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 (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Variable Interest Entities [Line Items]
Maximum Exposure To Loss	$ 21,639	$ 25,112
Insurance Assets	490	2,195
Insurance Liabilities	840	1,023
Derivative Liabilities	1
Variable Interest Entity [Member]
Variable Interest Entities [Line Items]
Derivative Liabilities		5
Other Comprehensive Income (Loss), Financial Liability, Fair Value Option, Reclassification Adjustment from AOCI for Derecognition, before Tax	(1)	5	$ (3)
Global Structured Finance [Member]
Variable Interest Entities [Line Items]
Maximum Exposure To Loss	3,642	4,879
Insurance Assets	274	1,949
Insurance Liabilities	628	765
Derivative Liabilities	1	5
Global Structured Finance [Member] | Residential Mortgage-Backed Securities [Member]
Variable Interest Entities [Line Items]
Maximum Exposure To Loss	2,559	3,265
Insurance Assets	266	1,929
Insurance Liabilities	400	521
Derivative Liabilities	0	0
Global Structured Finance [Member] | Other Consumer Asset-Backed [Member]
Variable Interest Entities [Line Items]
Maximum Exposure To Loss	652	788
Insurance Assets	6	17
Insurance Liabilities	225	234
Derivative Liabilities	0	0
Global Structured Finance [Member] | Other Debt Obligations [Member]
Variable Interest Entities [Line Items]
Maximum Exposure To Loss	430	826
Insurance Assets	2	3
Insurance Liabilities	2	10
Derivative Liabilities	1	5
Global Public Finance [Member]
Variable Interest Entities [Line Items]
Maximum Exposure To Loss	17,997	20,233
Insurance Assets	216	246
Insurance Liabilities	212	257
Derivative Liabilities	$ 0	$ 0