Financial Guarantee Insurance Contracts - Summary of Information Related to Policies Currently Included in Ambac's Loss Reserves or Subrogation Recoverable (Detail2) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Schedule Of Insured Financial Obligations With Credit Deterioration Reductions Of Gross Claim Liability Rmbs Subrogation	$ 1,727	$ 1,771
Loss Reserves Ceded To Reinsurers	26	23	$ 41	$ 31
Loss and loss expense reserves	1,548	1,826
Subrogation recoverable	(2,029)	(1,933)
Liability for Claims	(482)	(107)	$ 4,114	$ 3,696
Surveillance Category One [Member]
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Schedule Of Insured Financial Obligations With Credit Deterioration Reductions Of Gross Claim Liability Rmbs Subrogation	0	0
Surveillance Category Onea [Member]
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Schedule Of Insured Financial Obligations With Credit Deterioration Reductions Of Gross Claim Liability Rmbs Subrogation	0	0
Surveillance Category Two [Member]
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Schedule Of Insured Financial Obligations With Credit Deterioration Reductions Of Gross Claim Liability Rmbs Subrogation	0	0
Surveillance Category Three [Member]
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Schedule Of Insured Financial Obligations With Credit Deterioration Reductions Of Gross Claim Liability Rmbs Subrogation	0	0
Surveillance Category Four [Member]
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Schedule Of Insured Financial Obligations With Credit Deterioration Reductions Of Gross Claim Liability Rmbs Subrogation	1,727	1,771
Surveillance Category Five [Member]
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Schedule Of Insured Financial Obligations With Credit Deterioration Reductions Of Gross Claim Liability Rmbs Subrogation	$ 0	$ 0