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 (Detail) - USD ($) $ in Millions	Mar. 31, 2020	Dec. 31, 2019
Variable Interest Entities [Line Items]
Maximum Exposure To Loss	$ 30,018	$ 31,506
Insurance Assets	2,399	2,247
Insurance Liabilities	1,193	1,083
Derivative Liabilities	7
Variable Interest Entity, Primary Beneficiary [Member]
Variable Interest Entities [Line Items]
Derivative Liabilities		7
Global Public Finance [Member]
Variable Interest Entities [Line Items]
Maximum Exposure To Loss	22,533	23,341
Insurance Assets	285	287
Insurance Liabilities	329	321
Derivative Liabilities	(2)	0
Global Structured Finance [Member]
Variable Interest Entities [Line Items]
Maximum Exposure To Loss	7,485	8,165
Insurance Assets	2,114	1,961
Insurance Liabilities	864	762
Derivative Liabilities	9	8
Global Structured Finance [Member] | Residential Mortgage-Backed Securities [Member]
Variable Interest Entities [Line Items]
Maximum Exposure To Loss	5,063	5,373
Insurance Assets	2,075	1,913
Insurance Liabilities	609	523
Derivative Liabilities	0	0
Global Structured Finance [Member] | Other Consumer Asset-Backed [Member]
Variable Interest Entities [Line Items]
Maximum Exposure To Loss	1,276	1,373
Insurance Assets	30	31
Insurance Liabilities	235	216
Derivative Liabilities	0	0
Global Structured Finance [Member] | Other Commercial Asset-Backed [Member]
Variable Interest Entities [Line Items]
Maximum Exposure To Loss	100	314
Insurance Assets	3	9
Insurance Liabilities	2	6
Derivative Liabilities	0	0
Global Structured Finance [Member] | Other [Member]
Variable Interest Entities [Line Items]
Maximum Exposure To Loss	1,046	1,107
Insurance Assets	6	7
Insurance Liabilities	18	18
Derivative Liabilities	$ 9	$ 8