Represents the period over which the conditional default rate on a pool of insured obligations is assumed to improve from the current level to an intermediate conditional default rate.

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

- Definition

Represents the period for which estimated default rates for a pool of insured obligations are calculated.

+ Details

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

- Definition

Represents the final conditional default rate on a pool of insured obligations as a percentage of the plateau conditional default rate.

+ Details

Name:	ago_FinalConditionalDefaultRateAsAPercentageOfPlateauConditionalDefaultRate
Namespace Prefix:	ago
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Represents the intermediate conditional default rate on a pool of insured obligations as a percentage of the plateau conditional default rate.

+ Details

Name:	ago_IntermediateConditionalDefaultRateAsAPercentageOfPlateauConditionalDefaultRate
Namespace Prefix:	ago
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

The period for which the intermediate conditional default rate on a pool of insured obligations is held constant.

+ Details

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

- Definition

Represents the liquidation rate used to estimate the number of insured obligations in the 30 - 59 days delinquent loan category that will default.

+ Details

Name:	ago_LiquidationRateFor30To59DaysDelinquentCategory
Namespace Prefix:	ago
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Represents the liquidation rate used to estimate the number of insured obligations in the 60 - 89 days delinquent loan category that will default.

+ Details

Name:	ago_LiquidationRateFor60To89DaysDelinquentCategory
Namespace Prefix:	ago
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Represents the liquidation rate used to estimate the number of insured obligations in the 90 days - bankruptcy delinquent loan category that will default.

+ Details

Name:	ago_LiquidationRateFor90ToBankruptcyDelinquentCategory
Namespace Prefix:	ago
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Represents the liquidation rate used to estimate the number of insured obligations in foreclosure that will default.

+ Details

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

- Definition

Represents the liquidation rate used to estimate the number of insured obligations related to real estate owned by the entity that will default.

+ Details

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

- Definition

Represents the present value of the final level of assumed lifetime losses on a pool of insured obligations as a percentage of principal balance.

+ Details

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

- Definition

Represents the period over which the loss severity is assumed to decline to a final level.

+ Details

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

- Definition

Represents the minimum percentage of mortgage loans originally included in the pool of insured obligations with an option to make a minimum payment that has the potential to negatively amortize the loan required to categorize a transaction as an Option ARM.

+ Details

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

- Definition

Represents the period after which defaults projected to occur on a pool of insured obligations represent defaults that can be attributed to obligations that are currently performing.

+ Details

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

- Definition

Represents the period over which the application of the constant conditional default rate would produce approximately the amount of defaults calculated for each delinquency category of insured financial obligations.

+ Details

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

- Definition

Represents the period over which defaults projected to occur on a pool of insured obligations represent defaults that can be attributed to obligations that are currently delinquent or in foreclosure.

+ Details

Name:	ago_PeriodForDefaultsFromDelinquentOrForeclosureCategory
Namespace Prefix:	ago
Data Type:	xbrli:integerItemType
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.

+ Details

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

Financial Guaranty Insurance Contracts (Details 6)	9 Months Ended
Financial Guaranty Insurance Contracts (Details 6)	Sep. 30, 2011 Total first lien Y M transaction loan	Sep. 30, 2011 Alt-A first lien, Option ARM and Prime	Jun. 30, 2011 Alt-A first lien, Option ARM and Prime	Mar. 31, 2011 Alt-A first lien, Option ARM and Prime	Dec. 31, 2010 Alt-A first lien, Option ARM and Prime	Sep. 30, 2011 Subprime	Jun. 30, 2011 Subprime	Mar. 31, 2011 Subprime	Dec. 31, 2010 Subprime
Financial guarantee obligations
Minimum percentage for categorizing an Option ARM (as a percent)	66.00%
Projected period of loan liquidations (in years)	2
Liquidation rate for 30 - 59 days delinquent category (as a percent)		50.00%	50.00%	50.00%	50.00%	45.00%	45.00%	45.00%	45.00%
Liquidation rate for 60 - 89 days delinquent category (as a percent)		65.00%	65.00%	65.00%	65.00%	65.00%	65.00%	65.00%	65.00%
Liquidation rate for 90 - bankruptcy delinquent category (as a percent)		75.00%	75.00%	75.00%	75.00%	70.00%	70.00%	70.00%	70.00%
Liquidation rate for foreclosure category (as a percent)		85.00%	85.00%	85.00%	85.00%	85.00%	85.00%	85.00%	85.00%
Liquidation rate for real estate owned (as a percent)		100.00%	100.00%	100.00%	100.00%	100.00%	100.00%	100.00%	100.00%
Period for constant conditional default rate (in months)	24
Period from plateau to intermediate conditional default rate (in months)	12
Intermediate conditional default rate as a percentage of plateau conditional default rate (as a percent)	15.00%
Period of constant intermediate conditional default rate (in months)	36
Final conditional default rate as a percentage of plateau conditional default rate (as a percent)	5.00%
Period for which estimated defaults are attributed to loans currently delinquent or in foreclosure (in months)	24
Period after which estimated defaults are attributed to currently performing borrowers (in months)	24
Period for decline in loss severity (in years)	2
Loss severity (as a percent)	40.00%