Subsidiary Information - Condensed Consolidating Statement of Operations (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Revenues
Net earned premiums	$ 123	$ 123	$ 112	$ 118	$ 125	$ 142	$ 136	$ 145	$ 476	$ 548	$ 690
Net investment income	82	88	110	98	98	99	98	100	378	395	417
Asset management fees	22	0	0	0					22	0	0
Net realized investment gains (losses)	10	16	8	(12)	(18)	(7)	(2)	(5)	22	(32)	40
Net change in fair value of credit derivatives	19	5	(8)	(22)	9	21	48	34	(6)	112	111
Bargain purchase gain and settlement of pre-existing relationships									0	0	58
Other									71	(22)	423
Total revenues									963	1,001	1,739
Expenses
Loss and LAE	18	30	(1)	46	21	17	44	(18)	93	64	388
Amortization of deferred acquisition costs	5	3	4	6	4	3	4	5	18	16	19
Interest expense	22	22	22	23	23	23	24	24	89	94	97
Other operating expenses									303	248	244
Total expenses									503	422	748
Equity in net earnings of investees	1	0	1	2	1	(1)	1	0	4	1	0
Income (loss) before income taxes and equity in net earnings of investees	137	86	181	56	100	176	86	217	460	579	991
Income (Loss) from Continuing Operations before Income Taxes, Noncontrolling Interest	138	86	182	58	101	175	87	217	464	580	991
Total (provision) benefit for income taxes	(2)	(17)	(40)	(4)	(13)	(14)	(12)	(20)	(63)	(59)	(261)
Equity in net earnings of subsidiaries									0	0	0
Net income (loss)	136	69	142	54	88	161	75	197	401	521	730
Less: noncontrolling interests	(1)	0	0	0	0	0	0	0	(1)	0	0
Net income (loss) attributable to Assured Guaranty Ltd.	$ 137	$ 69	$ 142	$ 54	$ 88	$ 161	$ 75	$ 197	402	521	730
Comprehensive income (loss)									650	274	897
Reportable Legal Entities [Member] | Assured Guaranty Ltd. (Parent) [Member]
Revenues
Net earned premiums									0	0	0
Net investment income									0	1	0
Asset management fees									0
Net realized investment gains (losses)									0	0	0
Net change in fair value of credit derivatives									0	0	0
Bargain purchase gain and settlement of pre-existing relationships											0
Other									0	12	10
Total revenues									0	13	10
Expenses
Loss and LAE									0	0	0
Amortization of deferred acquisition costs									0	0	0
Interest expense									0	0	0
Other operating expenses									31	41	38
Total expenses									31	41	38
Equity in net earnings of investees									0	0	0
Income (loss) before income taxes and equity in net earnings of investees									(31)	(28)	(28)
Total (provision) benefit for income taxes									0	0	0
Equity in net earnings of subsidiaries									433	549	758
Net income (loss)									402	521	730
Less: noncontrolling interests									0	0	0
Net income (loss) attributable to Assured Guaranty Ltd.									402	521	730
Comprehensive income (loss)									651	274	897
Reportable Legal Entities [Member] | AGUS (Issuer) [Member]
Revenues
Net earned premiums									0	0	0
Net investment income									11	9	2
Asset management fees									0
Net realized investment gains (losses)									0	0	0
Net change in fair value of credit derivatives									0	0	0
Bargain purchase gain and settlement of pre-existing relationships											0
Other									0	0	0
Total revenues									11	9	2
Expenses
Loss and LAE									0	0	0
Amortization of deferred acquisition costs									0	0	0
Interest expense									51	49	47
Other operating expenses									10	10	12
Total expenses									61	59	59
Equity in net earnings of investees									0	0	0
Income (loss) before income taxes and equity in net earnings of investees									(50)	(50)	(57)
Total (provision) benefit for income taxes									10	52	17
Equity in net earnings of subsidiaries									422	412	636
Net income (loss)									382	414	596
Less: noncontrolling interests									0	0	0
Net income (loss) attributable to Assured Guaranty Ltd.									382	414	596
Comprehensive income (loss)									553	218	754
Reportable Legal Entities [Member] | AGMH (Issuer) [Member]
Revenues
Net earned premiums									0	0	0
Net investment income									1	1	0
Asset management fees									0
Net realized investment gains (losses)									0	0	0
Net change in fair value of credit derivatives									0	0	0
Bargain purchase gain and settlement of pre-existing relationships											0
Other									0	0	0
Total revenues									1	1	0
Expenses
Loss and LAE									0	0	0
Amortization of deferred acquisition costs									0	0	0
Interest expense									54	54	54
Other operating expenses									0	0	1
Total expenses									54	54	55
Equity in net earnings of investees									0	0	0
Income (loss) before income taxes and equity in net earnings of investees									(53)	(53)	(55)
Total (provision) benefit for income taxes									11	11	54
Equity in net earnings of subsidiaries									327	277	395
Net income (loss)									285	235	394
Less: noncontrolling interests									0	0	0
Net income (loss) attributable to Assured Guaranty Ltd.									285	235	394
Comprehensive income (loss)									440	107	482
Reportable Legal Entities [Member] | Other Entities [Member]
Revenues
Net earned premiums									486	563	728
Net investment income									388	398	426
Asset management fees									22
Net realized investment gains (losses)									22	(32)	45
Net change in fair value of credit derivatives									(6)	112	111
Bargain purchase gain and settlement of pre-existing relationships											58
Other									78	192	609
Total revenues									990	1,233	1,977
Expenses
Loss and LAE									98	70	327
Amortization of deferred acquisition costs									22	21	26
Interest expense									11	10	11
Other operating expenses									266	394	394
Total expenses									397	495	758
Equity in net earnings of investees									2	1	0
Income (loss) before income taxes and equity in net earnings of investees									595	739	1,219
Total (provision) benefit for income taxes									(84)	(123)	(359)
Equity in net earnings of subsidiaries									18	24	32
Net income (loss)									529	640	892
Less: noncontrolling interests									18	24	32
Net income (loss) attributable to Assured Guaranty Ltd.									511	616	860
Comprehensive income (loss)									781	395	1,084
Consolidation, Eliminations [Member]
Revenues
Net earned premiums									(10)	(15)	(38)
Net investment income									(22)	(14)	(11)
Asset management fees									0
Net realized investment gains (losses)									0	0	(5)
Net change in fair value of credit derivatives									0	0	0
Bargain purchase gain and settlement of pre-existing relationships											0
Other									(7)	(226)	(196)
Total revenues									(39)	(255)	(250)
Expenses
Loss and LAE									(5)	(6)	61
Amortization of deferred acquisition costs									(4)	(5)	(7)
Interest expense									(27)	(19)	(15)
Other operating expenses									(4)	(197)	(201)
Total expenses									(40)	(227)	(162)
Equity in net earnings of investees									2	0	0
Income (loss) before income taxes and equity in net earnings of investees									3	(28)	(88)
Total (provision) benefit for income taxes									0	1	27
Equity in net earnings of subsidiaries									(1,200)	(1,262)	(1,821)
Net income (loss)									(1,197)	(1,289)	(1,882)
Less: noncontrolling interests									(19)	(24)	(32)
Net income (loss) attributable to Assured Guaranty Ltd.									(1,178)	(1,265)	(1,850)
Comprehensive income (loss)									$ (1,775)	$ (720)	$ (2,320)