Condensed Consolidated Statements of Comprehensive Income (Loss) (Unaudited) (Parenthetical) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
Jun. 30, 2024	Jun. 30, 2023	Jun. 30, 2024	Jun. 30, 2023
Statement of Comprehensive Income [Abstract]
Investments with no credit impairment, tax	$ (4)	$ (6)	$ (9)	$ 8
Investments with credit impairment, tax	$ 1	$ (3)	$ 1	$ (2)

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