Subsidiary Information - Condensed Consolidating Statement of Operations (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Revenues
Net earned premiums	$ 112	$ 136	$ 230	$ 281
Net investment income	110	98	208	198
Net realized investment gains (losses)	8	(2)	(4)	(7)
Net change in fair value of credit derivatives	(8)	48	(30)	82
Other	44	(60)	57	(41)
Total revenues	266	220	461	513
Expenses
Loss and LAE	(1)	44	45	26
Amortization of deferred acquisition costs	4	4	10	9
Interest expense	22	24	45	48
Other operating expenses	60	62	124	127
Total expenses	85	134	224	210
Equity in net earnings of investees	1	1	3	1
Total before tax	182	87	240	304
Total (provision) benefit for income taxes	(40)	(12)	(44)	(32)
Equity in net earnings of subsidiaries	0	0	0	0
Net income (loss)	142	75	196	272
Less: noncontrolling interest	0	0	0	0
Net income (loss) attributable to Assured Guaranty Ltd.	142	75	196	272
Comprehensive income (loss)	176	13	398	84
Reportable Legal Entities [Member] | Assured Guaranty Ltd. (Parent) [Member]
Revenues
Net earned premiums	0	0	0	0
Net investment income	0	0	0	0
Net realized investment gains (losses)	0	0	0	0
Net change in fair value of credit derivatives	0	0	0	0
Other	2	3	4	6
Total revenues	2	3	4	6
Expenses
Loss and LAE	0	0	0	0
Amortization of deferred acquisition costs	0	0	0	0
Interest expense	0	0	0	0
Other operating expenses	10	10	20	20
Total expenses	10	10	20	20
Equity in net earnings of investees	0	0	0	0
Total before tax	(8)	(7)	(16)	(14)
Total (provision) benefit for income taxes	0	0	0	0
Equity in net earnings of subsidiaries	150	82	212	286
Net income (loss)	142	75	196	272
Less: noncontrolling interest	0	0	0	0
Net income (loss) attributable to Assured Guaranty Ltd.	142	75	196	272
Comprehensive income (loss)	176	13	398	84
Reportable Legal Entities [Member] | Issuer Subsidiary [Member] | AGUS [Member]
Revenues
Net earned premiums	0	0	0	0
Net investment income	4	2	6	4
Net realized investment gains (losses)	0	0	0	0
Net change in fair value of credit derivatives	0	0	0	0
Other	0	0	0	0
Total revenues	4	2	6	4
Expenses
Loss and LAE	0	0	0	0
Amortization of deferred acquisition costs	0	0	0	0
Interest expense	13	12	25	24
Other operating expenses	1	0	2	3
Total expenses	14	12	27	27
Equity in net earnings of investees	(1)	0	0	0
Total before tax	(11)	(10)	(21)	(23)
Total (provision) benefit for income taxes	2	2	4	5
Equity in net earnings of subsidiaries	154	64	204	225
Net income (loss)	145	56	187	207
Less: noncontrolling interest	0	0	0	0
Net income (loss) attributable to Assured Guaranty Ltd.	145	56	187	207
Comprehensive income (loss)	145	7	315	74
Reportable Legal Entities [Member] | Issuer Subsidiary [Member] | AGMH [Member]
Revenues
Net earned premiums	0	0	0	0
Net investment income	0	0	0	0
Net realized investment gains (losses)	0	0	0	0
Net change in fair value of credit derivatives	0	0	0	0
Other	0	0	0	0
Total revenues	0	0	0	0
Expenses
Loss and LAE	0	0	0	0
Amortization of deferred acquisition costs	0	0	0	0
Interest expense	14	14	27	27
Other operating expenses	0	1	0	1
Total expenses	14	15	27	28
Equity in net earnings of investees	0	0	0	0
Total before tax	(14)	(15)	(27)	(28)
Total (provision) benefit for income taxes	3	3	6	6
Equity in net earnings of subsidiaries	95	9	163	111
Net income (loss)	84	(3)	142	89
Less: noncontrolling interest	0	0	0	0
Net income (loss) attributable to Assured Guaranty Ltd.	84	(3)	142	89
Comprehensive income (loss)	109	(53)	261	(7)
Reportable Legal Entities [Member] | Other Entities [Member]
Revenues
Net earned premiums	114	139	235	288
Net investment income	110	99	211	200
Net realized investment gains (losses)	8	(2)	(4)	(7)
Net change in fair value of credit derivatives	(8)	48	(30)	82
Other	93	(9)	159	65
Total revenues	317	275	571	628
Expenses
Loss and LAE	5	46	52	30
Amortization of deferred acquisition costs	5	5	12	11
Interest expense	1	2	5	5
Other operating expenses	98	95	205	200
Total expenses	109	148	274	246
Equity in net earnings of investees	2	1	3	1
Total before tax	210	128	300	383
Total (provision) benefit for income taxes	(44)	(20)	(53)	(45)
Equity in net earnings of subsidiaries	5	6	9	13
Net income (loss)	171	114	256	351
Less: noncontrolling interest	5	6	9	13
Net income (loss) attributable to Assured Guaranty Ltd.	166	108	247	338
Comprehensive income (loss)	206	52	460	163
Consolidating Adjustments [Member]
Revenues
Net earned premiums	(2)	(3)	(5)	(7)
Net investment income	(4)	(3)	(9)	(6)
Net realized investment gains (losses)	0	0	0	0
Net change in fair value of credit derivatives	0	0	0	0
Other	(51)	(54)	(106)	(112)
Total revenues	(57)	(60)	(120)	(125)
Expenses
Loss and LAE	(6)	(2)	(7)	(4)
Amortization of deferred acquisition costs	(1)	(1)	(2)	(2)
Interest expense	(6)	(4)	(12)	(8)
Other operating expenses	(49)	(44)	(103)	(97)
Total expenses	(62)	(51)	(124)	(111)
Equity in net earnings of investees	0	0	0	0
Total before tax	5	(9)	4	(14)
Total (provision) benefit for income taxes	(1)	3	(1)	2
Equity in net earnings of subsidiaries	(404)	(161)	(588)	(635)
Net income (loss)	(400)	(167)	(585)	(647)
Less: noncontrolling interest	(5)	(6)	(9)	(13)
Net income (loss) attributable to Assured Guaranty Ltd.	(395)	(161)	(576)	(634)
Comprehensive income (loss)	$ (460)	$ (6)	$ (1,036)	$ (230)