Financial Guarantee Insurance Contracts - Summary of Information Related to Policies Currently Included in Ambac's Loss Reserves or Subrogation Recoverable (Details) $ in Millions	3 Months Ended	6 Months Ended
	Mar. 31, 2020	Jun. 30, 2021 USD ($) Policies	Dec. 31, 2020 USD ($) Policies	Jun. 30, 2020 USD ($)	Dec. 31, 2019 USD ($)
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Loss Reserves Ceded To Reinsurers		$ 30	$ 33	$ 35	$ 26
Ceded Loss And Loss Expenses Paid Not Yet Recovered		$ 1	$ 1	$ 1
Number of policies | Policies		242	232
Remaining weighted-average contract period (in years)	14 years	13 years
Gross insured contractual payments outstanding:
Principal		$ 6,545	$ 7,573
Interest		3,028	3,443
Total		9,573	11,016
Gross undiscounted claim liability		2,239	2,395
Discount, gross claim liability		320	303
Gross claim liability before all subrogation and before reinsurance		1,919	2,092
Less:
Gross RMBS subrogation		(1,742)	(1,753)
Discount, RMBS subrogation		2	3
Discounted RMBS subrogation, before reinsurance		(1,741)	(1,751)
Less:
Gross other subrogation		(654)	(755)
Discount, other subrogation		27	20
Discounted other subrogation, before reinsurance		(627)	(735)
Gross claim liability, net of all subrogation and discounts, before reinsurance		(449)	(394)
Less: Unearned premium reserves		(59)	(72)
Plus: Loss adjustment expenses reserves		52	68
Claim liability reported on Balance Sheet, before reinsurance		(456)	(397)
Reinsurance recoverable reported on Balance Sheet		$ 30	$ 33
I/SL [Member]
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Number of policies | Policies		40	40
Remaining weighted-average contract period (in years)	10 years	10 years
Gross insured contractual payments outstanding:
Principal		$ 894	$ 842
Interest		328	279
Total		1,222	1,121
Gross undiscounted claim liability		4	3
Discount, gross claim liability		0	0
Gross claim liability before all subrogation and before reinsurance		4	3
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	3
Less: Unearned premium reserves		(2)	(2)
Plus: Loss adjustment expenses reserves		2	1
Claim liability reported on Balance Sheet, before reinsurance		3	2
Reinsurance recoverable reported on Balance Sheet		$ 1	$ 0
IA [Member]
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Number of policies | Policies		27	25
Remaining weighted-average contract period (in years)	18 years	14 years
Gross insured contractual payments outstanding:
Principal		$ 508	$ 1,375
Interest		716	1,011
Total		1,224	2,386
Gross undiscounted claim liability		13	49
Discount, gross claim liability		1	2
Gross claim liability before all subrogation and before reinsurance		12	47
Less:
Gross RMBS subrogation		0	0
Discount, RMBS subrogation		0	0
Discounted RMBS subrogation, before reinsurance		0	0
Less:
Gross other subrogation		(6)	0
Discount, other subrogation		0	0
Discounted other subrogation, before reinsurance		(6)	0
Gross claim liability, net of all subrogation and discounts, before reinsurance		6	47
Less: Unearned premium reserves		(4)	(16)
Plus: Loss adjustment expenses reserves		0	2
Claim liability reported on Balance Sheet, before reinsurance		2	32
Reinsurance recoverable reported on Balance Sheet		$ 1	$ 6
II [Member]
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Number of policies | Policies		20	15
Remaining weighted-average contract period (in years)	8 years	14 years
Gross insured contractual payments outstanding:
Principal		$ 705	$ 595
Interest		425	484
Total		1,130	1,079
Gross undiscounted claim liability		59	40
Discount, gross claim liability		3	1
Gross claim liability before all subrogation and before reinsurance		56	40
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		56	39
Less: Unearned premium reserves		(9)	(5)
Plus: Loss adjustment expenses reserves		1	1
Claim liability reported on Balance Sheet, before reinsurance		47	35
Reinsurance recoverable reported on Balance Sheet		$ 13	$ 9
III [Member]
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Number of policies | Policies		16	15
Remaining weighted-average contract period (in years)	16 years	15 years
Gross insured contractual payments outstanding:
Principal		$ 1,389	$ 1,469
Interest		185	215
Total		1,574	1,685
Gross undiscounted claim liability		559	541
Discount, gross claim liability		114	85
Gross claim liability before all subrogation and before reinsurance		445	456
Less:
Gross RMBS subrogation		0	0
Discount, RMBS subrogation		0	0
Discounted RMBS subrogation, before reinsurance		0	0
Less:
Gross other subrogation		(35)	(36)
Discount, other subrogation		2	1
Discounted other subrogation, before reinsurance		(33)	(35)
Gross claim liability, net of all subrogation and discounts, before reinsurance		413	421
Less: Unearned premium reserves		(15)	(17)
Plus: Loss adjustment expenses reserves		4	5
Claim liability reported on Balance Sheet, before reinsurance		402	409
Reinsurance recoverable reported on Balance Sheet		$ 23	$ 24
IV [Member]
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Number of policies | Policies		134	132
Remaining weighted-average contract period (in years)	14 years	13 years
Gross insured contractual payments outstanding:
Principal		$ 3,009	$ 3,246
Interest		1,350	1,427
Total		4,359	4,673
Gross undiscounted claim liability		1,539	1,690
Discount, gross claim liability		198	213
Gross claim liability before all subrogation and before reinsurance		1,341	1,477
Less:
Gross RMBS subrogation		(1,742)	(1,753)
Discount, RMBS subrogation		2	3
Discounted RMBS subrogation, before reinsurance		(1,741)	(1,751)
Less:
Gross other subrogation		(601)	(706)
Discount, other subrogation		23	18
Discounted other subrogation, before reinsurance		(578)	(689)
Gross claim liability, net of all subrogation and discounts, before reinsurance		(977)	(963)
Less: Unearned premium reserves		(28)	(30)
Plus: Loss adjustment expenses reserves		45	59
Claim liability reported on Balance Sheet, before reinsurance		(960)	(933)
Reinsurance recoverable reported on Balance Sheet		$ (8)	$ (6)
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		$ 40	$ 47
Interest		24	26
Total		65	72
Gross undiscounted claim liability		65	72
Discount, gross claim liability		4	3
Gross claim liability before all subrogation and before reinsurance		60	69
Less:
Gross RMBS subrogation		0	0
Discount, RMBS subrogation		0	0
Discounted RMBS subrogation, before reinsurance		0	0
Less:
Gross other subrogation		(12)	(12)
Discount, other subrogation		2	1
Discounted other subrogation, before reinsurance		(10)	(11)
Gross claim liability, net of all subrogation and discounts, before reinsurance		50	58
Less: Unearned premium reserves		(1)	(1)
Plus: Loss adjustment expenses reserves		0	0
Claim liability reported on Balance Sheet, before reinsurance		49	57
Reinsurance recoverable reported on Balance Sheet		$ 0	$ 0