Represents the number of plaintiffs who filed a consolidated putative class action lawsuit no longer naming the entity as defendant but describing some of the entity's activities.

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

- Definition

Represents the amount of the underlying obligation insured by financial guarantors including the entity, which is the subject matter of a putative class action.

+ Details

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

- Definition

Represents the minimum number of proceedings for recoveries of losses, or failure to obtain recoveries, to have a material impact on the entity's results of operations in a particular quarter or fiscal year.

+ Details

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

- Definition

Represents the number of non-class action lawsuits for which the entity requested dismissal and the dismissal was denied by the court.

+ Details

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

- Definition

Represents number of commutations disputed.

+ Details

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

- Definition

Represents the number of consolidated complaints.

+ Details

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

- Definition

Represents the number of consolidated complaints which added credit rating agencies as defendants in addition to financial guaranty insurers.

+ Details

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

- Definition

Represents the number of consolidated complaints ordered by the court to be filed by a group of plaintiffs.

+ Details

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

- Definition

Represents the number of credit rating agencies added as new defendants in the consolidated complaints filed.

+ Details

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

- Definition

Represents the number of non-class action lawsuits filed naming the entity among other defendants.

+ Details

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

- Definition

Represents the number of putative class action lawsuits that were filed in a federal court alleging federal antitrust violations in the municipal derivatives industry.

+ Details

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

- Definition

Represents the percentage of amount in segregated account payable in cash.

+ Details

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

- Definition

Represents the percentage of amount in segregated account payable in surplus notes issued.

+ Details

Name:	ago_PercentageOfSegregatedAccountPayableInNotesIssued
Namespace Prefix:	ago
Data Type:	num:percentItemType
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_LossContingenciesLineItems
Namespace Prefix:	us-gaap
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

The value (monetary amount) of the award the plaintiff seeks in the legal matter.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 450
-SubTopic 20
-Section 50
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=6952336&loc=d3e14326-108349

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 450
-SubTopic 20
-Section 50
-Paragraph 4
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=6952336&loc=d3e14435-108349

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Statement of Financial Accounting Standard (FAS)
-Number 5
-Paragraph 9
-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 4: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 450
-SubTopic 20
-Section 50
-Paragraph 9
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=6952336&loc=d3e14557-108349

Reference 5: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Statement of Financial Accounting Standard (FAS)
-Number 5
-Paragraph 11
-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 6: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Statement of Financial Accounting Standard (FAS)
-Number 5
-Paragraph 10
-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 7: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Statement of Financial Accounting Standard (FAS)
-Number 5
-Paragraph 12
-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.

+ Details

Name:	us-gaap_LossContingencyDamagesSoughtValue
Namespace Prefix:	us-gaap
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Reflects the estimated amount of loss from the specified contingency as of the balance sheet date.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Statement of Financial Accounting Standard (FAS)
-Number 5
-Paragraph 9, 10, 11, 12
-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 FASB
-Name Accounting Standards Codification
-Topic 450
-SubTopic 20
-Section 50
-Paragraph 4
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=6952336&loc=d3e14435-108349

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 450
-SubTopic 20
-Section 50
-Paragraph 9
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=6952336&loc=d3e14557-108349

+ Details

Name:	us-gaap_LossContingencyEstimateOfPossibleLoss
Namespace Prefix:	us-gaap
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

Commitments and Contingencies (Details) (USD $)	12 Months Ended	1 Months Ended		1 Months Ended			6 Months Ended	12 Months Ended	1 Months Ended	12 Months Ended	1 Months Ended		12 Months Ended
Commitments and Contingencies (Details) (USD $)	Dec. 31, 2008 Lawsuits	Mar. 31, 2010 AGM and AGC Litigation California Lawsuits	Oct. 13, 2010 AGM and AGC Litigation California Defendants Lawsuits	Aug. 31, 2008 AGM Litigation Alabama	Mar. 31, 2011 AGM Litigation New York	Mar. 29, 2011 AGM Litigation New York	Jun. 30, 2011 AG Re and AGC Litigation Wisconsin	Dec. 31, 2008 AGM and AGMH Lawsuits	Sep. 30, 2009 AGMH (Issuer)	Dec. 31, 2008 AGMH (Issuer) Lawsuits	May 31, 2010 AGM and AGUS Lawsuits	Apr. 30, 2010 AGM and AGUS Lawsuits	Dec. 31, 2009 AGM and AGUS Lawsuits	Dec. 31, 2011 Legal Proceedings Lawsuits
Commitments and Contingencies Legal Proceedings
Minimum number of proceedings for recovery effort outcomes to have potential material impact														1
Number of consolidated complaints ordered		1
Number of consolidated complaints			3
Number of consolidated complaints which added new defendants			2
Number of credit rating agencies added as defendants			3
Amount of sewer debt obligation related to putative class action				$ 3,200,000,000
Amount to be repaid to the MARM 2006-OA2 trust as per court order					7,200,000
Estimated unreimbursed claims						$ 144,000,000
Percentage of amount in segregated account payable in cash (as a percent)							25.00%
Percentage of amount in segregated account payable in surplus notes issued (as a percent)							75.00%
Number of commutations disputed							1
Number of putative class action lawsuits filed in federal court	9							5		4
Number of plaintiffs filing consolidated complaints									4
Number of non-class action lawsuits filed								5			5		6
Number of non-class action lawsuits for which dismissal was denied												11