Insurance Contracts - Summary of Information Related to Policies Currently Included in Ambac's Loss Reserves or Subrogation Recoverable (Details) $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2023 USD ($) Policies	Dec. 31, 2022 USD ($) Policies	Sep. 30, 2022 USD ($)
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Ceded Loss And Loss Expenses Paid Not Yet Recovered			$ 1
Number of policies | Policies	162	162
Remaining weighted-average contract period (in years)	12 years	14 years
Gross insured contractual payments outstanding:
Principal	$ 4,076	$ 4,047
Interest	1,512	1,750
Total	5,588	5,797
Gross undiscounted claim liability	1,325	1,279
Discount, gross claim liability	557	496
Gross claim liability before all subrogation and before reinsurance	(769)	783
Less:
Gross RMBS subrogation		(140)
Discount, RMBS subrogation		0
Discounted RMBS subrogation, before reinsurance	0	(140)
Less:
Gross other subrogation	(313)	(233)
Discount, other subrogation	79	54
Discounted other subrogation, before reinsurance	(234)	(179)
Gross claim liability, net of all subrogation and discounts, before reinsurance	(535)	464
Less: Unearned premium reserves	(29)	(28)
Plus: Loss adjustment expenses reserves	4	8
Claim liability reported on Balance Sheet, before reinsurance	510	444
Reinsurance recoverable reported on Balance Sheet	32	33
Financial Guarantee
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Ceded Loss And Loss Expenses Paid Not Yet Recovered	7
Financial Guarantee
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Loss Reserves Ceded To Reinsurers	32	33
Ceded Loss And Loss Expenses Paid Not Yet Recovered	$ 0	$ 0
I/SL [Member]
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Number of policies | Policies	27	37
Remaining weighted-average contract period (in years)	9 years	7 years
Gross insured contractual payments outstanding:
Principal	$ 492	$ 709
Interest	115	526
Total	606	1,235
Gross undiscounted claim liability	2	4
Discount, gross claim liability	0	1
Gross claim liability before all subrogation and before reinsurance	(1)	3
Less:
Gross RMBS subrogation		0
Discount, RMBS subrogation		0
Discounted RMBS subrogation, before reinsurance		0
Less:
Gross other subrogation	(13)	(14)
Discount, other subrogation	3	2
Discounted other subrogation, before reinsurance	(10)	(12)
Gross claim liability, net of all subrogation and discounts, before reinsurance	9	9
Less: Unearned premium reserves	(1)	(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	12	6
Remaining weighted-average contract period (in years)	10 years	19 years
Gross insured contractual payments outstanding:
Principal	$ 1,185	$ 200
Interest	354	198
Total	1,539	399
Gross undiscounted claim liability	26	4
Discount, gross claim liability	4	1
Gross claim liability before all subrogation and before reinsurance	(22)	3
Less:
Gross RMBS subrogation		0
Discount, RMBS subrogation		0
Discounted RMBS subrogation, before reinsurance		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	(20)	0
Less: Unearned premium reserves	(13)	(2)
Plus: Loss adjustment expenses reserves	3	1
Claim liability reported on Balance Sheet, before reinsurance	9	(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	10	9
Remaining weighted-average contract period (in years)	13 years	14 years
Gross insured contractual payments outstanding:
Principal	$ 455	$ 459
Interest	275	286
Total	730	745
Gross undiscounted claim liability	42	43
Discount, gross claim liability	8	7
Gross claim liability before all subrogation and before reinsurance	(34)	36
Less:
Gross RMBS subrogation		0
Discount, RMBS subrogation		0
Discounted RMBS subrogation, before reinsurance		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)	14 years	14 years
Gross insured contractual payments outstanding:
Principal	$ 401	$ 1,000
Interest	143	156
Total	543	1,156
Gross undiscounted claim liability	432	446
Discount, gross claim liability	172	162
Gross claim liability before all subrogation and before reinsurance	(259)	284
Less:
Gross RMBS subrogation		0
Discount, RMBS subrogation		0
Discounted RMBS subrogation, before reinsurance		0
Less:
Gross other subrogation	(28)	(31)
Discount, other subrogation	5	5
Discounted other subrogation, before reinsurance	(23)	(26)
Gross claim liability, net of all subrogation and discounts, before reinsurance	(236)	258
Less: Unearned premium reserves	0	(8)
Plus: Loss adjustment expenses reserves	1	2
Claim liability reported on Balance Sheet, before reinsurance	236	252
Reinsurance recoverable reported on Balance Sheet	$ 20	$ 21
IV [Member]
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Number of policies | Policies	95	93
Remaining weighted-average contract period (in years)	12 years	12 years
Gross insured contractual payments outstanding:
Principal	$ 1,516	$ 1,646
Interest	607	565
Total	2,123	2,210
Gross undiscounted claim liability	778	729
Discount, gross claim liability	363	316
Gross claim liability before all subrogation and before reinsurance	(415)	413
Less:
Gross RMBS subrogation		(140)
Discount, RMBS subrogation		0
Discounted RMBS subrogation, before reinsurance		(140)
Less:
Gross other subrogation	(257)	(172)
Discount, other subrogation	67	42
Discounted other subrogation, before reinsurance	(190)	(130)
Gross claim liability, net of all subrogation and discounts, before reinsurance	(225)	143
Less: Unearned premium reserves	(10)	(10)
Plus: Loss adjustment expenses reserves	1	4
Claim liability reported on Balance Sheet, before reinsurance	217	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	$ 28	$ 34
Interest	18	19
Total	46	53
Gross undiscounted claim liability	46	53
Discount, gross claim liability	10	9
Gross claim liability before all subrogation and before reinsurance	(36)	43
Less:
Gross RMBS subrogation		0
Discount, RMBS subrogation		0
Discounted RMBS subrogation, before reinsurance		0
Less:
Gross other subrogation	(12)	(12)
Discount, other subrogation	4	4
Discounted other subrogation, before reinsurance	(8)	(8)
Gross claim liability, net of all subrogation and discounts, before reinsurance	(29)	35
Less: Unearned premium reserves	(1)	(1)
Plus: Loss adjustment expenses reserves	0	0
Claim liability reported on Balance Sheet, before reinsurance	28	34
Reinsurance recoverable reported on Balance Sheet	$ 0	$ 0