Subsidiary Information - Condensed Consolidating Statement of Operations (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Revenues
Net earned premiums	$ 123	$ 142	$ 353	$ 423
Net investment income	88	99	296	297
Net realized investment gains (losses)	16	(7)	12	(14)
Net change in fair value of credit derivatives	5	21	(25)	103
Other	(26)	20	31	(21)
Total revenues	206	275	667	788
Expenses
Loss and LAE	30	17	75	43
Amortization of deferred acquisition costs	3	3	13	12
Interest expense	22	23	67	71
Other operating expenses	65	56	189	183
Total expenses	120	99	344	309
Equity in net earnings of investees	0	(1)	3	0
Total before tax	86	175	326	479
Total (provision) benefit for income taxes	(17)	(14)	(61)	(46)
Equity in net earnings of subsidiaries	0	0	0	0
Net income (loss)	69	161	265	433
Less: noncontrolling interest	0	0	0	0
Net income (loss) attributable to Assured Guaranty Ltd.	69	161	265	433
Comprehensive income (loss)	93	87	491	171
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	3	3	7	9
Total revenues	3	3	7	9
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	30	30
Total expenses	10	10	30	30
Equity in net earnings of investees	0	0	0	0
Total before tax	(7)	(7)	(23)	(21)
Total (provision) benefit for income taxes	0	0	0	0
Equity in net earnings of subsidiaries	76	168	288	454
Net income (loss)	69	161	265	433
Less: noncontrolling interest	0	0	0	0
Net income (loss) attributable to Assured Guaranty Ltd.	69	161	265	433
Comprehensive income (loss)	93	87	491	171
Reportable Legal Entities [Member] | Issuer Subsidiary [Member] | AGUS [Member]
Revenues
Net earned premiums	0	0	0	0
Net investment income	2	2	8	6
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	2	2	8	6
Expenses
Loss and LAE	0	0	0	0
Amortization of deferred acquisition costs	0	0	0	0
Interest expense	12	13	37	37
Other operating expenses	1	6	3	9
Total expenses	13	19	40	46
Equity in net earnings of investees	0	0	0	0
Total before tax	(11)	(17)	(32)	(40)
Total (provision) benefit for income taxes	3	45	7	50
Equity in net earnings of subsidiaries	71	111	275	336
Net income (loss)	63	139	250	346
Less: noncontrolling interest	0	0	0	0
Net income (loss) attributable to Assured Guaranty Ltd.	63	139	250	346
Comprehensive income (loss)	73	76	388	150
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	13	13	40	40
Other operating expenses	0	(1)	0	0
Total expenses	13	12	40	40
Equity in net earnings of investees	0	0	0	0
Total before tax	(13)	(12)	(40)	(40)
Total (provision) benefit for income taxes	2	3	8	9
Equity in net earnings of subsidiaries	52	108	215	219
Net income (loss)	41	99	183	188
Less: noncontrolling interest	0	0	0	0
Net income (loss) attributable to Assured Guaranty Ltd.	41	99	183	188
Comprehensive income (loss)	38	61	299	54
Reportable Legal Entities [Member] | Other Entities [Member]
Revenues
Net earned premiums	126	146	361	434
Net investment income	91	101	302	301
Net realized investment gains (losses)	16	(7)	12	(14)
Net change in fair value of credit derivatives	5	21	(25)	103
Other	20	72	179	137
Total revenues	258	333	829	961
Expenses
Loss and LAE	32	18	84	48
Amortization of deferred acquisition costs	4	5	16	16
Interest expense	3	2	8	7
Other operating expenses	103	93	308	293
Total expenses	142	118	416	364
Equity in net earnings of investees	0	(1)	3	0
Total before tax	116	214	416	597
Total (provision) benefit for income taxes	(21)	(58)	(74)	(103)
Equity in net earnings of subsidiaries	5	7	14	20
Net income (loss)	100	163	356	514
Less: noncontrolling interest	5	7	14	20
Net income (loss) attributable to Assured Guaranty Ltd.	95	156	342	494
Comprehensive income (loss)	124	92	584	255
Consolidating Adjustments [Member]
Revenues
Net earned premiums	(3)	(4)	(8)	(11)
Net investment income	(5)	(4)	(14)	(10)
Net realized investment gains (losses)	0	0	0	0
Net change in fair value of credit derivatives	0	0	0	0
Other	(49)	(55)	(155)	(167)
Total revenues	(57)	(63)	(177)	(188)
Expenses
Loss and LAE	(2)	(1)	(9)	(5)
Amortization of deferred acquisition costs	(1)	(2)	(3)	(4)
Interest expense	(6)	(5)	(18)	(13)
Other operating expenses	(49)	(52)	(152)	(149)
Total expenses	(58)	(60)	(182)	(171)
Equity in net earnings of investees	0	0	0	0
Total before tax	1	(3)	5	(17)
Total (provision) benefit for income taxes	(1)	(4)	(2)	(2)
Equity in net earnings of subsidiaries	(204)	(394)	(792)	(1,029)
Net income (loss)	(204)	(401)	(789)	(1,048)
Less: noncontrolling interest	(5)	(7)	(14)	(20)
Net income (loss) attributable to Assured Guaranty Ltd.	(199)	(394)	(775)	(1,028)
Comprehensive income (loss)	$ (235)	$ (229)	$ (1,271)	$ (459)