Represents the high end of the range of the weighted average risk free rate used to discount the claim liability related to financial guaranty insurance contracts.

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

- Definition

Represents the low end of the range of the weighted average risk free rate used to discount the claim liability related to financial guaranty insurance contracts.

+ Details

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

- Details

Name:	ago_FinancialGuarantyInsuranceContractsSelectedInformationAbstract
Namespace Prefix:	ago
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount of unearned premium revenue in the statement of financial position for financial guarantee insurance contracts before adjustment for prepaid losses.

+ Details

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

- Definition

Represents the development (including breaches of representations and warranties) and accretion of the discount in entity's net expected loss to be paid, excluding paid claims during the period.

+ Details

Name:	ago_NetExpectedLossDevelopment
Namespace Prefix:	ago
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Represents the entity's estimate of the present value of future claim payments, net of reinsurance and net of salvage and subrogation which includes the present value benefit of estimated recoveries for breaches of R&W, including the effect of consolidating VIEs.

+ Details

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

- Details

Name:	ago_PresentValueOfNetExpectedLossAndLossAdjustmentExpensesToBePaidRollForward
Namespace Prefix:	ago
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

This element represents the weighted average period to collect the premium receivable for financial guarantee insurance contracts where premium is received over the contract period.

+ References

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

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 163

-Paragraph 30

-Subparagraph a(3)

-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 310

-Section 50

-Paragraph 3

-Subparagraph (a)(5)

-URI http://asc.fasb.org/extlink&oid=6933075&loc=SL5748726-161290

+ Details

Name:	us-gaap_FinancialGuaranteeInsuranceContractsPremiumReceivedOverContractPeriodPremiumReceivableWeightedAverageCollectionPeriod
Namespace Prefix:	us-gaap
Data Type:	xbrli:decimalItemType
Balance Type:	na
Period Type:	duration

- Definition

This element represents the weighted average risk free rate used to discount the premiums expected to be collected for financial guarantee insurance contracts where the premium is to be received over the contract period.

+ References

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

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 163

-Paragraph 30

-Subparagraph a(3)

-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 310

-Section 50

-Paragraph 3

-Subparagraph (a)(4)

-URI http://asc.fasb.org/extlink&oid=6933075&loc=SL5748726-161290

+ Details

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Namespace Prefix:	us-gaap
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

The amount of expense accrued as of the reporting date related to risk management activities (not including reinsurance) for the mitigating of claim liabilities related to financial guarantee insurance contracts.

+ References

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

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 163

-Paragraph 30

-Subparagraph g(3)

-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 50

-Paragraph 9

-Subparagraph (b)(4)

-URI http://asc.fasb.org/extlink&oid=6933314&loc=SL5749324-161292

+ Details

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

- Definition

Total amount of payments made during the reporting period to settle insured claims and pay costs incurred in the claims settlement process.

+ References

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

-Publisher AICPA

-Name Statement of Position (SOP)

-Number 94-5

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

-Publisher SEC

-Name Financial Reporting Release (FRR)

-Number 403

-Paragraph 04

-Subparagraph b

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 944

-SubTopic 40

-Section 50

-Paragraph 3

-Subparagraph (c)

-URI http://asc.fasb.org/extlink&oid=6482485&loc=d3e14764-158437

+ Details

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

- Definition

The carrying amount as of the balance sheet date due the entity from (a) agents and insureds, (b) uncollected premiums and (c) others, net of the allowance for doubtful accounts.

+ References

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

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Article 7

-Section 03

-Paragraph 5

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 944

-SubTopic 210

-Section S99

-Paragraph 1

-Subparagraph (SX 210.7-03.(a),5)

-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

+ Details

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Balance Type:	debit
Period Type:	instant

Financial Guaranty Insurance Contracts (Details 3) (USD $)	1 Months Ended	3 Months Ended		6 Months Ended																											6 Months Ended	12 Months Ended
	Dec. 31, 2010	Jun. 30, 2011	Jun. 30, 2010	Jun. 30, 2011	Jun. 30, 2010	Jun. 30, 2011 Public finance Financial Guaranty	Jun. 30, 2010 Public finance Financial Guaranty	Jun. 30, 2011 Total first lien Financial Guaranty	Jun. 30, 2010 Total first lien Financial Guaranty	Jun. 30, 2011 U.S. RMBS Financial Guaranty	Jun. 30, 2010 U.S. RMBS Financial Guaranty	Jun. 30, 2011 Prime first lien Financial Guaranty	Jun. 30, 2010 Prime first lien Financial Guaranty	Jun. 30, 2011 Alt-A first lien Financial Guaranty	Jun. 30, 2010 Alt-A first lien Financial Guaranty	Jun. 30, 2011 Option ARM Financial Guaranty	Jun. 30, 2010 Option ARM Financial Guaranty	Jun. 30, 2011 Subprime Financial Guaranty	Jun. 30, 2010 Subprime Financial Guaranty	Jun. 30, 2011 CES Financial Guaranty	Jun. 30, 2010 CES Financial Guaranty	Jun. 30, 2011 HELOCs Financial Guaranty	Jun. 30, 2010 HELOCs Financial Guaranty	Jun. 30, 2011 Second lien Financial Guaranty	Jun. 30, 2010 Second lien Financial Guaranty	Jun. 30, 2011 Other Structured finance. Financial Guaranty	Jun. 30, 2010 Other Structured finance. Financial Guaranty	Jun. 30, 2011 Financial Guaranty	Jun. 30, 2010 Financial Guaranty	Dec. 31, 2010 Financial Guaranty	Jun. 30, 2011 Policies Paid in Installments Y	Dec. 31, 2010 Policies Paid in Installments Y	Jun. 30, 2011 Other	Dec. 31, 2010 Other
Selected Information for Policies Paid in Installments
Premiums receivable, net of ceding commissions payable	$ 1,167,587,000	$ 1,059,461,000	$ 1,311,300,000	$ 1,059,461,000	$ 1,311,300,000																										$ 1,059,500,000	$ 1,167,600,000
Gross deferred premium revenue																												6,412,600,000		7,108,600,000	2,384,400,000	2,933,600,000
Weighted-average risk-free rate used to discount premiums (as a percent)																															3.60%	3.50%
Weighted-average period of premiums receivable (in years)																															10.1	10.1
Present Value of Net Expected Loss to be paid Roll Forward by Sector
Net Expected Loss to be Paid, at the beginning of the period						88,900,000	130,900,000	909,900,000	827,100,000	160,800,000	819,800,000	1,400,000		184,400,000	204,400,000	523,700,000	545,200,000	200,400,000	77,500,000	56,600,000	199,300,000	(805,700,000)	(206,600,000)	(749,100,000)	(7,300,000)	159,100,000	115,700,000	408,800,000	1,066,400,000				2,100,000	2,100,000
Development Economic Loss						(13,500,000)	(8,100,000)	(88,000,000)	160,200,000	(92,900,000)	148,500,000	1,800,000	400,000	21,800,000	15,400,000	(88,400,000)	75,100,000	(23,200,000)	69,300,000	(109,600,000)	(40,400,000)	104,700,000	28,700,000	(4,900,000)	(11,700,000)	24,500,000	36,000,000	(81,900,000)	176,400,000
Less: (Paid) Recovered Losses		564,700,000	(206,000,000)	398,300,000	(417,000,000)	(9,200,000)	(34,200,000)	(222,700,000)	(80,400,000)	398,300,000	(436,100,000)			(38,600,000)	(29,000,000)	(168,400,000)	(49,100,000)	(15,700,000)	(2,300,000)	(41,700,000)	(39,900,000)	662,700,000	(315,800,000)	621,000,000	(355,700,000)	(3,000,000)	(5,600,000)	386,100,000	(475,900,000)
Net Expected Loss to be Paid, at the end of the period						66,200,000	88,600,000	599,200,000	906,900,000	466,200,000	532,200,000	3,200,000	400,000	167,600,000	190,800,000	266,900,000	571,200,000	161,500,000	144,500,000	(94,700,000)	119,000,000	(38,300,000)	(493,700,000)	(133,000,000)	(374,700,000)	180,600,000	146,100,000	713,000,000	766,900,000				2,100,000	2,100,000
Expected LAE for mitigating claim liabilities	$ 17,200,000	$ 15,900,000		$ 15,900,000
Weighted-average risk free rate to discount expected loss to be paid, low end of range (as a percent)	0.00%			0.00%
Weighted-average risk free rate to discount expected loss to be paid, high end of range (as a percent)	5.34%			5.00%