Expected Losses to be Paid (Details 10) (USD $) In Millions, unless otherwise specified				9 Months Ended
Sep. 30, 2013	Dec. 31, 2012	Sep. 30, 2013 PUERTO RICO	Sep. 30, 2013 PUERTO RICO Sovereign Debt [Member]	Sep. 30, 2013 PUERTO RICO Structured finance	Sep. 30, 2013 PUERTO RICO Investment Grade [Member]	Sep. 30, 2013 PUERTO RICO Below Investment Grade [Member]
Guarantor Obligations [Line Items]
Insured Financial Obligations Outstanding Principal Amount	$ 474,667	$ 519,893	$ 5,497	$ 2,100	$ 2,900
Insured Financial Obligations, External Credit Rating, Percentage						92.00%	8.00%

Represents the outstanding principal amounts of the underlying insured obligations for financial guaranty contracts, net of ceded contracts.

No definition available.

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