Variable Interest Entities - Effect of Consolidating FG VIE's on Financial Information (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Variable Interest Entity [Line Items]
Net earned premiums	$ 107	$ 123	$ 331	$ 353
Net investment income	71	88	229	296
Fair value gains (losses) on FG VIEs	0	4	(8)	42
Fair value gains (losses) on consolidated investment vehicles	18	0	37	0
Loss and LAE	(73)	(30)	(130)	(75)
Other operating expenses	(41)	(27)	(128)	(71)
Equity in net earnings of investees	7	0	3	3
Income (loss) before income taxes and equity in net earnings of investees	72	86	236	323
Less: Tax provision (benefit)	(10)	17	20	61
Net income (loss)	89	69	219	265
Less: Noncontrolling interests	3	0	5	0
Net income (loss) attributable to Assured Guaranty Ltd.	86	69	214	265
Variable Interest Entity, Primary Beneficiary
Variable Interest Entity [Line Items]
Net earned premiums	(2)	(2)	(4)	(16)
Net investment income	(1)	(1)	(4)	(3)
Asset Management Fees	(2)	0	(4)	0
Fair value gains (losses) on FG VIEs	0	4	(8)	42
Fair value gains (losses) on consolidated investment vehicles	18	0	37	0
Loss and LAE	(1)	(3)	7	(18)
Other operating expenses	0	0	1	0
Equity in net earnings of investees	(13)	0	(29)	0
Income (loss) before income taxes and equity in net earnings of investees	(1)	(2)	(4)	5
Less: Tax provision (benefit)	(1)	0	(2)	1
Net income (loss)	0	(2)	(2)	4
Less: Noncontrolling interests	3	0	5	0
Net income (loss) attributable to Assured Guaranty Ltd.	$ (3)	$ (2)	$ (7)	$ 4