Fair Value Measurement - Fair Value Level 3 Rollforward Recurring Basis (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
Mar. 31, 2020	Mar. 31, 2019	Sep. 30, 2019
FG VIEs' assets, at fair value [Member]
Fair Value Level 3 Rollforward
Fair value at beginning of period	$ 442	$ 569	$ 569
Total Pretax Realized And Unrealized Gains (Losses) Recorded As Assets [Abstract]
Net Income (loss)	(37)	17
Other comprehensive income (loss)	0	0
Purchases	0	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales	0
Settlements	(37)	(26)
Transfers Into Level 3	0
Fair value at end of period	368	560
Change in unrealized gains/(losses) related to financial instruments held	(35)	20
Assets of Consolidated Investment Vehicles [Member]
Fair Value Level 3 Rollforward
Fair value at beginning of period	64
Total Pretax Realized And Unrealized Gains (Losses) Recorded As Assets [Abstract]
Net Income (loss)	1
Other comprehensive income (loss)	0
Purchases	45
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales	0
Settlements	(14)
Transfers Into Level 3	0
Fair value at end of period	96
Change in unrealized gains/(losses) related to financial instruments held	1
Other Assets and Other Invested Assets [Member]
Fair Value Level 3 Rollforward
Fair value at beginning of period	55	77	77
Total Pretax Realized And Unrealized Gains (Losses) Recorded As Assets [Abstract]
Net Income (loss)	48	(9)
Other comprehensive income (loss)	0	0
Purchases	0	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales	0
Settlements	0	0
Transfers Into Level 3	0
Fair value at end of period	103	68
Change in unrealized gains/(losses) related to financial instruments held	48	(9)
Obligations of state and political subdivisions [Member] \| Fixed Maturities [Member]
Fair Value Level 3 Rollforward
Fair value at beginning of period	107	99	99
Total Pretax Realized And Unrealized Gains (Losses) Recorded As Assets [Abstract]
Net Income (loss)	1	1
Other comprehensive income (loss)	(21)	5
Purchases	0	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales	0
Settlements	(1)	(1)
Transfers Into Level 3	0
Fair value at end of period	86	104
Corporate securities [Member] \| Fixed Maturities [Member]
Fair Value Level 3 Rollforward
Fair value at beginning of period	41	56	56
Total Pretax Realized And Unrealized Gains (Losses) Recorded As Assets [Abstract]
Net Income (loss)	(7)	(11)
Other comprehensive income (loss)	(8)	3
Purchases	0	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales	0
Settlements	0	0
Transfers Into Level 3	0
Fair value at end of period	26	48
RMBS [Member] \| Fixed Maturities [Member]
Fair Value Level 3 Rollforward
Fair value at beginning of period	308	309	309
Total Pretax Realized And Unrealized Gains (Losses) Recorded As Assets [Abstract]
Net Income (loss)	3	6
Other comprehensive income (loss)	(47)	5
Purchases	0	11
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales	0
Settlements	(11)	(13)
Transfers Into Level 3	0
Fair value at end of period	253	318
Asset-backed Securities [Member] \| Fixed Maturities [Member]
Fair Value Level 3 Rollforward
Fair value at beginning of period	658	947	947
Total Pretax Realized And Unrealized Gains (Losses) Recorded As Assets [Abstract]
Net Income (loss)	7	14
Other comprehensive income (loss)	(59)	(9)
Purchases	0	10
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales	(2)
Settlements	(7)	(4)
Transfers Into Level 3	(1)
Fair value at end of period	596	958
Other Comprehensive Income (Loss) [Member] \| Other Assets and Other Invested Assets [Member]
Total Pretax Realized And Unrealized Gains (Losses) Recorded As Assets [Abstract]
Change in unrealized gains/(losses) related to financial instruments held		0
Other Comprehensive Income (Loss) [Member] \| Obligations of state and political subdivisions [Member] \| Fixed Maturities [Member]
Total Pretax Realized And Unrealized Gains (Losses) Recorded As Assets [Abstract]
Change in unrealized gains/(losses) related to financial instruments held	(21)	5
Other Comprehensive Income (Loss) [Member] \| Corporate securities [Member] \| Fixed Maturities [Member]
Total Pretax Realized And Unrealized Gains (Losses) Recorded As Assets [Abstract]
Change in unrealized gains/(losses) related to financial instruments held	(8)	3
Other Comprehensive Income (Loss) [Member] \| RMBS [Member] \| Fixed Maturities [Member]
Total Pretax Realized And Unrealized Gains (Losses) Recorded As Assets [Abstract]
Change in unrealized gains/(losses) related to financial instruments held	(46)	5
Other Comprehensive Income (Loss) [Member] \| Asset-backed Securities [Member] \| Fixed Maturities [Member]
Total Pretax Realized And Unrealized Gains (Losses) Recorded As Assets [Abstract]
Change in unrealized gains/(losses) related to financial instruments held	(58)	(8)
FG VIEs' liabilities with recourse, at fair value [Member]
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Purchases		0
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value at beginning of period	(367)	(517)	(517)
Total Pretax Realized And Unrealized Gains (Losses) Recorded As Liabilities [Abstract]
Net income (loss)		(11)	16
Other comprehensive income (loss)		0	13
Settlements		23	26
Fair value at end of period	(312)	(505)
Change in unrealized gains/(losses) included in earnings related to financial instruments	15	(11)
FG VIEs' liabilities with recourse, at fair value [Member] \| Other Comprehensive Income (Loss) [Member]
Total Pretax Realized And Unrealized Gains (Losses) Recorded As Liabilities [Abstract]
Change in unrealized gains/(losses) included in earnings related to financial instruments	13	0
Variable Interest Liabilities without Recourse [Member]
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Purchases		0
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value at beginning of period	(102)	(102)	(102)
Total Pretax Realized And Unrealized Gains (Losses) Recorded As Liabilities [Abstract]
Net income (loss)	11	(4)
Other comprehensive income (loss)	0	0
Settlements	9	2
Fair value at end of period	(82)	(104)
Change in unrealized gains/(losses) included in earnings related to financial instruments	11	(3)
Liabilities of Consolidated Investment Vehicles [Member]
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value at beginning of period	(481)
Total Pretax Realized And Unrealized Gains (Losses) Recorded As Liabilities [Abstract]
Net income (loss)	55
Other comprehensive income (loss)	0
Settlements	0
Fair value at end of period	(426)
Change in unrealized gains/(losses) included in earnings related to financial instruments	55
Credit Risk Contract [Member]
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Fair value at start of period	(185)	(207)	$ (207)
Total Pretax Realized And Unrealized Gains (Losses) Recorded As Net Derivative Asset (Liability) [Abstract]
Net income (loss)	(77)	(22)
Other comprehensive income (loss)	0	0
Purchases		0
Settlements	0	1
Fair value at end of period	(262)	(228)
Change in unrealized gains/(losses) related to financial instruments held as of end of year	$ (73)	$ (21)

Total Pretax Realized And Unrealized Gains (Losses) Recorded As Assets [Abstract]

No definition available.

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

- Definition

Total Pretax Realized And Unrealized Gains (Losses) Recorded As Liabilities [Abstract]

+ References

No definition available.

+ Details

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

- Definition

Total Pretax Realized And Unrealized Gains (Losses) Recorded As Net Derivative Asset (Liability) [Abstract]

+ References

No definition available.

+ Details

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

- Definition

Amount of unrealized gain (loss) recognized in income from asset measured at fair value on recurring basis using unobservable input (level 3) and still held.

+ References

+ Details

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

- Definition

A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

+ References

No definition available.

+ Details

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

- Definition

Amount of unrealized gain (loss) recognized in income from liability measured at fair value on recurring basis using unobservable input (level 3) and still held.

+ References

+ Details

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

- Definition

A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

+ References

No definition available.

+ Details

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

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

- Definition

Amount of gain (loss) recognized in income from asset measured at fair value on recurring basis using unobservable input (level 3).

+ References

+ Details

Name:	us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInEarnings1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Amount of gain (loss) recognized in other comprehensive income (OCI) from asset measured at fair value on recurring basis using unobservable input (level 3).

+ References

+ Details

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

- Definition

Amount of purchases of financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

+ References

+ Details

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

- Definition

Amount of sale of financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

+ References

+ Details

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

- Definition

Amount of settlement of financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

+ References

+ Details

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

- Definition

Amount of transfers of financial instrument classified as an asset out of level 3 of the fair value hierarchy.

+ References

+ Details

Name:	us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetTransfersOutOfLevel3
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Fair value of financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

+ References

+ Details

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

- Definition

Amount of gain (loss) recognized in income from liability measured at fair value on recurring basis using unobservable input (level 3).

+ References

+ Details

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

- Definition

Amount of gain (loss) recognized in other comprehensive income (OCI) from liability measured at fair value on recurring basis using unobservable input (level 3).

+ References

+ Details

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

- Definition

Amount of purchases of financial instrument classified as a liability measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

+ References

+ Details

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

- Definition

Amount of settlements of financial instrument classified as a liability measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

+ References

+ Details

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

- Definition

Fair value of financial instrument classified as a liability measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

+ References

+ Details

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

- Definition

Amount of unrealized gain (loss) recognized in income for derivative asset (liability) after deduction of derivative liability (asset), measured at fair value using unobservable input (level 3) and still held.

+ References

+ Details

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

- Definition

A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

+ References

No definition available.

+ Details

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

- Definition

Amount of gain (loss) recognized in income from derivative asset (liability) after deduction of derivative liability (asset), measured at fair value on recurring basis using unobservable input (level 3).

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)(1)
-URI http://asc.fasb.org/extlink&oid=117815213&loc=d3e19207-110258

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 3
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=117815213&loc=d3e19279-110258

+ Details

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

- Definition

Amount of gain (loss) recognized in other comprehensive income from derivative asset (liability) after deduction of derivative liability (asset), measured at fair value on recurring basis using unobservable input (level 3).

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 3
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=117815213&loc=d3e19279-110258

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)(1a)
-URI http://asc.fasb.org/extlink&oid=117815213&loc=d3e19207-110258

+ Details

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

- Definition

Amount of purchases of financial instrument classified as a derivative asset (liability) after deduction of derivative liability (asset), measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 3
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=117815213&loc=d3e19279-110258

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)(2)
-URI http://asc.fasb.org/extlink&oid=117815213&loc=d3e19207-110258

+ Details

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

- Definition

Amount of settlements of financial instrument classified as a derivative asset (liability) after deduction of derivative liability, measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 3
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=117815213&loc=d3e19279-110258

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)(2)
-URI http://asc.fasb.org/extlink&oid=117815213&loc=d3e19207-110258

+ Details

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

- Definition

Fair value of financial instrument classified as derivative asset (liability) after deduction of derivative liability (asset), measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=117815213&loc=d3e19207-110258

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 3
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=117815213&loc=d3e19279-110258

+ Details

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

- Details

Name:	us-gaap_FairValueByAssetClassAxis=ago_VariableInterestEntityAssetsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FairValueByAssetClassAxis=ago_AssetsofConsolidatedInvestmentVehiclesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FairValueByAssetClassAxis=ago_OtherAssetsandOtherInvestedAssetsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_USStatesAndPoliticalSubdivisionsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FairValueByAssetClassAxis=us-gaap_FixedMaturitiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_CorporateDebtSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_ResidentialMortgageBackedSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_AssetBackedSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_OtherComprehensiveIncomeLocationAxis=us-gaap_OtherComprehensiveIncomeMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FairValueByLiabilityClassAxis=ago_VariableInterestLiabilitieswithRecourseMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FairValueByLiabilityClassAxis=ago_VariableInterestLiabilitieswithoutRecourseMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FairValueByLiabilityClassAxis=ago_LiabilitiesofConsolidatedInvestmentVehiclesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DerivativeInstrumentRiskAxis=us-gaap_CreditRiskContractMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: