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, 2022	Dec. 31, 2021
Variable Interest Entities [Line Items]
Maximum Exposure To Loss	$ 24,517	$ 25,112
Insurance Assets	1,899	2,195
Insurance Liabilities	962	1,023
Derivative Liabilities	2
Variable Interest Entity, Primary Beneficiary [Member]
Variable Interest Entities [Line Items]
Derivative Liabilities		5
Global Public Finance [Member]
Variable Interest Entities [Line Items]
Maximum Exposure To Loss	19,910	20,233
Insurance Assets	244	246
Insurance Liabilities	255	257
Derivative Liabilities	(1)	0
Global Structured Finance [Member]
Variable Interest Entities [Line Items]
Maximum Exposure To Loss	4,608	4,879
Insurance Assets	1,655	1,949
Insurance Liabilities	706	765
Derivative Liabilities	3	5
Global Structured Finance [Member] | Residential Mortgage-Backed Securities [Member]
Variable Interest Entities [Line Items]
Maximum Exposure To Loss	3,089	3,265
Insurance Assets	1,637	1,929
Insurance Liabilities	466	521
Derivative Liabilities	0	0
Global Structured Finance [Member] | Other Consumer Asset-Backed [Member]
Variable Interest Entities [Line Items]
Maximum Exposure To Loss	743	788
Insurance Assets	16	17
Insurance Liabilities	231	234
Derivative Liabilities	0	0
Global Structured Finance [Member] | Other [Member]
Variable Interest Entities [Line Items]
Maximum Exposure To Loss	775	826
Insurance Assets	2	3
Insurance Liabilities	9	10
Derivative Liabilities	$ 3	$ 5