Derivatives
Quoted price of CDS contract (as a percent)	1.40%	1.59%	0.41%	0.56%	0.56%	1.10%
One-year CDS spread \| AGC
Credit Derivatives
Quoted price of CDS contract (as a percent)	0.27%	0.32%	0.09%	0.14%	0.13%	0.22%