Condensed Consolidated Statements of Comprehensive Income (Loss) (Unaudited) - 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]
Net income (loss)	$ 20	$ 89	$ 137	$ 219
Change in net unrealized gains (losses) on:
Investments with no credit impairment, net of tax provision (benefit) of $(8), $7, $(18) and $11	(66)	67	(134)	80
Investments with credit impairment, net of tax provision (benefit) of $(2), $2, $3 and $(5)	(6)	8	12	(19)
Change in net unrealized gains (losses) on investments	(72)	75	(122)	61
Change in instrument-specific credit risk on financial guaranty variable interest entities' liabilities with recourse, net of tax	2	(4)	(1)	1
Other, net of tax provision (benefit)	(1)	0	(1)	0
Other comprehensive income (loss)	(71)	71	(124)	62
Comprehensive income (loss)	(51)	160	13	281
Less: Comprehensive income (loss) attributable to noncontrolling interests	3	3	11	5
Comprehensive income (loss) attributable to Assured Guaranty Ltd.	$ (54)	$ 157	$ 2	$ 276