Condensed Consolidated Statements of Comprehensive Income (Loss) (Unaudited) (Parenthetical) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Statement of Comprehensive Income [Abstract]
Investments with no credit impairment, tax	$ (8)	$ 7	$ (18)	$ 11
Investments with credit impairment, tax	$ (2)	$ 2	$ 3	$ (5)

Other Comprehensive Income (Loss), Available-for-sale Securities, Without Credit Impairment, Tax

No definition available.

Name:	ago_OtherComprehensiveIncomeLossAvailableforsaleSecuritiesWithoutCreditImpairmentTax
Namespace Prefix:	ago_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration