Variable Interest Entities - Effect of Consolidating FG VIE's on Financial Information (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Variable Interest Entity [Line Items]
Net earned premiums	$ 112	$ 136	$ 230	$ 281
Net investment income	110	98	208	198
Fair value gains (losses) on FG VIEs (1)	33	2	38	6
Loss and LAE	1	(44)	(45)	(26)
Income (loss) before income taxes and equity in net earnings of investees	181	86	237	303
Less: Tax provision (benefit)	40	12	44	32
Net income (loss) attributable to Assured Guaranty Ltd.	142	75	196	272
Effect on OCI	34	(62)	202	(188)
Net cash flows provided by (used in) operating activities			(198)	444
Variable Interest Entity, Primary Beneficiary [Member]
Variable Interest Entity [Line Items]
Net earned premiums	(11)	(3)	(14)	(6)
Net investment income	(1)	(1)	(2)	(2)
Fair value gains (losses) on FG VIEs (1)	33	2	38	6
Loss and LAE	(14)	(3)	(15)	3
Income (loss) before income taxes and equity in net earnings of investees	7	(5)	7	1
Less: Tax provision (benefit)	1	(1)	1	0
Net income (loss) attributable to Assured Guaranty Ltd.	6	(4)	6	1
Effect on OCI	(1)	4	(2)	2
Net cash flows provided by (used in) operating activities	$ (2)	$ 4	$ (1)	$ 6