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, 2020	Dec. 31, 2019
Variable Interest Entities [Line Items]
Maximum Exposure To Loss	$ 28,881	$ 31,506
Insurance Assets	2,361	2,247
Insurance Liabilities	1,177	1,083
Derivative Liabilities	7
Variable Interest Entity, Primary Beneficiary [Member]
Variable Interest Entities [Line Items]
Derivative Liabilities		7
Global Public Finance [Member]
Variable Interest Entities [Line Items]
Maximum Exposure To Loss	22,183	23,341
Insurance Assets	268	287
Insurance Liabilities	316	321
Derivative Liabilities	(1)	0
Global Structured Finance [Member]
Variable Interest Entities [Line Items]
Maximum Exposure To Loss	6,698	8,165
Insurance Assets	2,093	1,961
Insurance Liabilities	860	762
Derivative Liabilities	8	8
Global Structured Finance [Member] | Residential Mortgage-Backed Securities [Member]
Variable Interest Entities [Line Items]
Maximum Exposure To Loss	4,535	5,373
Insurance Assets	2,063	1,913
Insurance Liabilities	604	523
Derivative Liabilities	0	0
Global Structured Finance [Member] | Other Consumer Asset-Backed [Member]
Variable Interest Entities [Line Items]
Maximum Exposure To Loss	1,120	1,373
Insurance Assets	26	31
Insurance Liabilities	240	216
Derivative Liabilities	0	0
Global Structured Finance [Member] | Other Commercial Asset-Backed [Member]
Variable Interest Entities [Line Items]
Maximum Exposure To Loss	52	314
Insurance Assets	3	9
Insurance Liabilities	1	6
Derivative Liabilities	0	0
Global Structured Finance [Member] | Other [Member]
Variable Interest Entities [Line Items]
Maximum Exposure To Loss	991	1,107
Insurance Assets	0	7
Insurance Liabilities	15	18
Derivative Liabilities	$ 8	$ 8