Variable Interest Entities - Effect of Consolidating FG VIE's on Financial Information (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Variable Interest Entity [Line Items]
Fair value gains (losses) on financial guaranty variable interest entities	$ 5	$ 0	$ 18	$ (8)
Fair value gains (losses) on consolidated investment vehicles	16	18	53	37
Equity in earnings of investees	23	7	66	3
Income (loss) before income taxes and equity in earnings of investees	(18)	72	79	236
Less: Tax provision (benefit)	(15)	(10)	8	20
Net income (loss)	20	89	137	219
Less: Noncontrolling interests	3	3	11	5
Net income (loss) attributable to Assured Guaranty Ltd.	17	86	126	214
Variable Interest Entity, Primary Beneficiary | Financial Guaranty Variable Interest Entities And Consolidated Investment Vehicles
Variable Interest Entity [Line Items]
Fair value gains (losses) on financial guaranty variable interest entities	5	0	18	(8)
Fair value gains (losses) on consolidated investment vehicles	16	18	53	37
Equity in earnings of investees	(11)	(13)	(35)	(29)
Other	(13)	(6)	(26)	(4)
Income (loss) before income taxes and equity in earnings of investees	(3)	(1)	10	(4)
Less: Tax provision (benefit)	(2)	(1)	(1)	(2)
Net income (loss)	(1)	0	11	(2)
Less: Noncontrolling interests	3	3	11	5
Net income (loss) attributable to Assured Guaranty Ltd.	$ (4)	$ (3)	$ 0	$ (7)