Variable Interest Entities - Effect of Consolidating FG VIE's on Financial Information (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Variable Interest Entity [Line Items]
Net earned premiums	$ 123	$ 142	$ 353	$ 423
Net investment income	88	99	296	297
Fair value gains (losses) on FG VIEs	4	5	42	11
Loss and LAE	(30)	(17)	(75)	(43)
Income (loss) before income taxes and equity in net earnings of investees	86	176	323	479
Less: Tax provision (benefit)	17	14	61	46
Net income (loss) attributable to Assured Guaranty Ltd.	69	161	265	433
Effect on OCI	24	(74)	226	(262)
Net cash flows provided by (used in) operating activities			(365)	352
Variable Interest Entity, Primary Beneficiary [Member]
Variable Interest Entity [Line Items]
Net earned premiums	(2)	(3)	(16)	(9)
Net investment income	(1)	(1)	(3)	(3)
Fair value gains (losses) on FG VIEs	4	5	42	11
Loss and LAE	(3)	(3)	(18)	0
Income (loss) before income taxes and equity in net earnings of investees	(2)	(2)	5	(1)
Less: Tax provision (benefit)	0	0	1	0
Net income (loss) attributable to Assured Guaranty Ltd.	(2)	(2)	4	(1)
Effect on OCI	1	(2)	(1)	0
Net cash flows provided by (used in) operating activities	$ 1	$ 1	$ 0	$ 7