Condensed Consolidated Statements of Comprehensive Income (Loss) (unaudited) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Statement of Comprehensive Income [Abstract]
Net income (loss)	$ 89	$ 69	$ 219	$ 265
Change in net unrealized gains (losses) on:
Investments with no credit impairment recognized in the statement of operations, net of tax provision (benefit) of $7, $7, $11 and $51	67	33	80	275
Investments with credit impairment recognized in the statement of operations, net of tax provision (benefit) of $2, $(2), $(5) and $(14)	8	(9)	(19)	(52)
Change in net unrealized gains (losses) on investments	75	24	61	223
Change in net unrealized gains (losses) on financial guaranty variable interest entities' liabilities with recourse, net of tax	(4)	0	1	4
Other, net of tax provision (benefit)	0	0	0	(1)
Other comprehensive income (loss)	71	24	62	226
Comprehensive income (loss)	160	93	281	491
Less: Comprehensive income (loss) attributable to noncontrolling interest	3	0	5	0
Comprehensive income (loss) attributable to Assured Guaranty Ltd.	$ 157	$ 93	$ 276	$ 491