Insurance Contracts - Summary of Information Related to Policies Currently Included in Ambac's Loss Reserves or Subrogation Recoverable (Details) $ in Millions	6 Months Ended	9 Months Ended
	Jun. 30, 2024	Sep. 30, 2024 USD ($) Policies	Dec. 31, 2023 USD ($) Policies	Sep. 30, 2023 USD ($)
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Ceded Loss And Loss Expenses Paid Not Yet Recovered				$ 7
Number of policies | Policies		158	141
Remaining weighted-average contract period (in years)	12 years	11 years
Gross insured contractual payments outstanding:
Principal		$ 2,736	$ 3,838
Interest		1,152	1,421
Total		3,887	5,259
Gross undiscounted claim liability		1,103	1,202
Discount, gross claim liability		404	426
Gross claim liability before all subrogation and before reinsurance		(700)	777
Less:
Gross other subrogation		(263)	(263)
Discount, other subrogation		69	69
Discounted other subrogation, before reinsurance		(195)	(194)
Gross claim liability, net of all subrogation and discounts, before reinsurance		(505)	583
Less: Unearned premium reserves		(15)	(28)
Plus: Loss adjustment expenses reserves		1	4
Claim liability reported on Balance Sheet, before reinsurance		491	559
Reinsurance recoverable reported on Balance Sheet		27	30
Financial Guarantee
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Ceded Loss And Loss Expenses Paid Not Yet Recovered		42
Financial Guarantee
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Loss Reserves Ceded To Reinsurers		26	30
Ceded Loss And Loss Expenses Paid Not Yet Recovered		$ 1	$ (1)
I/SL [Member]
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Number of policies | Policies		25	18
Remaining weighted-average contract period (in years)	9 years	5 years
Gross insured contractual payments outstanding:
Principal		$ 428	$ 429
Interest		68	75
Total		497	505
Gross undiscounted claim liability		1	1
Discount, gross claim liability		0	0
Gross claim liability before all subrogation and before reinsurance		(1)	1
Less:
Gross other subrogation		(12)	(13)
Discount, other subrogation		2	2
Discounted other subrogation, before reinsurance		(10)	(11)
Gross claim liability, net of all subrogation and discounts, before reinsurance		9	10
Less: Unearned premium reserves		(1)	0
Plus: Loss adjustment expenses reserves		0	0
Claim liability reported on Balance Sheet, before reinsurance		(10)	(10)
Reinsurance recoverable reported on Balance Sheet		$ 0	$ 1
IA [Member]
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Number of policies | Policies		6	8
Remaining weighted-average contract period (in years)	9 years	18 years
Gross insured contractual payments outstanding:
Principal		$ 198	$ 1,084
Interest		180	328
Total		379	1,412
Gross undiscounted claim liability		9	19
Discount, gross claim liability		2	2
Gross claim liability before all subrogation and before reinsurance		(7)	17
Less:
Gross other subrogation		(3)	(2)
Discount, other subrogation		0	0
Discounted other subrogation, before reinsurance		(3)	(2)
Gross claim liability, net of all subrogation and discounts, before reinsurance		(4)	(15)
Less: Unearned premium reserves		(2)	(12)
Plus: Loss adjustment expenses reserves		0	3
Claim liability reported on Balance Sheet, before reinsurance		2	6
Reinsurance recoverable reported on Balance Sheet		$ 0	$ 0
II [Member]
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Number of policies | Policies		13	9
Remaining weighted-average contract period (in years)	13 years	12 years
Gross insured contractual payments outstanding:
Principal		$ 437	$ 430
Interest		252	262
Total		689	692
Gross undiscounted claim liability		39	41
Discount, gross claim liability		6	7
Gross claim liability before all subrogation and before reinsurance		(33)	34
Less:
Gross other subrogation		(1)	0
Discount, other subrogation		0	0
Discounted other subrogation, before reinsurance		0	0
Gross claim liability, net of all subrogation and discounts, before reinsurance		(33)	34
Less: Unearned premium reserves		(4)	(4)
Plus: Loss adjustment expenses reserves		0	0
Claim liability reported on Balance Sheet, before reinsurance		29	30
Reinsurance recoverable reported on Balance Sheet		$ 8	$ 8
III [Member]
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Number of policies | Policies		11	13
Remaining weighted-average contract period (in years)	13 years	10 years
Gross insured contractual payments outstanding:
Principal		$ 260	$ 394
Interest		91	139
Total		351	534
Gross undiscounted claim liability		220	324
Discount, gross claim liability		65	86
Gross claim liability before all subrogation and before reinsurance		(155)	239
Less:
Gross other subrogation		(32)	(27)
Discount, other subrogation		3	4
Discounted other subrogation, before reinsurance		(28)	(23)
Gross claim liability, net of all subrogation and discounts, before reinsurance		(127)	215
Less: Unearned premium reserves		2	0
Plus: Loss adjustment expenses reserves		0	0
Claim liability reported on Balance Sheet, before reinsurance		128	215
Reinsurance recoverable reported on Balance Sheet		$ 13	$ 18
IV [Member]
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Number of policies | Policies		99	88
Remaining weighted-average contract period (in years)	12 years	12 years
Gross insured contractual payments outstanding:
Principal		$ 1,390	$ 1,473
Interest		545	600
Total		1,935	2,073
Gross undiscounted claim liability		797	772
Discount, gross claim liability		324	323
Gross claim liability before all subrogation and before reinsurance		(473)	450
Less:
Gross other subrogation		(205)	(208)
Discount, other subrogation		60	60
Discounted other subrogation, before reinsurance		(145)	(149)
Gross claim liability, net of all subrogation and discounts, before reinsurance		(329)	301
Less: Unearned premium reserves		(9)	(10)
Plus: Loss adjustment expenses reserves		0	1
Claim liability reported on Balance Sheet, before reinsurance		320	292
Reinsurance recoverable reported on Balance Sheet		$ 5	$ 3
V [Member]
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Number of policies | Policies		4	5
Remaining weighted-average contract period (in years)	7 years	8 years
Gross insured contractual payments outstanding:
Principal		$ 21	$ 27
Interest		16	17
Total		37	44
Gross undiscounted claim liability		37	44
Discount, gross claim liability		7	8
Gross claim liability before all subrogation and before reinsurance		(30)	36
Less:
Gross other subrogation		(11)	(11)
Discount, other subrogation		3	3
Discounted other subrogation, before reinsurance		(8)	(8)
Gross claim liability, net of all subrogation and discounts, before reinsurance		(22)	28
Less: Unearned premium reserves		(1)	(1)
Plus: Loss adjustment expenses reserves		0	0
Claim liability reported on Balance Sheet, before reinsurance		21	27
Reinsurance recoverable reported on Balance Sheet		$ 0	$ 0