Contracts Accounted for as Credit Derivatives - Collateral Posting Requirements on Credit Derivatives (Details) - USD ($) $ in Millions	Sep. 30, 2017	Dec. 31, 2016
Credit Derivatives
Collateral posted	$ 18	$ 116
Collateral Debt Obligations, Collateral Requirement [Member]
Credit Derivatives
Collateral posting requirement.	502	690
Collateral Debt Obligations, Collateral Cap Negotiated [Member]
Credit Derivatives
Collateral posting requirement.	469	674
Collateral Debt Obligations, No Cap Negotiated [Member]
Credit Derivatives
Collateral posted	$ 18	$ 116

Aggregate notional amount specified by the derivative(s). Expressed as an absolute value.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher SEC
-Name Regulation S-X (SX)
-Number 210
-Article 12
-Section 13
-Sentence Column B

+ Details

Name:	invest_DerivativeNotionalAmount
Namespace Prefix:	invest_
Data Type:	xbrli:monetaryItemType
Balance Type:	na
Period Type:	instant

- Definition

The aggregate fair value of assets that are already posted, at the end of the reporting period, as collateral for derivative instruments with credit-risk-related contingent features.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 4H
-Subparagraph (d)
-URI http://asc.fasb.org/extlink&oid=84234895&loc=SL5624258-113959

+ Details

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

- 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.

+ References

No definition available.

+ Details

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

- Details

Name:	us-gaap_UnderlyingAssetClassAxis=ago_CollateralDebtObligationsCreditDerivativeContractsGrossParwithCollateralPostingRequirementsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_UnderlyingAssetClassAxis=ago_CollateralDebtObligationsMaximumPostingRequirementMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_UnderlyingAssetClassAxis=ago_CollateralDebtObligationsCollateralPostedMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: