Variable Interest Entities - Effect of Consolidating FG VIE's on Financial Information (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Variable Interest Entity [Line Items]
Net earned premiums	$ 121	$ 112	$ 224	$ 230
Net investment income	78	110	158	208
Fair value gains (losses) on FG VIEs	1	33	(8)	38
Fair value gains (losses) on consolidated investment vehicles	31	0	19	0
Loss and LAE	(37)	1	(57)	(45)
Other operating expenses	(42)	(21)	(87)	(44)
Equity in net earnings of investees	0	1	(4)	3
Income (loss) before income taxes and equity in net earnings of investees	222	181	164	237
Less: Tax provision (benefit)	34	40	30	44
Net income (loss)	188	142	130	196
Less: Noncontrolling interests	5	0	2	0
Net income (loss) attributable to Assured Guaranty Ltd.	183	142	128	196
Net cash flows provided by (used in) operating activities			(445)	(198)
Variable Interest Entity, Primary Beneficiary [Member]
Variable Interest Entity [Line Items]
Net earned premiums	(1)	(11)	(2)	(14)
Net investment income	(2)	(1)	(3)	(2)
Asset Management Fees	(1)	0	(2)	0
Fair value gains (losses) on FG VIEs	1	33	(8)	38
Fair value gains (losses) on consolidated investment vehicles	31	0	19	0
Loss and LAE	2	(14)	8	(15)
Other operating expenses	1	0	1	0
Equity in net earnings of investees	(26)	0	(16)	0
Income (loss) before income taxes and equity in net earnings of investees	5	7	(3)	7
Less: Tax provision (benefit)	0	1	(1)	1
Net income (loss)	5	6	(2)	6
Less: Noncontrolling interests	5	0	2	0
Net income (loss) attributable to Assured Guaranty Ltd.	$ 0	$ 6	$ (4)	$ 6