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	Mar. 31, 2021	Dec. 31, 2020
Variable Interest Entities [Line Items]
Maximum Exposure To Loss	$ 27,357	$ 27,998
Insurance Assets	2,243	2,314
Insurance Liabilities	1,057	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	21,330	21,646
Insurance Assets	266	263
Insurance Liabilities	291	287
Derivative Liabilities	0	0
Global Structured Finance [Member]
Variable Interest Entities [Line Items]
Maximum Exposure To Loss	6,027	6,352
Insurance Assets	1,977	2,051
Insurance Liabilities	766	834
Derivative Liabilities	7	8
Global Structured Finance [Member] | Residential Mortgage-Backed Securities [Member]
Variable Interest Entities [Line Items]
Maximum Exposure To Loss	4,065	4,308
Insurance Assets	1,951	2,024
Insurance Liabilities	513	580
Derivative Liabilities	0	0
Global Structured Finance [Member] | Other Consumer Asset-Backed [Member]
Variable Interest Entities [Line Items]
Maximum Exposure To Loss	981	1,050
Insurance Assets	23	24
Insurance Liabilities	239	239
Derivative Liabilities	0	0
Global Structured Finance [Member] | Other Commercial Asset-Backed [Member]
Variable Interest Entities [Line Items]
Maximum Exposure To Loss	23	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	958	970
Insurance Assets	0	0
Insurance Liabilities	13	13
Derivative Liabilities	$ 7	$ 8