Name:	ago_DeferredTaxAssetsRealizationAssessmentPeriodForWhichCumulativeOperatingIncomeConsidered
Namespace Prefix:	ago_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

[Line Items] for Income Taxes [Table]

No definition available.

Name:	ago_IncomeTaxesLineItems
Namespace Prefix:	ago_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

Percentage of domestic federal statutory tax rate applicable to pretax income (loss).

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 740
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SAB Topic 6.I.Fact.4)
-URI https://asc.fasb.org/extlink&oid=122134291&loc=d3e330036-122817

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 235
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.4-08(h)(2))
-URI https://asc.fasb.org/extlink&oid=120395691&loc=d3e23780-122690

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 740
-SubTopic 10
-Section 50
-Paragraph 12
-URI https://asc.fasb.org/extlink&oid=121826272&loc=d3e32687-109319

Name:	us-gaap_EffectiveIncomeTaxRateReconciliationAtFederalStatutoryIncomeTaxRate
Namespace Prefix:	us-gaap_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	duration

Amount of operating loss carryforward, before tax effects, available to reduce future taxable income under enacted tax laws.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 740
-SubTopic 10
-Section 50
-Paragraph 3
-Subparagraph (a)
-URI https://asc.fasb.org/extlink&oid=121826272&loc=d3e32559-109319

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

Amount of valuation allowance pertaining to the deferred tax asset representing potential future taxable deductions from tax credit carryforwards for which it is more likely than not that a tax benefit will not be realized.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 740
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)
-URI https://asc.fasb.org/extlink&oid=121826272&loc=d3e32537-109319

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

Amount of decrease in unrecognized tax benefits resulting from settlements with taxing authorities.

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 740
-SubTopic 10
-Section 50
-Paragraph 15A
-Subparagraph (a)(3)
-URI https://asc.fasb.org/extlink&oid=121826272&loc=SL6600010-109319

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

Amount of expense for interest on an underpayment of income taxes and penalties related to a tax position claimed or expected to be claimed in the tax return.

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 740
-SubTopic 10
-Section 50
-Paragraph 15
-Subparagraph (c)
-URI https://asc.fasb.org/extlink&oid=121826272&loc=d3e32718-109319

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

The total amount of unrecognized tax benefits that, if recognized, would affect the effective tax rate.

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 740
-SubTopic 10
-Section 50
-Paragraph 15A
-Subparagraph (b)
-URI https://asc.fasb.org/extlink&oid=121826272&loc=SL6600010-109319

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

Amount of increase (decrease) in the valuation allowance for a specified deferred tax asset.

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 740
-SubTopic 10
-Section 50
-Paragraph 2
-URI https://asc.fasb.org/extlink&oid=121826272&loc=d3e32537-109319

Name:	us-gaap_ValuationAllowanceDeferredTaxAssetChangeInAmount
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

Name:	us-gaap_BusinessAcquisitionAxis=ago_CIFGHoldingInc.Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Name:	us-gaap_IncomeTaxAuthorityAxis=country_GB
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Name:	us-gaap_IncomeTaxAuthorityAxis=country_FR
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: