Expected Loss to be Paid - Narrative Net Expected Recoveries from Breaches of R&W (Details) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
RMBS [Member] \| United States [Member]
Schedule of Expected Losses to be Paid [Line Items]
Future net R&W benefit	$ 13	$ 117

For guarantor obligations, represents the reduction to estimated losses from anticipated recoveries on defaulted insured obligations with breached representations and warranties.

No definition available.

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