Derecognized Deferred Rent And Lease Incentive Liabilities

No definition available.

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

- Definition

Represents the amount of payment on the termination of credit derivatives transactions which the defendant owes to plaintiff as per the calculation of plaintiff.

+ References

No definition available.

+ Details

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

- Definition

Represents the amount of payment on the termination of credit derivatives transactions which the plaintiff owes to defendant as per the calculation of defendant.

+ References

No definition available.

+ Details

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

- Definition

Gain Contingency, Allegations, Payment For Termination of Other Credit Derivative Transactions Owed by Plaintiff to Defendant As Per Calculation of Defendant

+ References

No definition available.

+ Details

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

- Definition

Represents the number of credit derivative transactions which the entity has improperly terminated as per the allegations of the plaintiff.

+ References

No definition available.

+ Details

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

- Definition

Represents the number of credit derivative transactions which the entity has improperly calculated termination payment as per the allegations of the plaintiff.

+ References

No definition available.

+ Details

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

- Definition

Operating Leases, Rented Area

+ References

No definition available.

+ Details

Name:	ago_OperatingLeasesRentedArea
Namespace Prefix:	ago_
Data Type:	num:areaItemType
Balance Type:	na
Period Type:	instant

- Definition

The amount or range of possible amounts of gain that could be realized upon the resolution of a contingency.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 450
-SubTopic 30
-Section 50
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=6397426&loc=d3e17499-108355

+ Details

Name:	us-gaap_GainContingencyUnrecordedAmount
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount of rent expense incurred for leased assets, including but not limited to, furniture and equipment, that is not directly or indirectly associated with the manufacture, sale or creation of a product or product line.

+ References

No definition available.

+ Details

Name:	us-gaap_LeaseAndRentalExpense
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Term of lessee's operating lease renewal, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 842
-SubTopic 20
-Section 50
-Paragraph 3
-Subparagraph (a)(3)
-URI http://asc.fasb.org/extlink&oid=77888426&loc=SL77918673-209980

+ Details

Name:	us-gaap_LesseeOperatingLeaseRenewalTerm
Namespace Prefix:	us-gaap_
Data Type:	xbrli:durationItemType
Balance Type:	na
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_LossContingenciesLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Present value of lessee's discounted obligation for lease payments from operating lease.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 842
-SubTopic 20
-Section 45
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=77888419&loc=SL77918627-209977

+ Details

Name:	us-gaap_OperatingLeaseLiability
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount of lessee's right to use underlying asset under operating lease.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 842
-SubTopic 20
-Section 45
-Paragraph 1
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=77888419&loc=SL77918627-209977

+ Details

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

- Details

Name:	dei_LegalEntityAxis=ago_AssuredGuarantyMunicipalCorpMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_LitigationCaseAxis=ago_LehmanBrothersInternationalEuropeVsAgFinancialProductsIncMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	dei_LegalEntityAxis=ago_AssuredGuarantyFinancialProductsIncMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_LossContingenciesByNatureOfContingencyAxis=us-gaap_GuaranteeObligationsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	dei_LegalEntityAxis=ago_LehmanBrothersInternationalEuropeMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_GainContingenciesByNatureAxis=us-gaap_PositiveOutcomeOfLitigationMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_LitigationStatusAxis=us-gaap_PendingLitigationMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_RangeAxis=srt_MinimumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_RangeAxis=srt_MaximumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_StatementGeographicalAxis=stpr_NY
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_StatementGeographicalAxis=ago_NewYork2Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	dei_LegalEntityAxis=ago_BlueMountainCapitalManagementLLCMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Commitments and Contingencies - Narrative (Details) $ in Millions	12 Months Ended
	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2019 USD ($) ft²	Jan. 01, 2019 USD ($)	Apr. 10, 2015 USD ($)	Nov. 28, 2011 USD ($) Transaction
Commitments and Contingencies Legal Proceedings
Operating lease liabilities			$ 130	$ 95
Operating lease assets			$ 100	69
Derecognized Deferred Rent And Lease Incentive Liabilities				$ 26
Rent expense	$ 9	$ 9
AGM [Member]
Commitments and Contingencies Legal Proceedings
Renewal term			5 years
LBIE vs. AG Financial Products [Member] \| AG Financial Products Inc. [Member] \| Guarantee Obligations [Member]
Commitments and Contingencies Legal Proceedings
Number of credit derivative transactions for which termination payment is alleged to be improperly calculated \| Transaction						9
LBIE vs. AG Financial Products [Member] \| Lehman Brothers International (Europe) [Member]
Commitments and Contingencies Legal Proceedings
Number of credit derivative transactions alleged to be improperly terminated \| Transaction						28
Positive Outcome of Litigation [Member] \| Pending Litigation [Member] \| LBIE vs. AG Financial Products [Member] \| AG Financial Products Inc. [Member]
Commitments and Contingencies Legal Proceedings
Termination payments which LBIE owes to AG Financial Products as per calculation of AG Financial Products						$ 4
Gain Contingency, Allegations, Payment For Termination of Other Credit Derivative Transactions Owed by Plaintiff to Defendant As Per Calculation of Defendant						25
Positive Outcome of Litigation [Member] \| Pending Litigation [Member] \| LBIE vs. AG Financial Products [Member] \| Lehman Brothers International (Europe) [Member]
Commitments and Contingencies Legal Proceedings
Termination payments which AG Financial Products owes to LBIE as per calculation of LBIE						$ 1,400
Positive Outcome of Litigation [Member] \| Minimum [Member] \| LBIE vs. AG Financial Products [Member] \| Lehman Brothers International (Europe) [Member]
Commitments and Contingencies Legal Proceedings
Gain contingency, unrecorded amount					$ 200
Positive Outcome of Litigation [Member] \| Maximum [Member] \| LBIE vs. AG Financial Products [Member] \| Lehman Brothers International (Europe) [Member]
Commitments and Contingencies Legal Proceedings
Gain contingency, unrecorded amount					$ 500
New York [Member] \| AGM [Member]
Commitments and Contingencies Legal Proceedings
Lease space \| ft²			103,500
New York, Second Location [Member] \| BlueMountain Capital Management, LLC [Member]
Commitments and Contingencies Legal Proceedings
Lease space \| ft²			78,400