Financial Guarantee Insurance Contracts - Summary of Information Related to Policies Currently Included in Ambac's Loss Reserves or Subrogation Recoverable (Detail) $ in Millions	12 Months Ended
	Dec. 31, 2023 USD ($) Policies	Dec. 31, 2022 USD ($) Policies	Dec. 31, 2021 USD ($)
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Number of policies | Policies	141	162
Remaining weighted-average contract period (in years)	12 years	14 years
Gross insured contractual payments outstanding:
Principal	$ 3,838	$ 4,047
Interest	1,421	1,750
Total	5,259	5,797
Gross undiscounted claim liability	1,202	1,279
Discount, gross claim liability	426	496
Gross claim liability before all subrogation and before reinsurance	777	783
Less:
Gross RMBS subrogation	0	(140)
Discount, RMBS subrogation	0	0
Schedule Of Insured Financial Obligations With Credit Deterioration Reductions Of Gross Claim Liability Rmbs Subrogation	0	(140)
Less:
Gross other subrogation	(263)	(233)
Discount, other subrogation	69	54
Discounted other subrogation, before reinsurance	(194)	(179)
Gross claim liability, net of all subrogation and discounts, before reinsurance	583	464
Less: Unearned premium reserves	(28)	(28)
Plus: Loss adjustment expenses reserves	4	8
Claim liability reported on Balance Sheet, before reinsurance		444
Reinsurance recoverable reported on Balance Sheet	30	33
Ceded Loss And Loss Expenses Paid Not Yet Recovered		0	$ 0
Loss Reserves Ceded To Reinsurers		$ 33
Financial Guarantee
Less:
Claim liability reported on Balance Sheet, before reinsurance	559
Ceded Loss And Loss Expenses Paid Not Yet Recovered	8
Loss Reserves Ceded To Reinsurers	$ 30
Loss Reserves [Member]
Less:
Weighted average risk-free rate used to discount loss reserves	3.90%	3.90%
Surveillance Category One [Member]
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Number of policies | Policies	18	37
Remaining weighted-average contract period (in years)	9 years	7 years
Gross insured contractual payments outstanding:
Principal	$ 429	$ 709
Interest	75	526
Total	505	1,235
Gross undiscounted claim liability	1	4
Discount, gross claim liability	0	1
Gross claim liability before all subrogation and before reinsurance	1	3
Less:
Gross RMBS subrogation	0	0
Discount, RMBS subrogation	0	0
Schedule Of Insured Financial Obligations With Credit Deterioration Reductions Of Gross Claim Liability Rmbs Subrogation	0	0
Less:
Gross other subrogation	(13)	(14)
Discount, other subrogation	2	2
Discounted other subrogation, before reinsurance	(11)	(12)
Gross claim liability, net of all subrogation and discounts, before reinsurance	(10)	(9)
Less: Unearned premium reserves	0	(2)
Plus: Loss adjustment expenses reserves	0	1
Claim liability reported on Balance Sheet, before reinsurance	(10)	(10)
Reinsurance recoverable reported on Balance Sheet	$ 1	$ 1
IA [Member]
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Number of policies | Policies	8	6
Remaining weighted-average contract period (in years)	9 years	19 years
Gross insured contractual payments outstanding:
Principal	$ 1,084	$ 200
Interest	328	198
Total	1,412	399
Gross undiscounted claim liability	19	4
Discount, gross claim liability	2	1
Gross claim liability before all subrogation and before reinsurance	17	3
Less:
Gross RMBS subrogation	0	0
Discount, RMBS subrogation	0	0
Schedule Of Insured Financial Obligations With Credit Deterioration Reductions Of Gross Claim Liability Rmbs Subrogation	0	0
Less:
Gross other subrogation	(2)	(4)
Discount, other subrogation	0	0
Discounted other subrogation, before reinsurance	(2)	(3)
Gross claim liability, net of all subrogation and discounts, before reinsurance	15	0
Less: Unearned premium reserves	(12)	(2)
Plus: Loss adjustment expenses reserves	3	1
Claim liability reported on Balance Sheet, before reinsurance	6	(2)
Reinsurance recoverable reported on Balance Sheet	$ 0	$ 0
II [Member]
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Number of policies | Policies	9	9
Remaining weighted-average contract period (in years)	13 years	14 years
Gross insured contractual payments outstanding:
Principal	$ 430	$ 459
Interest	262	286
Total	692	745
Gross undiscounted claim liability	41	43
Discount, gross claim liability	7	7
Gross claim liability before all subrogation and before reinsurance	34	36
Less:
Gross RMBS subrogation	0	0
Discount, RMBS subrogation	0	0
Schedule Of Insured Financial Obligations With Credit Deterioration Reductions Of Gross Claim Liability Rmbs Subrogation	0	0
Less:
Gross other subrogation	0	0
Discount, other subrogation	0	0
Discounted other subrogation, before reinsurance	0	0
Gross claim liability, net of all subrogation and discounts, before reinsurance	34	36
Less: Unearned premium reserves	(4)	(5)
Plus: Loss adjustment expenses reserves	0	0
Claim liability reported on Balance Sheet, before reinsurance	30	32
Reinsurance recoverable reported on Balance Sheet	$ 8	$ 8
III [Member]
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Number of policies | Policies	13	12
Remaining weighted-average contract period (in years)	13 years	14 years
Gross insured contractual payments outstanding:
Principal	$ 394	$ 1,000
Interest	139	156
Total	534	1,156
Gross undiscounted claim liability	324	446
Discount, gross claim liability	86	162
Gross claim liability before all subrogation and before reinsurance	239	284
Less:
Gross RMBS subrogation	0	0
Discount, RMBS subrogation	0	0
Schedule Of Insured Financial Obligations With Credit Deterioration Reductions Of Gross Claim Liability Rmbs Subrogation	0	0
Less:
Gross other subrogation	(27)	(31)
Discount, other subrogation	4	5
Discounted other subrogation, before reinsurance	(23)	(26)
Gross claim liability, net of all subrogation and discounts, before reinsurance	215	258
Less: Unearned premium reserves	0	(8)
Plus: Loss adjustment expenses reserves	0	2
Claim liability reported on Balance Sheet, before reinsurance	215	252
Reinsurance recoverable reported on Balance Sheet	$ 18	$ 21
IV [Member]
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Number of policies | Policies	88	93
Remaining weighted-average contract period (in years)	12 years	12 years
Gross insured contractual payments outstanding:
Principal	$ 1,473	$ 1,646
Interest	600	565
Total	2,073	2,210
Gross undiscounted claim liability	772	729
Discount, gross claim liability	323	316
Gross claim liability before all subrogation and before reinsurance	450	413
Less:
Gross RMBS subrogation	0	(140)
Discount, RMBS subrogation	0	0
Schedule Of Insured Financial Obligations With Credit Deterioration Reductions Of Gross Claim Liability Rmbs Subrogation	0	(140)
Less:
Gross other subrogation	(208)	(172)
Discount, other subrogation	60	42
Discounted other subrogation, before reinsurance	(149)	(130)
Gross claim liability, net of all subrogation and discounts, before reinsurance	301	143
Less: Unearned premium reserves	(10)	(10)
Plus: Loss adjustment expenses reserves	1	4
Claim liability reported on Balance Sheet, before reinsurance	292	137
Reinsurance recoverable reported on Balance Sheet	$ 3	$ 3
V [Member]
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Number of policies | Policies	5	5
Remaining weighted-average contract period (in years)	7 years	7 years
Gross insured contractual payments outstanding:
Principal	$ 27	$ 34
Interest	17	19
Total	44	53
Gross undiscounted claim liability	44	53
Discount, gross claim liability	8	9
Gross claim liability before all subrogation and before reinsurance	36	43
Less:
Gross RMBS subrogation	0	0
Discount, RMBS subrogation	0	0
Schedule Of Insured Financial Obligations With Credit Deterioration Reductions Of Gross Claim Liability Rmbs Subrogation	0	0
Less:
Gross other subrogation	(11)	(12)
Discount, other subrogation	3	4
Discounted other subrogation, before reinsurance	(8)	(8)
Gross claim liability, net of all subrogation and discounts, before reinsurance	28	35
Less: Unearned premium reserves	(1)	(1)
Plus: Loss adjustment expenses reserves	0	0
Claim liability reported on Balance Sheet, before reinsurance	27	34
Reinsurance recoverable reported on Balance Sheet	$ 0	$ 0