Condensed Consolidated Statements of Comprehensive Income (Loss) (Unaudited) (Parenthetical) - USD ($) $ in Millions	3 Months Ended
Mar. 31, 2025	Mar. 31, 2024
Statement of Comprehensive Income [Abstract]
Investments with no credit impairment, tax	$ 6	$ (5)
Investments with credit impairment, tax	$ 5	$ 0

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