Related Party Transactions (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
Dec. 31, 2011
Related party transactions
Net revenues (expenses) from transactions with related parties	$ (2.6)
Net income (expense) \| Related party \| Maximum
Related party transactions
Net revenues (expenses) from transactions with related parties	$ (1.9)
Entities affiliated to Wilbur L. Ross, Jr. and funds under his control
Related party transactions
% of Ownership	10.90%
Wellington
Related party transactions
% of Ownership	9.60%

Revenues, net of returns and allowances, realized from sales and other transactions net of expenses recognized (excluding transactions that are eliminated in consolidated or combined financial statements) with related party during the period.

No definition available.

Name:	ago_RelatedPartyTransactionRevenuesExpensesFromTransactionsWithRelatedParty
Namespace Prefix:	ago_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration