Financial Guaranty Insurance Contracts (Details 11) (USD $) In Millions, unless otherwise specified	Mar. 31, 2012	Dec. 31, 2011
Loss and LAE Reserve
Subtotal	$ 864.7	$ 671.3
Effect of consolidating FG VIEs	(63.1)	(61.6)
Total	801.6	609.7
Salvage and Subrogation Recoverable
Subtotal	616.6	585.2
Effect of consolidating FG VIEs	(292.2)	(258.1)
Total	324.4	327.1
Net
Subtotal	248.1	86.1
Effect of consolidating FG VIEs	229.1	196.5
Total	477.2	282.6
Financial Guaranty
Loss and LAE Reserve
Subtotal	862.8	669.4
Total	951.3
Salvage and Subrogation Recoverable
Subtotal	616.6	585.2
Net
Subtotal	246.2	84.2
Effect of consolidating FG VIEs	229.1	196.5
Total	475.3	280.7
Financial Guaranty \| U.S. RMBS
Loss and LAE Reserve
Subtotal	324.3	336.4
Salvage and Subrogation Recoverable
Subtotal	531.2	509.4
Net
Subtotal	(206.9)	(173.0)
Financial Guaranty \| Total first lien
Loss and LAE Reserve
Subtotal	260.2	264.1
Salvage and Subrogation Recoverable
Subtotal	204.9	196.0
Net
Subtotal	55.3	68.1
Financial Guaranty \| Prime first lien
Loss and LAE Reserve
Subtotal	1.6	1.2
Net
Subtotal	1.6	1.2
Financial Guaranty \| Alt-A first lien
Loss and LAE Reserve
Subtotal	59.9	69.8
Salvage and Subrogation Recoverable
Subtotal	57.1	55.4
Net
Subtotal	2.8	14.4
Financial Guaranty \| Option ARM
Loss and LAE Reserve
Subtotal	139.0	141.7
Salvage and Subrogation Recoverable
Subtotal	147.6	140.3
Net
Subtotal	(8.6)	1.4
Financial Guaranty \| Subprime
Loss and LAE Reserve
Subtotal	59.7	51.4
Salvage and Subrogation Recoverable
Subtotal	0.2	0.3
Net
Subtotal	59.5	51.1
Financial Guaranty \| Second lien
Loss and LAE Reserve
Subtotal	64.1	72.3
Salvage and Subrogation Recoverable
Subtotal	326.3	313.4
Net
Subtotal	(262.2)	(241.1)
Financial Guaranty \| CES
Loss and LAE Reserve
Subtotal	9.3	11.2
Salvage and Subrogation Recoverable
Subtotal	139.8	136.2
Net
Subtotal	(130.5)	(125.0)
Financial Guaranty \| HELOCs
Loss and LAE Reserve
Subtotal	54.8	61.1
Salvage and Subrogation Recoverable
Subtotal	186.5	177.2
Net
Subtotal	(131.7)	(116.1)
Financial Guaranty \| Structured Finance
Loss and LAE Reserve
Subtotal	176.2	233.0
Salvage and Subrogation Recoverable
Subtotal	9.8	5.9
Net
Subtotal	166.4	227.1
Financial Guaranty \| Public finance
Loss and LAE Reserve
Subtotal	362.3	100.0
Salvage and Subrogation Recoverable
Subtotal	75.6	69.9
Net
Subtotal	286.7	30.1
Financial Guaranty \| Sovereign debt of Greece
Net
Subtotal	275.5	32.6
Other
Loss and LAE Reserve
Subtotal	1.9	1.9
Net
Subtotal	1.9	1.9
Total	$ 1.9	$ 1.9

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

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

- Definition

Represents the effect on the aggregate amount of policy reserves (provided for future obligations including unpaid claims and claims adjustment expenses) and policy benefits (liability for future policy benefits) relating to consolidating VIEs.

+ References

No definition available.

+ Details

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

- Definition

Alternate concept name for the aggregate amount of policy reserves (provided for future obligations including unpaid claims and claims adjustment expenses) and policy benefits (liability for future policy benefits) as of the balance sheet date, net of reinsurance recoverable on unpaid losses.

+ References

No definition available.

+ Details

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

- Definition

Represents the reserve for losses and loss adjustment expenses, net of reinsurance recoverable on unpaid losses and net of salvage and subrogation recoverable.

+ References

No definition available.

+ Details

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

- Definition

Represents the effect on reserve for losses and loss adjustment expenses, net of reinsurance recoverable on unpaid losses and net of salvage and subrogation recoverable related to consolidating VIEs.

+ References

No definition available.

+ Details

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

- Definition

Represents the reserve for losses and loss adjustment expenses, net of reinsurance recoverable on unpaid losses, net of salvage and subrogation recoverable and before adjusting for the effect of consolidating VIEs.

+ References

No definition available.

+ Details

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

- Definition

+ References

No definition available.

+ Details

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

- Definition

Represents the effect on the salvage and subrogation recoverable related to consolidating VIEs.

+ References

No definition available.

+ Details

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

- Definition

The estimated amount as of the balance sheet date to be received by an insurer from the sale of property (usually damaged) on which the insurer has paid a total claim to the insured and has obtained title to the property and the right of an insurer to pursue any course of recovery of damages, in its name or in the name of the policyholder, against a third party who is liable for costs relating to an insured event that have been paid by the insurer, before adjusting for the effect of consolidating VIEs.

+ References

No definition available.

+ Details

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

- Details

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

- Definition

The estimated amount as of the balance sheet date to be received by an insurer from the sale of property (usually damaged) on which the insurer has paid a total claim to the insured and has obtained title to the property and the right of an insurer to pursue any course of recovery of damages, in its name or in the name of the policyholder, against a third party who is liable for costs relating to an insured event that have been paid by the insurer. Estimated salvage and subrogation is generally deducted from the liability for unpaid claims.

+ References

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

-Publisher AICPA

-Name Audit and Accounting Guide (AAG)

-Number AAG-PLI

-Paragraph 32

-IssueDate 2006-09-01

-Chapter 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 FASB

-Name Accounting Standards Codification

-Topic 944

-SubTopic 40

-Section 30

-Paragraph 2

-URI http://asc.fasb.org/extlink&oid=6482241&loc=d3e13779-158432

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 944

-SubTopic 40

-Section 30

-Paragraph 3

-URI http://asc.fasb.org/extlink&oid=6482241&loc=d3e13783-158432

Reference 4: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 60

-Paragraph 18

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