Business and Basis of Presentation (Details)	Mar. 31, 2013 Company Security
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Financial guaranty contracts accounted for as credit derivatives, minimum number of credit events triggering obligations	1
Financial guaranty contracts accounted for as credit derivatives, minimum number of third party referenced securities or loans	1
Number of holding companies having public debt outstanding	2

Represents the minimum number of defined credit events, upon the occurrence of which the entity's obligation under financial guaranty contracts accounted for as credit derivatives is triggered.

No definition available.

Name:	ago_CreditDerivativesMinimumNumberOfEventsTriggeringGuarantyObligation
Namespace Prefix:	ago_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant