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, 2023 USD ($) Policies	Sep. 30, 2022	Dec. 31, 2022 USD ($) Policies	Jun. 30, 2022 USD ($)
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Ceded Loss And Loss Expenses Paid Not Yet Recovered				$ 0
Number of policies | Policies	163		162
Remaining weighted-average contract period (in years)	12 years	14 years
Gross insured contractual payments outstanding:
Principal	$ 4,258		$ 4,047
Interest	1,784		1,750
Total	6,042		5,797
Gross undiscounted claim liability	1,296		1,279
Discount, gross claim liability	486		496
Gross claim liability before all subrogation and before reinsurance	(810)		783
Less:
Gross RMBS subrogation			(140)
Discount, RMBS subrogation			0
Discounted RMBS subrogation, before reinsurance	0		(140)
Less:
Gross other subrogation	(257)		(233)
Discount, other subrogation	67		54
Discounted other subrogation, before reinsurance	(190)		(179)
Gross claim liability, net of all subrogation and discounts, before reinsurance	(620)		464
Less: Unearned premium reserves	(31)		(28)
Plus: Loss adjustment expenses reserves	5		8
Claim liability reported on Balance Sheet, before reinsurance	594		444
Reinsurance recoverable reported on Balance Sheet	33		33
Financial Guarantee
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Ceded Loss And Loss Expenses Paid Not Yet Recovered	5
Financial Guarantee
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Loss Reserves Ceded To Reinsurers	33		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	32		37
Remaining weighted-average contract period (in years)	8 years	7 years
Gross insured contractual payments outstanding:
Principal	$ 573		$ 709
Interest	389		526
Total	962		1,235
Gross undiscounted claim liability	3		4
Discount, gross claim liability	1		1
Gross claim liability before all subrogation and before reinsurance	(3)		3
Less:
Gross RMBS subrogation			0
Discount, RMBS subrogation			0
Discounted RMBS subrogation, before reinsurance			0
Less:
Gross other subrogation	(15)		(14)
Discount, other subrogation	3		2
Discounted other subrogation, before reinsurance	(12)		(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	9		6
Remaining weighted-average contract period (in years)	13 years	19 years
Gross insured contractual payments outstanding:
Principal	$ 633		$ 200
Interest	352		198
Total	986		399
Gross undiscounted claim liability	12		4
Discount, gross claim liability	2		1
Gross claim liability before all subrogation and before reinsurance	(10)		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	(7)		0
Less: Unearned premium reserves	(6)		(2)
Plus: Loss adjustment expenses reserves	1		1
Claim liability reported on Balance Sheet, before reinsurance	2		(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	12		9
Remaining weighted-average contract period (in years)	12 years	14 years
Gross insured contractual payments outstanding:
Principal	$ 1,048		$ 459
Interest	286		286
Total	1,334		745
Gross undiscounted claim liability	56		43
Discount, gross claim liability	9		7
Gross claim liability before all subrogation and before reinsurance	(47)		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	(47)		36
Less: Unearned premium reserves	(12)		(5)
Plus: Loss adjustment expenses reserves	1		0
Claim liability reported on Balance Sheet, before reinsurance	37		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	12		12
Remaining weighted-average contract period (in years)	14 years	14 years
Gross insured contractual payments outstanding:
Principal	$ 408		$ 1,000
Interest	146		156
Total	553		1,156
Gross undiscounted claim liability	409		446
Discount, gross claim liability	145		162
Gross claim liability before all subrogation and before reinsurance	(263)		284
Less:
Gross RMBS subrogation			0
Discount, RMBS subrogation			0
Discounted RMBS subrogation, before reinsurance			0
Less:
Gross other subrogation	(29)		(31)
Discount, other subrogation	5		5
Discounted other subrogation, before reinsurance	(24)		(26)
Gross claim liability, net of all subrogation and discounts, before reinsurance	(239)		258
Less: Unearned premium reserves	0		(8)
Plus: Loss adjustment expenses reserves	1		2
Claim liability reported on Balance Sheet, before reinsurance	239		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	93		93
Remaining weighted-average contract period (in years)	12 years	12 years
Gross insured contractual payments outstanding:
Principal	$ 1,568		$ 1,646
Interest	593		565
Total	2,161		2,210
Gross undiscounted claim liability	769		729
Discount, gross claim liability	320		316
Gross claim liability before all subrogation and before reinsurance	(449)		413
Less:
Gross RMBS subrogation			(140)
Discount, RMBS subrogation			0
Discounted RMBS subrogation, before reinsurance			(140)
Less:
Gross other subrogation	(200)		(172)
Discount, other subrogation	56		42
Discounted other subrogation, before reinsurance	(144)		(130)
Gross claim liability, net of all subrogation and discounts, before reinsurance	(305)		143
Less: Unearned premium reserves	(10)		(10)
Plus: Loss adjustment expenses reserves	1		4
Claim liability reported on Balance Sheet, before reinsurance	297		137
Reinsurance recoverable reported on Balance Sheet	$ 4		$ 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	19		19
Total	46		53
Gross undiscounted claim liability	46		53
Discount, gross claim liability	9		9
Gross claim liability before all subrogation and before reinsurance	(37)		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