Condensed Consolidated Statements of Comprehensive Income (Loss) (Unaudited) (Parenthetical) - USD ($) $ in Millions	3 Months Ended
Mar. 31, 2024	Mar. 31, 2023
Statement of Comprehensive Income [Abstract]
Investments with no credit impairment, tax	$ (5)	$ 14
Investments with credit impairment, tax	$ 0	$ 1

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