Condensed Consolidated Statements of Comprehensive Income (unaudited) (Parenthetical) - USD ($) $ in Millions	3 Months Ended
Mar. 31, 2020	Mar. 31, 2019
Statement of Comprehensive Income [Abstract]
Investments with no credit impairment, tax provision (benefit)	$ (27)	$ 25
Investments with credit impairment, tax provision (benefit)	(13)	0
Other Comprehensive Income (Loss), Other, Tax, Portion Attributable To Parent	$ 0	$ 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