Special Purpose Entities, Including Variable Interest Entities - Summary of Carrying Amount of Assets, Liabilities and Maximum Exposure to Loss of Ambac's Variable Interests in Non-Consolidated Variable Interest Entities (Details) - USD ($) $ in Millions	Sep. 30, 2023	Dec. 31, 2022
Variable Interest Entities [Line Items]
Maximum Exposure To Loss	$ 20,971	$ 21,639
Insurance Assets	392	490
Insurance Liabilities	833	840
Derivative Liabilities	0
Variable Interest Entity, Primary Beneficiary [Member]
Variable Interest Entities [Line Items]
Derivative Liabilities		1
Global Public Finance [Member]
Variable Interest Entities [Line Items]
Maximum Exposure To Loss	17,543	17,997
Insurance Assets	210	216
Insurance Liabilities	208	212
Derivative Liabilities	0	0
Global Structured Finance [Member]
Variable Interest Entities [Line Items]
Maximum Exposure To Loss	3,428	3,642
Insurance Assets	182	274
Insurance Liabilities	624	628
Derivative Liabilities	0	1
Global Structured Finance [Member] | Residential Mortgage-Backed Securities [Member]
Variable Interest Entities [Line Items]
Maximum Exposure To Loss	2,441	2,559
Insurance Assets	175	266
Insurance Liabilities	396	400
Derivative Liabilities	0	0
Global Structured Finance [Member] | Other Consumer Asset-Backed [Member]
Variable Interest Entities [Line Items]
Maximum Exposure To Loss	566	652
Insurance Assets	5	6
Insurance Liabilities	227	225
Derivative Liabilities	0	0
Global Structured Finance [Member] | Other [Member]
Variable Interest Entities [Line Items]
Maximum Exposure To Loss	421	430
Insurance Assets	2	2
Insurance Liabilities	2	2
Derivative Liabilities	$ 0	$ 1