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	Dec. 31, 2020	Dec. 31, 2019
Global Structured Finance [Member]
Variable Interest Entities [Line Items]
Maximum Exposure To Loss	$ 6,352	$ 8,165
Insurance Assets	2,051	1,961
Insurance Liabilities	834	762
Derivative Liabilities	8	8
Global Structured Finance [Member] | Residential Mortgage-Backed Securities [Member]
Variable Interest Entities [Line Items]
Maximum Exposure To Loss	4,308	5,373
Insurance Assets	2,024	1,913
Insurance Liabilities	580	523
Derivative Liabilities	0	0
Global Structured Finance [Member] | Other Consumer Asset-Backed [Member]
Variable Interest Entities [Line Items]
Maximum Exposure To Loss	1,050	1,373
Insurance Assets	24	31
Insurance Liabilities	239	216
Derivative Liabilities	0	0
Global Structured Finance [Member] | Other Commercial Asset-Backed [Member]
Variable Interest Entities [Line Items]
Maximum Exposure To Loss	24	314
Insurance Assets	3	9
Insurance Liabilities	1	6
Derivative Liabilities	0	0
Global Structured Finance [Member] | Other Debt Obligations [Member]
Variable Interest Entities [Line Items]
Maximum Exposure To Loss	970	1,107
Insurance Assets	0	7
Insurance Liabilities	13	18
Derivative Liabilities	8	8
Global Public Finance [Member]
Variable Interest Entities [Line Items]
Maximum Exposure To Loss	21,646	23,341
Insurance Assets	263	287
Insurance Liabilities	287	321
Derivative Liabilities	0	0
Maximum Exposure To Loss	27,998	31,506
Insurance Assets	2,314	2,247
Insurance Liabilities	1,122	1,083
Derivative Liabilities	$ 8
Variable Interest Entity [Member]
Variable Interest Entities [Line Items]
Derivative Liabilities		$ 7