Financial Guarantee Insurance Contracts - Summary of Information Related to Policies Currently Included in Ambac's Loss Reserves or Subrogation Recoverable (Detail) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2020 USD ($) Policies	Dec. 31, 2019 USD ($) Policies	Mar. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Loss Reserves Ceded To Reinsurers	$ 35	$ 26	$ 26	$ 23
Ceded Loss And Loss Expenses Paid Not Yet Recovered	$ 1	$ 0	$ 1
Number of policies | Policies	224	221
Remaining weighted-average contract period (in years)	15 years	15 years
Gross insured contractual payments outstanding:
Principal	$ 7,046	$ 6,168
Interest	3,358	3,029
Total	10,404	9,197
Gross undiscounted claim liability	2,450	2,434
Discount, gross claim liability	259	541
Gross claim liability before all subrogation and before reinsurance	2,191	1,893
Less:
Gross RMBS subrogation	(1,771)	(1,777)
Discount, RMBS subrogation	7	49
Discounted RMBS subrogation, before reinsurance	(1,764)	(1,727)
Less:
Gross other subrogation	(829)	(720)
Discount, other subrogation	21	53
Discounted other subrogation, before reinsurance	(809)	(666)
Gross claim liability, net of all subrogation and discounts, before reinsurance	(381)	(501)
Less: Unearned premium reserves	(70)	(54)
Plus: Loss adjustment expenses reserves	55	73
Claim liability reported on Balance Sheet, before reinsurance	(395)	(482)
Reinsurance recoverable reported on Balance Sheet	$ 36	$ 26
I/SL [Member]
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Number of policies | Policies	33	34
Remaining weighted-average contract period (in years)	24 years	8 years
Gross insured contractual payments outstanding:
Principal	$ 727	$ 668
Interest	394	340
Total	1,121	1,007
Gross undiscounted claim liability	18	2
Discount, gross claim liability	1	0
Gross claim liability before all subrogation and before reinsurance	17	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	17	2
Less: Unearned premium reserves	(2)	(1)
Plus: Loss adjustment expenses reserves	0	1
Claim liability reported on Balance Sheet, before reinsurance	15	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	22	18
Remaining weighted-average contract period (in years)	21 years	21 years
Gross insured contractual payments outstanding:
Principal	$ 483	$ 510
Interest	507	507
Total	991	1,016
Gross undiscounted claim liability	44	44
Discount, gross claim liability	2	5
Gross claim liability before all subrogation and before reinsurance	42	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	42	39
Less: Unearned premium reserves	(9)	(9)
Plus: Loss adjustment expenses reserves	1	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	14	11
Remaining weighted-average contract period (in years)	9 years	9 years
Gross insured contractual payments outstanding:
Principal	$ 607	$ 277
Interest	512	128
Total	1,119	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	41	20
Less: Unearned premium reserves	(5)	(1)
Plus: Loss adjustment expenses reserves	1	1
Claim liability reported on Balance Sheet, before reinsurance	36	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	16	16
Remaining weighted-average contract period (in years)	17 years	17 years
Gross insured contractual payments outstanding:
Principal	$ 1,524	$ 857
Interest	326	366
Total	1,850	1,223
Gross undiscounted claim liability	521	541
Discount, gross claim liability	71	152
Gross claim liability before all subrogation and before reinsurance	450	389
Less:
Gross RMBS subrogation	0	0
Discount, RMBS subrogation	0	0
Discounted RMBS subrogation, before reinsurance	0	0
Less:
Gross other subrogation	(39)	(41)
Discount, other subrogation	1	4
Discounted other subrogation, before reinsurance	(38)	(37)
Gross claim liability, net of all subrogation and discounts, before reinsurance	412	353
Less: Unearned premium reserves	(19)	(7)
Plus: Loss adjustment expenses reserves	4	4
Claim liability reported on Balance Sheet, before reinsurance	397	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	136	139
Remaining weighted-average contract period (in years)	15 years	14 years
Gross insured contractual payments outstanding:
Principal	$ 3,667	$ 3,819
Interest	1,608	1,678
Total	5,275	5,498
Gross undiscounted claim liability	1,778	1,778
Discount, gross claim liability	184	381
Gross claim liability before all subrogation and before reinsurance	1,594	1,397
Less:
Gross RMBS subrogation	(1,771)	(1,777)
Discount, RMBS subrogation	7	49
Discounted RMBS subrogation, before reinsurance	(1,764)	(1,727)
Less:
Gross other subrogation	(777)	(666)
Discount, other subrogation	18	47
Discounted other subrogation, before reinsurance	(759)	(620)
Gross claim liability, net of all subrogation and discounts, before reinsurance	(929)	(950)
Less: Unearned premium reserves	(34)	(35)
Plus: Loss adjustment expenses reserves	50	67
Claim liability reported on Balance Sheet, before reinsurance	(914)	(918)
Reinsurance recoverable reported on Balance Sheet	$ (7)	$ (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	$ 37	$ 37
Interest	11	11
Total	48	48
Gross undiscounted claim liability	48	48
Discount, gross claim liability	0	2
Gross claim liability before all subrogation and before reinsurance	47	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	36	36
Less: Unearned premium reserves	0	0
Plus: Loss adjustment expenses reserves	0	0
Claim liability reported on Balance Sheet, before reinsurance	36	36
Reinsurance recoverable reported on Balance Sheet	$ 0	$ 0