Commutation Agreement Exposure, Percent

No definition available.

Name:	ago_CommutationAgreementExposurePercent
Namespace Prefix:	ago_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Derivative, Gain (Loss) On Reinsurance Derivative

+ References

No definition available.

+ Details

Name:	ago_DerivativeGainLossOnReinsuranceDerivative
Namespace Prefix:	ago_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Represents the gains (losses) from commutations to recapture business previously ceded to other insurers.

+ References

No definition available.

+ Details

Name:	ago_GainLossFromCommutations
Namespace Prefix:	ago_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Change during the period in the net amount of insured risk resulting from commutations to recapture business previously ceded to other insurers.

+ References

No definition available.

+ Details

Name:	ago_IncreaseDecreaseInNetParDueToCommutations
Namespace Prefix:	ago_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Represents the amount of losses, over and above the minimum losses required for the activation or attachment of excess of loss reinsurance facility, which are covered by the facility.

+ References

No definition available.

+ Details

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

- Definition

Represents the remaining amount of losses which will be assumed by the reinsurers under the excess of loss reinsurance facility.

+ References

No definition available.

+ Details

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

- Definition

Represents the minimum amount of net losses, net of other reinsurance and recoveries, required for the activation or attachment of excess of loss reinsurance facility.

+ References

No definition available.

+ Details

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

- Definition

Reinsurance Retention Policy, Reinsured Risk, Potential Payments Due to Third Parties, Amount

+ References

No definition available.

+ Details

Name:	ago_ReinsuranceRetentionPolicyReinsuredRiskPotentialPaymentsDuetoThirdPartiesAmount
Namespace Prefix:	ago_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Amount of debt securities categorized neither as held-to-maturity nor trading.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 25
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=75030427&loc=d3e22054-111558

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=82896770&loc=d3e26610-111562

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (aa)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27161-111563

Reference 4: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 320
-Section 50
-Paragraph 2
-Subparagraph (b)-(g)
-URI http://asc.fasb.org/extlink&oid=75019621&loc=d3e62557-112803

+ Details

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

- Definition

This item represents a non-contingent liability for the fair value of an obligation to stand ready to perform over the term of a guaranty issued in the event that specified triggering events or conditions occur.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 460
-SubTopic 10
-Section 25
-Paragraph 4
-URI http://asc.fasb.org/extlink&oid=82913062&loc=d3e11049-110243

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03.15)
-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03.15)
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

Reference 4: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 460
-SubTopic 10
-Section 25
-Paragraph 3
-URI http://asc.fasb.org/extlink&oid=82913062&loc=d3e11019-110243

+ Details

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

- Definition

Cash payments made for reinsurance during the current period.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 25
-Subparagraph (g)
-URI http://asc.fasb.org/extlink&oid=98514028&loc=d3e3536-108585

+ Details

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

- Definition

Amount of risk reinsured in excess of stated retention per company policy.

+ References

No definition available.

+ Details

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

- Details

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

- Details

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

- Details

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

- Details

Name:	us-gaap_CededCreditRiskAxis=ago_NationalPublicFinanceGuaranteeCorporationMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_CededCreditRiskAxis=ago_AmbacAssuranceCorporationMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_CededCreditRiskAxis=ago_OtherReinsurerMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_CededCreditRiskAxis=ago_FGICUKLimitedMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_ReinsurancePolicyTypeAxis=ago_ExcessofLossReinsuranceFacilityMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

Name:	us-gaap_CollateralAxis=us-gaap_UncollateralizedMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_CreditRatingAMBestAxis=us-gaap_AMBestAMinusRatingMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_CreditRatingStandardPoorsAxis=us-gaap_StandardPoorsARatingMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Reinsurance and Other Monoline Exposures - Narrative (Details) - USD ($)		3 Months Ended			6 Months Ended		12 Months Ended
	Jan. 01, 2016	Jun. 30, 2017	Mar. 31, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016	Dec. 31, 2016
Ceded Credit Risk [Line Items]
Derivative losses on reinsurance derivatives		$ 33,000		$ 1,400,000	$ 600,000	$ 1,700,000
Fixed-maturity securities		$ 11,183,000,000			$ 11,183,000,000		$ 10,823,000,000
Net par due to commutations			$ 1,000,000,000
Commutation agreement exposure, percent (over)		97.00%			97.00%
Gain recorded to other income from commutations		$ 0	$ 73,000,000	$ 0		$ 0
Assured Guaranty Re [Member]
Ceded Credit Risk [Line Items]
Amounts could be required to pay if third party exercised right to recapture business					$ 51,000,000
AGC [Member]
Ceded Credit Risk [Line Items]
Amounts could be required to pay if third party exercised right to recapture business					17,000,000
Fixed Maturities [Member]
Ceded Credit Risk [Line Items]
Fixed-maturity securities		10,505,000,000			10,505,000,000		10,233,000,000
Fixed Maturities [Member] \| National [Member]
Ceded Credit Risk [Line Items]
Fixed-maturity securities		103,000,000			103,000,000
Fixed Maturities [Member] \| Ambac [Member]
Ceded Credit Risk [Line Items]
Fixed-maturity securities		69,000,000			69,000,000
Fixed Maturities [Member] \| Other [Member]
Ceded Credit Risk [Line Items]
Fixed-maturity securities		8,000,000			8,000,000
Fixed Maturities [Member] \| FGIC UK Limited [Member]
Ceded Credit Risk [Line Items]
Fixed-maturity securities		139,000,000			139,000,000
Excess of Loss Reinsurance Facility [Member] \| AGM, AGC and MAC [Member]
Ceded Credit Risk [Line Items]
Reinsurance retention policy, excess retention, amount reinsured	$ 360,000,000
Minimum net losses required for attachment of excess of loss reinsurance facility	1,250,000,000.00
Amount of losses covered under the facility	400,000,000
Remaining amount of losses covered under the facility	$ 40,000,000
Premiums paid during the period					9,000,000		$ 9,000,000
Uncollateralized [Member] \| AM Best, A- Rating [Member]
Ceded Credit Risk [Line Items]
Guaranty Liabilities		5,000,000			5,000,000
Uncollateralized [Member] \| Standard & Poor's, A Rating [Member]
Ceded Credit Risk [Line Items]
Guaranty Liabilities		$ 5,000,000			$ 5,000,000