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	Jun. 30, 2021	Dec. 31, 2020
Variable Interest Entities [Line Items]
Maximum Exposure To Loss	$ 26,325	$ 27,998
Insurance Assets	2,254	2,314
Insurance Liabilities	1,051	1,122
Derivative Liabilities	6
Variable Interest Entity, Primary Beneficiary [Member]
Variable Interest Entities [Line Items]
Derivative Liabilities		8
Global Public Finance [Member]
Variable Interest Entities [Line Items]
Maximum Exposure To Loss	20,802	21,646
Insurance Assets	264	263
Insurance Liabilities	278	287
Derivative Liabilities	0	0
Global Structured Finance [Member]
Variable Interest Entities [Line Items]
Maximum Exposure To Loss	5,523	6,352
Insurance Assets	1,989	2,051
Insurance Liabilities	772	834
Derivative Liabilities	7	8
Global Structured Finance [Member] | Residential Mortgage-Backed Securities [Member]
Variable Interest Entities [Line Items]
Maximum Exposure To Loss	3,747	4,308
Insurance Assets	1,967	2,024
Insurance Liabilities	527	580
Derivative Liabilities	0	0
Global Structured Finance [Member] | Other Consumer Asset-Backed [Member]
Variable Interest Entities [Line Items]
Maximum Exposure To Loss	894	1,050
Insurance Assets	19	24
Insurance Liabilities	234	239
Derivative Liabilities	0	0
Global Structured Finance [Member] | Other Commercial Asset-Backed [Member]
Variable Interest Entities [Line Items]
Maximum Exposure To Loss	1	24
Insurance Assets	3	3
Insurance Liabilities	1	1
Derivative Liabilities	0	0
Global Structured Finance [Member] | Other [Member]
Variable Interest Entities [Line Items]
Maximum Exposure To Loss	880	970
Insurance Assets	0	0
Insurance Liabilities	10	13
Derivative Liabilities	$ 7	$ 8