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, 2021	Dec. 31, 2020	Dec. 31, 2019
Variable Interest Entities [Line Items]
Maximum Exposure To Loss	$ 25,112	$ 27,998
Insurance Assets	2,195	2,314
Insurance Liabilities	1,023	1,122
Derivative Liabilities	5
Variable Interest Entity [Member]
Variable Interest Entities [Line Items]
Derivative Liabilities		8
Other Comprehensive Income (Loss), Financial Liability, Fair Value Option, Reclassification Adjustment from AOCI for Derecognition, before Tax	5	(3)	$ 14
Global Structured Finance [Member]
Variable Interest Entities [Line Items]
Maximum Exposure To Loss	4,879	6,352
Insurance Assets	1,949	2,051
Insurance Liabilities	765	834
Derivative Liabilities	5	8
Global Structured Finance [Member] | Residential Mortgage-Backed Securities [Member]
Variable Interest Entities [Line Items]
Maximum Exposure To Loss	3,265	4,308
Insurance Assets	1,929	2,024
Insurance Liabilities	521	580
Derivative Liabilities	0	0
Global Structured Finance [Member] | Other Consumer Asset-Backed [Member]
Variable Interest Entities [Line Items]
Maximum Exposure To Loss	788	1,050
Insurance Assets	17	24
Insurance Liabilities	234	239
Derivative Liabilities	0	0
Global Structured Finance [Member] | Other Debt Obligations [Member]
Variable Interest Entities [Line Items]
Maximum Exposure To Loss	826	994
Insurance Assets	3	3
Insurance Liabilities	10	15
Derivative Liabilities	5	8
Global Public Finance [Member]
Variable Interest Entities [Line Items]
Maximum Exposure To Loss	20,233	21,646
Insurance Assets	246	263
Insurance Liabilities	257	287
Derivative Liabilities	$ 0	$ 0