Business and Basis of Presentation (Details)	Sep. 30, 2017 Company
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Number of holding companies having outstanding public debt	2

Represents the number of holding companies in the entity's organizational structure having outstanding public debt.

No definition available.

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