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, 2020 USD ($) Policies	Dec. 31, 2019 USD ($) Policies	Dec. 31, 2018 USD ($)
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Number of policies | Policies	232	221
Remaining weighted-average contract period (in years)	14 years	15 years
Gross insured contractual payments outstanding:
Principal	$ 7,573	$ 6,168
Interest	3,443	3,029
Total	11,016	9,197
Gross undiscounted claim liability	2,395	2,434
Discount, gross claim liability	303	541
Gross claim liability before all subrogation and before reinsurance	2,092	1,893
Less:
Gross RMBS subrogation	(1,753)	(1,777)
Discount, RMBS subrogation	3	49
Schedule Of Insured Financial Obligations With Credit Deterioration Reductions Of Gross Claim Liability Rmbs Subrogation	(1,751)	(1,727)
Less:
Gross other subrogation	(755)	(720)
Discount, other subrogation	20	53
Discounted other subrogation, before reinsurance	(735)	(666)
Gross claim liability, net of all subrogation and discounts, before reinsurance	(394)	(501)
Less: Unearned premium reserves	(72)	(54)
Plus: Loss adjustment expenses reserves	68	73
Claim liability reported on Balance Sheet, before reinsurance	(397)	(482)
Reinsurance recoverable reported on Balance Sheet	33	26
Ceded Loss And Loss Expenses Paid Not Yet Recovered	$ 1	$ 0	$ 1
Loss Reserves [Member]
Less:
Weighted average risk-free rate used to discount loss reserves	1.10%	2.10%
Surveillance Category One [Member]
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Number of policies | Policies	40	34
Remaining weighted-average contract period (in years)	10 years	8 years
Gross insured contractual payments outstanding:
Principal	$ 842	$ 668
Interest	279	340
Total	1,121	1,007
Gross undiscounted claim liability	3	2
Discount, gross claim liability	0	0
Gross claim liability before all subrogation and before reinsurance	3	2
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	3	2
Less: Unearned premium reserves	(2)	(1)
Plus: Loss adjustment expenses reserves	1	1
Claim liability reported on Balance Sheet, before reinsurance	2	1
Reinsurance recoverable reported on Balance Sheet	$ 0	$ 0
IA [Member]
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Number of policies | Policies	25	18
Remaining weighted-average contract period (in years)	18 years	21 years
Gross insured contractual payments outstanding:
Principal	$ 1,375	$ 510
Interest	1,011	507
Total	2,386	1,016
Gross undiscounted claim liability	49	44
Discount, gross claim liability	2	5
Gross claim liability before all subrogation and before reinsurance	47	39
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	47	39
Less: Unearned premium reserves	(16)	(9)
Plus: Loss adjustment expenses reserves	2	1
Claim liability reported on Balance Sheet, before reinsurance	32	30
Reinsurance recoverable reported on Balance Sheet	$ 6	$ 6
II [Member]
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Number of policies | Policies	15	11
Remaining weighted-average contract period (in years)	8 years	9 years
Gross insured contractual payments outstanding:
Principal	$ 595	$ 277
Interest	484	128
Total	1,079	404
Gross undiscounted claim liability	40	21
Discount, gross claim liability	1	1
Gross claim liability before all subrogation and before reinsurance	40	20
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	39	20
Less: Unearned premium reserves	(5)	(1)
Plus: Loss adjustment expenses reserves	1	1
Claim liability reported on Balance Sheet, before reinsurance	35	20
Reinsurance recoverable reported on Balance Sheet	$ 9	$ 7
III [Member]
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Number of policies | Policies	15	16
Remaining weighted-average contract period (in years)	16 years	17 years
Gross insured contractual payments outstanding:
Principal	$ 1,469	$ 857
Interest	215	366
Total	1,685	1,223
Gross undiscounted claim liability	541	541
Discount, gross claim liability	85	152
Gross claim liability before all subrogation and before reinsurance	456	389
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	(36)	(41)
Discount, other subrogation	1	4
Discounted other subrogation, before reinsurance	(35)	(37)
Gross claim liability, net of all subrogation and discounts, before reinsurance	421	353
Less: Unearned premium reserves	(17)	(7)
Plus: Loss adjustment expenses reserves	5	4
Claim liability reported on Balance Sheet, before reinsurance	409	349
Reinsurance recoverable reported on Balance Sheet	$ 24	$ 24
IV [Member]
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Number of policies | Policies	132	139
Remaining weighted-average contract period (in years)	14 years	14 years
Gross insured contractual payments outstanding:
Principal	$ 3,246	$ 3,819
Interest	1,427	1,678
Total	4,673	5,498
Gross undiscounted claim liability	1,690	1,778
Discount, gross claim liability	213	381
Gross claim liability before all subrogation and before reinsurance	1,477	1,397
Less:
Gross RMBS subrogation	(1,753)	(1,777)
Discount, RMBS subrogation	3	49
Schedule Of Insured Financial Obligations With Credit Deterioration Reductions Of Gross Claim Liability Rmbs Subrogation	(1,751)	(1,727)
Less:
Gross other subrogation	(706)	(666)
Discount, other subrogation	18	47
Discounted other subrogation, before reinsurance	(689)	(620)
Gross claim liability, net of all subrogation and discounts, before reinsurance	(963)	(950)
Less: Unearned premium reserves	(30)	(35)
Plus: Loss adjustment expenses reserves	59	67
Claim liability reported on Balance Sheet, before reinsurance	(933)	(918)
Reinsurance recoverable reported on Balance Sheet	$ (6)	$ (10)
V [Member]
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Number of policies | Policies	5	3
Remaining weighted-average contract period (in years)	7 years	3 years
Gross insured contractual payments outstanding:
Principal	$ 47	$ 37
Interest	26	11
Total	72	48
Gross undiscounted claim liability	72	48
Discount, gross claim liability	3	2
Gross claim liability before all subrogation and before reinsurance	69	46
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	(12)	(13)
Discount, other subrogation	1	3
Discounted other subrogation, before reinsurance	(11)	(10)
Gross claim liability, net of all subrogation and discounts, before reinsurance	58	36
Less: Unearned premium reserves	(1)	0
Plus: Loss adjustment expenses reserves	0	0
Claim liability reported on Balance Sheet, before reinsurance	57	36
Reinsurance recoverable reported on Balance Sheet	$ 0	$ 0