Related Party Transactions (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
Dec. 31, 2012
Related Party Transaction [Line Items]
Expenses from transactions with related party	$ (3.4)
WLR Funds [Member]
Related Party Transaction [Line Items]
Percentage of wwnership by related party	10.20%
Wellington Management Company LLP [Member]
Related Party Transaction [Line Items]
Percentage of wwnership by related party	8.60%
Maximum
Related Party Transaction [Line Items]
Expenses from transactions with related party	$ (2.0)

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