Subsidiary Information - Condensed Consolidating Statement of Operations (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Revenues
Net earned premiums	$ 142	$ 186	$ 423	$ 512
Net investment income	98	99	298	322
Net realized investment gains (losses)	(7)	7	(14)	54
Net change in fair value of credit derivatives	21	58	103	106
Bargain purchase gain and settlement of pre-existing relationships	0	0	0	58
Other	20	273	(22)	406
Total revenues	274	623	788	1,458
Expenses
Loss and LAE	17	223	43	354
Amortization of deferred acquisition costs	3	5	12	13
Interest expense	23	24	71	73
Other operating expenses	56	58	183	183
Total expenses	99	310	309	623
Income (loss) before income taxes	175	313	479	835
Total (provision) benefit for income taxes	(14)	(105)	(46)	(157)
Equity in net earnings of subsidiaries	0	0	0	0
Net income (loss)	161	208	433	678
Less: noncontrolling interest	0	0	0	0
Net income (loss)	161	208	433	678
Comprehensive income (loss)	87	219	171	855
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
Bargain purchase gain and settlement of pre-existing relationships				0
Other	3	3	9	8
Total revenues	3	3	9	8
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
Income (loss) before income taxes	(7)	(7)	(21)	(22)
Total (provision) benefit for income taxes	0	0	0	0
Equity in net earnings of subsidiaries	168	215	454	700
Net income (loss)	161	208	433	678
Less: noncontrolling interest	0	0	0	0
Net income (loss)	161	208	433	678
Comprehensive income (loss)	87	219	171	855
Reportable Legal Entities [Member] | Issuer Subsidiary [Member] | AGUS [Member]
Revenues
Net earned premiums	0	0	0	0
Net investment income	2	0	6	1
Net realized investment gains (losses)	0	0	0	0
Net change in fair value of credit derivatives	0	0	0	0
Bargain purchase gain and settlement of pre-existing relationships				0
Other	0	0	0	0
Total revenues	2	0	6	1
Expenses
Loss and LAE	0	0	0	0
Amortization of deferred acquisition costs	0	0	0	0
Interest expense	13	12	37	36
Other operating expenses	6	1	9	8
Total expenses	19	13	46	44
Income (loss) before income taxes	(17)	(13)	(40)	(43)
Total (provision) benefit for income taxes	45	4	50	13
Equity in net earnings of subsidiaries	111	237	336	574
Net income (loss)	139	228	346	544
Less: noncontrolling interest	0	0	0	0
Net income (loss)	139	228	346	544
Comprehensive income (loss)	76	237	150	705
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
Bargain purchase gain and settlement of pre-existing relationships				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	(1)	0	0	1
Total expenses	12	13	40	41
Income (loss) before income taxes	(12)	(13)	(40)	(41)
Total (provision) benefit for income taxes	3	4	9	15
Equity in net earnings of subsidiaries	108	178	219	422
Net income (loss)	99	169	188	396
Less: noncontrolling interest	0	0	0	0
Net income (loss)	99	169	188	396
Comprehensive income (loss)	61	178	54	484
Reportable Legal Entities [Member] | Other Entities [Member]
Revenues
Net earned premiums	146	190	434	523
Net investment income	100	101	302	325
Net realized investment gains (losses)	(7)	(9)	(14)	53
Net change in fair value of credit derivatives	21	58	103	106
Bargain purchase gain and settlement of pre-existing relationships				58
Other	72	302	136	527
Total revenues	332	642	961	1,592
Expenses
Loss and LAE	18	250	48	301
Amortization of deferred acquisition costs	5	8	16	19
Interest expense	2	3	7	8
Other operating expenses	93	91	293	286
Total expenses	118	352	364	614
Income (loss) before income taxes	214	290	597	978
Total (provision) benefit for income taxes	(58)	(94)	(103)	(194)
Equity in net earnings of subsidiaries	7	8	20	22
Net income (loss)	163	204	514	806
Less: noncontrolling interest	7	8	20	22
Net income (loss)	156	196	494	784
Comprehensive income (loss)	92	274	255	1,004
Consolidating Adjustments [Member]
Revenues
Net earned premiums	(4)	(4)	(11)	(11)
Net investment income	(4)	(2)	(10)	(4)
Net realized investment gains (losses)	0	16	0	1
Net change in fair value of credit derivatives	0	0	0	0
Bargain purchase gain and settlement of pre-existing relationships				0
Other	(55)	(32)	(167)	(129)
Total revenues	(63)	(22)	(188)	(143)
Expenses
Loss and LAE	(1)	(27)	(5)	53
Amortization of deferred acquisition costs	(2)	(3)	(4)	(6)
Interest expense	(5)	(4)	(13)	(11)
Other operating expenses	(52)	(44)	(149)	(142)
Total expenses	(60)	(78)	(171)	(106)
Income (loss) before income taxes	(3)	56	(17)	(37)
Total (provision) benefit for income taxes	(4)	(19)	(2)	9
Equity in net earnings of subsidiaries	(394)	(638)	(1,029)	(1,718)
Net income (loss)	(401)	(601)	(1,048)	(1,746)
Less: noncontrolling interest	(7)	(8)	(20)	(22)
Net income (loss)	(394)	(593)	(1,028)	(1,724)
Comprehensive income (loss)	$ (229)	$ (689)	$ (459)	$ (2,193)