Financial Guaranty Insurance Losses - Narrative (Details) (USD $)	3 Months Ended	12 Months Ended
Mar. 31, 2015	Dec. 31, 2014
Guarantor Obligations [Line Items]
Financial guaranty insurance expected LAE reserve, amount	10,000,000	12,000,000
Guaranteed investment contract, aggregate accreted balance	2,100,000,000
Assets of GIC issuers, aggregate accreted principal	3,300,000,000
Assets of GIC issuers, aggregate market value	3,200,000,000
Variable rate demand obligations
Guarantor Obligations [Line Items]
Rate basis for bank bond rate	prime rate
Minimum [Member]
Guarantor Obligations [Line Items]
Weighted average risk-free rates for U.S. dollar denominated financial guaranty insurance obligations	0.00%	0.00%
Minimum [Member] \| Variable rate demand obligations
Guarantor Obligations [Line Items]
Bank bond rate (as a percent)	2.00%
Threshold period of bonds held by bank for right of accelerated repayment (in days)	90 days
Maximum [Member]
Guarantor Obligations [Line Items]
Weighted average risk-free rates for U.S. dollar denominated financial guaranty insurance obligations	2.89%	2.95%
Maximum [Member] \| Variable rate demand obligations
Guarantor Obligations [Line Items]
Bank bond rate (as a percent)	3.00%
Bank bond capped rate (as a percent)	25.00%
Threshold period of bonds held by bank for right of accelerated repayment (in days)	180 days
Cash \| Minimum [Member]
Guarantor Obligations [Line Items]
Collateralization as a percentage of GIC balance	100.00%
Asset-backed Securities [Member] \| Maximum [Member]
Guarantor Obligations [Line Items]
Collateralization as a percentage of GIC balance	108.00%
Termination of Swap Obligation due to Rating Downgrade [Member] \| AGM [Member]
Guarantor Obligations [Line Items]
Maximum possible estimated termination payment on swap obligations after taking account the rating downgrade in January 2013	163,000,000
Termination of Swap Obligation due to Further Rating Downgrade [Member] \| AGM [Member]
Guarantor Obligations [Line Items]
Maximum possible estimated termination payment on swap obligations after taking account the rating downgrade in January 2013	416,000,000
Financial Guarantee [Member] \| AGM and AGC [Member]
Guarantor Obligations [Line Items]
Financial guarantee insurance contracts principal amount insured	5,900,000,000
Internal Credit, B Minus Rating [Member] \| Financial Guarantee [Member] \| AGM and AGC [Member]
Guarantor Obligations [Line Items]
Financial guarantee insurance contracts principal amount insured	300,000,000

Collateralization as a Percentage of GIC Balance

No definition available.

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Data Type:	num:percentItemType
Balance Type:	na
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- Definition

Represents the typical percentage points added to the reference rate on bonds insured by the entity and held by the bank.

+ References

No definition available.

+ Details

Name:	ago_FinancialGuarantyInsuranceContractsBankBondRate
Namespace Prefix:	ago_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Represents the typical maximum interest rate on bonds insured by the entity and held by the bank.

+ References

No definition available.

+ Details

Name:	ago_FinancialGuarantyInsuranceContractsBankBondRateCapped
Namespace Prefix:	ago_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

The reference rate for the variable interest rate typically accrued on bonds insured by the entity and held by the bank.

+ References

No definition available.

+ Details

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Namespace Prefix:	ago_
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Balance Type:	na
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- Definition

Represents the low end of the range of the period of time after which the bank has the right to demand accelerated repayment on bonds insured by the entity and held by the bank.

+ References

No definition available.

+ Details

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Namespace Prefix:	ago_
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- Definition

Guaranteed Investment Contract, Accreted Principal

+ References

No definition available.

+ Details

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

- Definition

Guaranteed Investment Contract, Aggregate Accreted Balance

+ References

No definition available.

+ Details

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Namespace Prefix:	ago_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
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- Definition

Guaranteed Investment Contract, Aggregate Market Value

+ References

No definition available.

+ Details

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

- Definition

Guarantor Obligations, Face Amount Outstanding, Net of Reinsurance Recoverable

+ References

No definition available.

+ Details

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

- Definition

This element represents the weighted average risk free rate used to discount the claim liability related to financial guarantee insurance contracts.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 944

-SubTopic 40

-Section 50

-Paragraph 9

-Subparagraph (a)(1)

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

+ 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 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

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Balance Type:	credit
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- Details

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

- Definition

The estimated maximum amount of possible loss from the contingency.

+ References

Reference 1: 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=25496072&loc=d3e14435-108349

Reference 2: 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=25496072&loc=d3e14557-108349

+ Details

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- Details

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- Details

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- Details

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- Details

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