Variable Interest Entities - Effect of Consolidating FG VIE's on Financial Information (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Variable Interest Entity [Line Items]
Net earned premiums	$ 118	$ 145
Net investment income	98	100
Fair value gains (losses) on FG VIEs (1)	5	4
Loss and LAE	(46)	18
Income (loss) before income taxes and equity in net earnings of investees	56	217
Less: tax provision (benefit)	4	20
Net income (loss) attributable to Assured Guaranty Ltd.	54	197
Effect on OCI	0	(2)
Net cash flows provided by (used in) operating activities	(332)	27
Variable Interest Entity, Primary Beneficiary [Member]
Variable Interest Entity [Line Items]
Net earned premiums	(3)	(3)
Net investment income	(1)	(1)
Fair value gains (losses) on FG VIEs (1)	5	4
Loss and LAE	(1)	6
Income (loss) before income taxes and equity in net earnings of investees	0	6
Less: tax provision (benefit)	0	1
Net income (loss) attributable to Assured Guaranty Ltd.	0	5
Effect on OCI	(1)	(2)
Net cash flows provided by (used in) operating activities	$ 1	$ 2