Subsidiary Information - Condensed Consolidating Statement of Operations (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Revenues
Net earned premiums	$ 125	$ 142	$ 136	$ 145	$ 178	$ 186	$ 162	$ 164	$ 548	$ 690	$ 864
Net investment income	100	98	99	101	96	99	101	122	398	418	408
Net realized investment gains (losses)	(18)	(7)	(2)	(5)	(14)	7	15	32	(32)	40	(29)
Net change in fair value of credit derivatives	9	21	48	34	5	58	(6)	54	112	111	98
Bargain purchase gain and settlement of pre-existing relationships					0	0	0	58	0	58	259
Other									(24)	422	77
Total revenues									1,002	1,739	1,677
Expenses
Loss and LAE	21	17	44	(18)	34	223	72	59	64	388	295
Amortization of DAC	4	3	4	5	6	5	4	4	16	19	18
Interest expense	23	23	24	24	24	24	25	24	94	97	102
Other operating expenses	65	56	62	65	61	58	57	68	248	244	245
Total expenses									422	748	660
Income (loss) before income taxes	101	175	87	217	156	313	150	372	580	991	1,017
Total (provision) benefit for income taxes	(13)	(14)	(12)	(20)	(104)	(105)	3	(55)	(59)	(261)	(136)
Equity in net earnings of subsidiaries									0	0	0
Net income (loss)									521	730	881
Less: noncontrolling interest									0	0	0
Net income (loss)	$ 88	$ 161	$ 75	$ 197	$ 52	$ 208	$ 153	$ 317	521	730	881
Comprehensive income (loss)									274	897	793
Reportable Legal Entities [Member] | Assured Guaranty Ltd. (Parent) [Member]
Revenues
Net earned premiums									0	0	0
Net investment income									1	0	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	0
Other									12	10	0
Total revenues									13	10	0
Expenses
Loss and LAE									0	0	0
Amortization of DAC									0	0	0
Interest expense									0	0	0
Other operating expenses									41	38	29
Total expenses									41	38	29
Income (loss) before income taxes									(28)	(28)	(29)
Total (provision) benefit for income taxes									0	0	0
Equity in net earnings of subsidiaries									549	758	910
Net income (loss)									521	730	881
Less: noncontrolling interest									0	0	0
Net income (loss)									521	730	881
Comprehensive income (loss)									274	897	793
Reportable Legal Entities [Member] | AGUS (Issuer) [Member]
Revenues
Net earned premiums									0	0	0
Net investment income									9	2	0
Net realized investment gains (losses)									0	0	2
Net change in fair value of credit derivatives									0	0	0
Bargain purchase gain and settlement of pre-existing relationships										0	0
Other									0	0	0
Total revenues									9	2	2
Expenses
Loss and LAE									0	0	0
Amortization of DAC									0	0	0
Interest expense									49	47	52
Other operating expenses									10	12	2
Total expenses									59	59	54
Income (loss) before income taxes									(50)	(57)	(52)
Total (provision) benefit for income taxes									52	17	18
Equity in net earnings of subsidiaries									412	636	794
Net income (loss)									414	596	760
Less: noncontrolling interest									0	0	0
Net income (loss)									414	596	760
Comprehensive income (loss)									218	754	685
Reportable Legal Entities [Member] | AGMH (Issuer) [Member]
Revenues
Net earned premiums									0	0	0
Net investment income									1	0	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	0
Other									0	0	0
Total revenues									1	0	0
Expenses
Loss and LAE									0	0	0
Amortization of DAC									0	0	0
Interest expense									54	54	54
Other operating expenses									0	1	2
Total expenses									54	55	56
Income (loss) before income taxes									(53)	(55)	(56)
Total (provision) benefit for income taxes									11	54	20
Equity in net earnings of subsidiaries									277	395	274
Net income (loss)									235	394	238
Less: noncontrolling interest									0	0	0
Net income (loss)									235	394	238
Comprehensive income (loss)									107	482	144
Reportable Legal Entities [Member] | Other Entities [Member]
Revenues
Net earned premiums									563	728	892
Net investment income									401	427	412
Net realized investment gains (losses)									(32)	45	(28)
Net change in fair value of credit derivatives									112	111	98
Bargain purchase gain and settlement of pre-existing relationships										58	257
Other									190	608	78
Total revenues									1,234	1,977	1,709
Expenses
Loss and LAE									70	327	296
Amortization of DAC									21	26	30
Interest expense									10	11	10
Other operating expenses									394	394	217
Total expenses									495	758	553
Income (loss) before income taxes									739	1,219	1,156
Total (provision) benefit for income taxes									(123)	(359)	(175)
Equity in net earnings of subsidiaries									24	32	44
Net income (loss)									640	892	1,025
Less: noncontrolling interest									24	32	44
Net income (loss)									616	860	981
Comprehensive income (loss)									395	1,084	953
Consolidating Adjustments [Member]
Revenues
Net earned premiums									(15)	(38)	(28)
Net investment income									(14)	(11)	(4)
Net realized investment gains (losses)									0	(5)	(3)
Net change in fair value of credit derivatives									0	0	0
Bargain purchase gain and settlement of pre-existing relationships										0	2
Other									(226)	(196)	(1)
Total revenues									(255)	(250)	(34)
Expenses
Loss and LAE									(6)	61	(1)
Amortization of DAC									(5)	(7)	(12)
Interest expense									(19)	(15)	(14)
Other operating expenses									(197)	(201)	(5)
Total expenses									(227)	(162)	(32)
Income (loss) before income taxes									(28)	(88)	(2)
Total (provision) benefit for income taxes									1	27	1
Equity in net earnings of subsidiaries									(1,262)	(1,821)	(2,022)
Net income (loss)									(1,289)	(1,882)	(2,023)
Less: noncontrolling interest									(24)	(32)	(44)
Net income (loss)									(1,265)	(1,850)	(1,979)
Comprehensive income (loss)									$ (720)	$ (2,320)	$ (1,782)