Represents the amount cumulative strip par exposure that has been terminated on a consensual basis.

Name:	ago_CumulativeStripParExposureTerminated
Namespace Prefix:	ago
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Represents the amount to be added to the calculation for the minimum consolidated net worth required to be maintained per the terms of the credit agreement, in the event that the borrowing capacity has already been reduced.

+ Details

Name:	ago_LineOfCreditFacilityCovenantTermsAdditionalNetWorthRequirementUnderReducedBorrowingCapacity
Namespace Prefix:	ago
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Represents the threshold level of cumulative municipal losses, net of recoveries as a percent of the average annual debt service of the covered portfolio, which must be incurred by the entity to trigger availability of the credit facility.

+ Details

Name:	ago_LineOfCreditFacilityCovenantTermsCumulativeMunicipalLossesAsPercentOfDebtService
Namespace Prefix:	ago
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Represents the threshold amount of cumulative municipal losses, net of recoveries, which must be incurred by the entity to trigger availability of the credit facility.

+ Details

Name:	ago_LineOfCreditFacilityCovenantTermsCumulativeMunicipalLossesOnPortfolioNet
Namespace Prefix:	ago
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Represents the maximum debt-to-capital ratio which the entity is required to maintain per the terms of the credit agreement.

+ Details

Name:	ago_LineOfCreditFacilityCovenantTermsMaximumRatioOfIndebtednessToNetCapital
Namespace Prefix:	ago
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Represents the percentage of aggregate consolidated net income for a specified period to be added to the calculation for the minimum consolidated net worth required to be maintained per the terms of the credit agreement.

+ Details

Name:	ago_LineOfCreditFacilityCovenantTermsMinimumPercentageOfConsolidatedNetIncomeOrLoss
Namespace Prefix:	ago
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Represents the minimum net worth required to be maintained per the terms of the credit agreement, as a percentage of the consolidated net worth on a specified date.

+ Details

Name:	ago_LineOfCreditFacilityCovenantTermsMinimumPercentageOfNetWorthRequiredForCompliance
Namespace Prefix:	ago
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Represents the maximum borrowing capacity to which the credit facility may be reduced if a specified consolidated net worth is not maintained by the entity.

+ Details

Name:	ago_LineOfCreditFacilityReducedBorrowingCapacityIfNetWorthNotMaintained
Namespace Prefix:	ago
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Represents the threshold level of reduced borrowing capacity which will trigger the termination of the credit facility.

+ Details

Name:	ago_LineOfCreditFacilityReducedBorrowingCapacityToTriggerTermination
Namespace Prefix:	ago
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Represents the term of the credit facility.

+ Details

Name:	ago_LineOfCreditFacilityTerm
Namespace Prefix:	ago
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- Definition

Represents the liquidity claims on additional gross exposure.

+ Details

Name:	ago_LiquidityClaimsOnAdditionalGrossExposure
Namespace Prefix:	ago
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Represents the liquidity claims on gross exposure.

+ Details

Name:	ago_LiquidityClaimsOnGrossExposure
Namespace Prefix:	ago
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Represents the number of leveraged lease transactions due to 2008 downgrade.

+ Details

Name:	ago_NumberOfLeveragedLeaseTransactionsDueTo2008Downgrade
Namespace Prefix:	ago
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

- Definition

Represents the number of leveraged lease transactions due to 2011 downgrade.

+ Details

Name:	ago_NumberOfLeveragedLeaseTransactionsDueTo2011Downgrade
Namespace Prefix:	ago
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

- Definition

Represents the period after end of each fiscal quarter which quarterly financial statements needs to be submitted of both the entity and its subsidiaries.

+ Details

Name:	ago_PeriodAfterEndOfEachFiscalQuarterWhichQuarterlyFinancialStatementsNeedsToBeSubmitted
Namespace Prefix:	ago
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- Definition

Represents the period after end of each fiscal year which quarterly financial statements needs to be submitted of both the entity and its subsidiaries.

+ Details

Name:	ago_PeriodAfterEndOfEachFiscalYearWhichQuarterlyFinancialStatementsNeedsToBeSubmitted
Namespace Prefix:	ago
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- Definition

The total amount of the contingent obligation under letters of credit outstanding as of the reporting date.

+ Details

Name:	us-gaap_LettersOfCreditOutstandingAmount
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.

+ Details

Name:	us-gaap_LineOfCreditFacilityLineItems
Namespace Prefix:	us-gaap
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Maximum borrowing capacity under the credit facility without consideration of any current restrictions on the amount that could be borrowed or the amounts currently outstanding under the facility.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 129

-Paragraph 2, 4

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 02

-Paragraph 19, 22

-Article 5

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 210

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SX 210.5-02.19(b),22(b))

-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

+ Details

Name:	us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity
Namespace Prefix:	us-gaap
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

Long-Term Debt and Credit Facilities (Details 2) (USD $)	1 Months Ended						9 Months Ended			9 Months Ended
	Nov. 30, 2006 2006 Credit Facility Y	Sep. 30, 2011 2006 Credit Facility	Dec. 31, 2010 2006 Credit Facility	Nov. 06, 2006 2006 Credit Facility	Sep. 30, 2011 AGM 2009 Strip Coverage Facility transaction	Jul. 01, 2009 AGM 2009 Strip Coverage Facility	Sep. 30, 2011 2009 Strip Coverage Facility D	Sep. 30, 2011 AG Re Credit Facility	Jul. 31, 2007 AG Re Credit Facility	Sep. 30, 2011 AGM Credit Facility	Apr. 30, 2005 AGM Credit Facility
Credit facilities
Period after end of each fiscal quarter which quarterly financial statements needs to be submitted (in days)							60
Period after end of each fiscal year which quarterly financial statements needs to be submitted (in days)							115
Maximum credit facility				$ 300,000,000		$ 1,000,000,000			$ 200,000,000		$ 297,500,000
Term of revolving credit facility (in years)	5
Minimum net worth which is required to be maintained (as a percent)						75.00%
Maximum debt-to-capital ratio which is required to be maintained (as a percent)					30.00%
Outstanding letters of credit		2,900,000	2,900,000
Number of leveraged lease transactions due to 2008 downgrade					45
Liquidity claims on gross exposure					1,100,000,000
Number of leveraged lease transactions due to 2011 downgrade					26
Liquidity claims on additional gross exposure					1,000,000,000
Cumulative strip par exposure terminated					600,000,000
Reduced maximum credit facility for 2014 if consolidated net worth not maintained					750,000,000
Reduced commitment amount to trigger termination					984,500,000
Minimum percentage of aggregate consolidate net income (or loss) for July 1, 2009 through June 30, 2014 to be added to net worth requirement (as a percent)					25.00%
Amount to be added to net worth requirement if borrowing capacity is reduced					750,000,000
Threshold cumulative municipal losses to utilize credit facility								$ 260,000,000		$ 297,500,000
Threshold cumulative municipal losses as a percent of debt service to utilize credit facility (as a percent)								4.50%		5.00%