-- None. No documentation exists for this element. --

No definition available.

Name:	ago_CreditRiskDerivativesAtFairValueBeforeEffectOfCreditSpreadNet1Abstract
Namespace Prefix:	ago_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Fair Value, Option, Liabilities with Recourse, Unpaid Principal Balance

+ References

No definition available.

+ Details

Name:	ago_FairValueOptionLiabilitieswithRecourseUnpaidPrincipalBalance
Namespace Prefix:	ago_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Fair Value, Option, Loans Held as Liabilities, 90 Days or More Past Due, Aggregate Difference

+ References

No definition available.

+ Details

Name:	ago_FairValueOptionLoansHeldasLiabilities90DaysorMorePastDueAggregateDifference
Namespace Prefix:	ago_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Fair Value, Option, Loans Held as Liabilities, Aggregate Difference

+ References

No definition available.

+ Details

Name:	ago_FairValueOptionLoansHeldasLiabilitiesAggregateDifference
Namespace Prefix:	ago_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

The amount of the variable interest entity's revenues consolidated into the statement of income, including interest income, net realized and unrealized gains (losses) on assets, and other income, net of the amount of the variable interest entity's expenses consolidated into the statement of income, including interest expense, net realized and unrealized gains (losses) on liabilities with and without recourse, and other expenses.

+ References

No definition available.

+ Details

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

- Definition

Variable Interest Entity, Number of Entities

+ References

No definition available.

+ Details

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

- Definition

This item represents the estimated amount of gains or losses included in earnings during the period attributable to changes in instrument-specific credit risk arising from loans and other receivables held as assets.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 50
-Paragraph 30
-Subparagraph (c)(1)
-URI http://asc.fasb.org/extlink&oid=75031198&loc=d3e14172-108612

+ Details

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

- Definition

This item represents the aggregate fair value of loans held as assets that are 90 days or more past due for which the fair value option has been elected.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 50
-Paragraph 28
-Subparagraph (e)(1)
-URI http://asc.fasb.org/extlink&oid=75031198&loc=d3e14064-108612

+ Details

Name:	us-gaap_FairValueOptionLoansHeldAsAssets90DaysOrMorePastDue
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

For loans held as assets for which the fair value option has been elected, this item represents the difference between the aggregate fair value and the aggregate unpaid principal balance of those loans that are 90 days or more past due.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 50
-Paragraph 28
-Subparagraph (e)(3)
-URI http://asc.fasb.org/extlink&oid=75031198&loc=d3e14064-108612

+ Details

Name:	us-gaap_FairValueOptionLoansHeldAsAssets90DaysOrMorePastDueAggregateDifference
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 50
-Paragraph 28
-Subparagraph (e)(3)
-URI http://asc.fasb.org/extlink&oid=75031198&loc=d3e14064-108612

+ Details

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

- Definition

The reporting entity's maximum amount of exposure to loss as a result of its involvement with the Variable Interest Entity (VIE).

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 810
-SubTopic 10
-Section 50
-Paragraph 4
-Subparagraph b
-URI http://asc.fasb.org/extlink&oid=77990344&loc=d3e5728-111685

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 810
-SubTopic 10
-Section 50
-Paragraph 6
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=77990344&loc=d3e5747-111685

+ Details

Name:	us-gaap_VariableInterestEntityEntityMaximumLossExposureAmount
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
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_VariableInterestEntityLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Details

Name:	us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis=us-gaap_VariableInterestEntityPrimaryBeneficiaryAggregatedDisclosureMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis=us-gaap_VariableInterestEntityNotPrimaryBeneficiaryAggregatedDisclosureMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

Consolidated Variable Interest Entities - Narrative (Details)	3 Months Ended		9 Months Ended
	Sep. 30, 2017 USD ($) Entity	Sep. 30, 2016 USD ($) Entity	Sep. 30, 2017 USD ($) Entity	Sep. 30, 2016 USD ($) Entity	Dec. 31, 2016 USD ($) Entity	Dec. 31, 2015 Entity
Variable Interest Entity [Line Items]
Net fair value gains and losses on FG VIEs are expected to reverse to zero at maturity of the VIE debt	$ 0		$ 0
Credit Risk Derivatives at Fair Value before Effect of Credit Spread Net 1 [Abstract]
Fair value gains (losses) on FG VIEs	3,000,000	$ (11,000,000)	25,000,000	$ 11,000,000
Variable Interest Entity, Primary Beneficiary [Member]
Credit Risk Derivatives at Fair Value before Effect of Credit Spread Net 1 [Abstract]
Fair value gains (losses) on FG VIEs	$ 3,000,000	$ (11,000,000)	$ 25,000,000	$ 11,000,000
Number of VIE that did not require consolidation \| Entity	31	32	31	32	32	34
Variable Interest Entity, Not Primary Beneficiary, Aggregated Disclosure [Member]
Credit Risk Derivatives at Fair Value before Effect of Credit Spread Net 1 [Abstract]
Number of VIE that did not require consolidation \| Entity	520		520		600
Residential Mortgage Backed Securities and Other Insurance Products [Member] \| Variable Interest Entity, Primary Beneficiary [Member]
Credit Risk Derivatives at Fair Value before Effect of Credit Spread Net 1 [Abstract]
Total unpaid principal balance for the VIEs' assets that were over 90 days or more past due	$ 102,000,000		$ 102,000,000		$ 137,000,000
Fair Value, Option, Loans Held as Assets, Aggregate Difference	369,000,000		369,000,000
Difference between the aggregate unpaid principal and aggregate fair value of the VIEs' Assets					432,000,000
Change in the instrument specific credit risk of the VIEs' assets	8,000,000	$ 1,000,000	32,000,000	$ 36,000,000
Unpaid principal for FG VIEs’ liabilities with recourse	705,000,000		705,000,000		871,000,000
Fair Value, Option, Loans Held as Liabilities, Aggregate Difference	$ 75,000,000		$ 75,000,000
Unpaid principal for FG VIEs' liabilities with and without recourse					$ 109,000,000