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, 2023	Dec. 31, 2022	Dec. 31, 2021
Variable Interest Entities [Line Items]
Maximum Exposure To Loss	$ 20,861	$ 21,639
Insurance Assets	351	490
Insurance Liabilities	836	840
Derivative Liabilities	0
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	71	106
Fixed income securities, amortized cost	2,197	2,041
Collateralized Debt Obligations [Member]
Variable Interest Entities [Line Items]
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	1	4
Fixed income securities, amortized cost	139	141
Residential Mortgage-Backed Securities [Member]
Variable Interest Entities [Line Items]
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	14	19
Fixed income securities, amortized cost	239	230
Municipal Bonds [Member]
Variable Interest Entities [Line Items]
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	1	2
Fixed income securities, amortized cost	72	44
Variable Interest Entity [Member]
Variable Interest Entities [Line Items]
Derivative Liabilities		1
Other Comprehensive Income (Loss), Financial Liability, Fair Value Option, Reclassification Adjustment from AOCI for Derecognition, before Tax	5	(1)	$ 5
Variable Interest Entity [Member] | Municipal Bonds [Member]
Variable Interest Entities [Line Items]
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	0	(4)
Fixed income securities, amortized cost	88	99
Global Structured Finance [Member]
Variable Interest Entities [Line Items]
Maximum Exposure To Loss	3,364	3,642
Insurance Assets	141	274
Insurance Liabilities	634	628
Derivative Liabilities	0	1
Global Structured Finance [Member] | Residential Mortgage-Backed Securities [Member]
Variable Interest Entities [Line Items]
Maximum Exposure To Loss	2,391	2,559
Insurance Assets	135	266
Insurance Liabilities	432	400
Derivative Liabilities	0	0
Global Structured Finance [Member] | Other Consumer Asset-Backed [Member]
Variable Interest Entities [Line Items]
Maximum Exposure To Loss	540	652
Insurance Assets	5	6
Insurance Liabilities	200	225
Derivative Liabilities	0	0
Global Structured Finance [Member] | Other Debt Obligations [Member]
Variable Interest Entities [Line Items]
Maximum Exposure To Loss	433	430
Insurance Assets	2	2
Insurance Liabilities	2	2
Derivative Liabilities	0	1
Global Public Finance [Member]
Variable Interest Entities [Line Items]
Maximum Exposure To Loss	17,498	17,997
Insurance Assets	209	216
Insurance Liabilities	202	212
Derivative Liabilities	$ 0	$ 0