Financial Guarantee 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, 2020 USD ($) Policies	Dec. 31, 2019 USD ($) Policies	Sep. 30, 2019 USD ($)	Dec. 31, 2018 USD ($)
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Loss Reserves Ceded To Reinsurers	$ 36	$ 26	$ 26	$ 23
Ceded Loss And Loss Expenses Paid Not Yet Recovered	$ 1	$ 0	$ 0
Number of policies | Policies	242	221
Remaining weighted-average contract period (in years)	15 years	15 years
Gross insured contractual payments outstanding:
Principal	$ 7,752	$ 6,168
Interest	3,639	3,029
Total	11,391	9,197
Gross undiscounted claim liability	2,404	2,434
Discount, gross claim liability	259	541
Gross claim liability before all subrogation and before reinsurance	2,145	1,893
Less:
Gross RMBS subrogation	(1,760)	(1,777)
Discount, RMBS subrogation	2	49
Discounted RMBS subrogation, before reinsurance	(1,757)	(1,727)
Less:
Gross other subrogation	(799)	(720)
Discount, other subrogation	18	53
Discounted other subrogation, before reinsurance	(781)	(666)
Gross claim liability, net of all subrogation and discounts, before reinsurance	(393)	(501)
Less: Unearned premium reserves	(80)	(54)
Plus: Loss adjustment expenses reserves	80	73
Claim liability reported on Balance Sheet, before reinsurance	(393)	(482)
Reinsurance recoverable reported on Balance Sheet	$ 37	$ 26
I/SL [Member]
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Number of policies | Policies	46	34
Remaining weighted-average contract period (in years)	10 years	8 years
Gross insured contractual payments outstanding:
Principal	$ 1,064	$ 668
Interest	329	340
Total	1,393	1,007
Gross undiscounted claim liability	4	2
Discount, gross claim liability	0	0
Gross claim liability before all subrogation and before reinsurance	4	2
Less:
Gross RMBS subrogation	0	0
Discount, RMBS subrogation	0	0
Discounted RMBS subrogation, before reinsurance	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	4	2
Less: Unearned premium reserves	(3)	(1)
Plus: Loss adjustment expenses reserves	2	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	26	18
Remaining weighted-average contract period (in years)	19 years	21 years
Gross insured contractual payments outstanding:
Principal	$ 1,146	$ 510
Interest	1,083	507
Total	2,229	1,016
Gross undiscounted claim liability	56	44
Discount, gross claim liability	2	5
Gross claim liability before all subrogation and before reinsurance	54	39
Less:
Gross RMBS subrogation	0	0
Discount, RMBS subrogation	0	0
Discounted RMBS subrogation, before reinsurance	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	54	39
Less: Unearned premium reserves	(22)	(9)
Plus: Loss adjustment expenses reserves	2	1
Claim liability reported on Balance Sheet, before reinsurance	34	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	16	11
Remaining weighted-average contract period (in years)	8 years	9 years
Gross insured contractual payments outstanding:
Principal	$ 614	$ 277
Interest	492	128
Total	1,106	404
Gross undiscounted claim liability	41	21
Discount, gross claim liability	1	1
Gross claim liability before all subrogation and before reinsurance	41	20
Less:
Gross RMBS subrogation	0	0
Discount, RMBS subrogation	0	0
Discounted RMBS subrogation, before reinsurance	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	40	20
Less: Unearned premium reserves	(5)	(1)
Plus: Loss adjustment expenses reserves	1	1
Claim liability reported on Balance Sheet, before reinsurance	37	20
Reinsurance recoverable reported on Balance Sheet	$ 10	$ 7
III [Member]
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Number of policies | Policies	16	16
Remaining weighted-average contract period (in years)	16 years	17 years
Gross insured contractual payments outstanding:
Principal	$ 1,511	$ 857
Interest	260	366
Total	1,771	1,223
Gross undiscounted claim liability	536	541
Discount, gross claim liability	71	152
Gross claim liability before all subrogation and before reinsurance	465	389
Less:
Gross RMBS subrogation	0	0
Discount, RMBS subrogation	0	0
Discounted RMBS subrogation, before reinsurance	0	0
Less:
Gross other subrogation	(37)	(41)
Discount, other subrogation	1	4
Discounted other subrogation, before reinsurance	(36)	(37)
Gross claim liability, net of all subrogation and discounts, before reinsurance	429	353
Less: Unearned premium reserves	(18)	(7)
Plus: Loss adjustment expenses reserves	7	4
Claim liability reported on Balance Sheet, before reinsurance	418	349
Reinsurance recoverable reported on Balance Sheet	$ 27	$ 24
IV [Member]
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Number of policies | Policies	135	139
Remaining weighted-average contract period (in years)	14 years	14 years
Gross insured contractual payments outstanding:
Principal	$ 3,386	$ 3,819
Interest	1,465	1,678
Total	4,851	5,498
Gross undiscounted claim liability	1,726	1,778
Discount, gross claim liability	185	381
Gross claim liability before all subrogation and before reinsurance	1,541	1,397
Less:
Gross RMBS subrogation	(1,760)	(1,777)
Discount, RMBS subrogation	2	49
Discounted RMBS subrogation, before reinsurance	(1,757)	(1,727)
Less:
Gross other subrogation	(749)	(666)
Discount, other subrogation	16	47
Discounted other subrogation, before reinsurance	(734)	(620)
Gross claim liability, net of all subrogation and discounts, before reinsurance	(950)	(950)
Less: Unearned premium reserves	(32)	(35)
Plus: Loss adjustment expenses reserves	68	67
Claim liability reported on Balance Sheet, before reinsurance	(913)	(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	3	3
Remaining weighted-average contract period (in years)	2 years	3 years
Gross insured contractual payments outstanding:
Principal	$ 32	$ 37
Interest	10	11
Total	41	48
Gross undiscounted claim liability	41	48
Discount, gross claim liability	0	2
Gross claim liability before all subrogation and before reinsurance	41	46
Less:
Gross RMBS subrogation	0	0
Discount, RMBS subrogation	0	0
Discounted RMBS subrogation, before reinsurance	0	0
Less:
Gross other subrogation	(13)	(13)
Discount, other subrogation	1	3
Discounted other subrogation, before reinsurance	(11)	(10)
Gross claim liability, net of all subrogation and discounts, before reinsurance	30	36
Less: Unearned premium reserves	0	0
Plus: Loss adjustment expenses reserves	0	0
Claim liability reported on Balance Sheet, before reinsurance	30	36
Reinsurance recoverable reported on Balance Sheet	$ 0	$ 0