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, 2019	Dec. 31, 2018
Variable Interest Entities [Line Items]
Maximum Exposure To Loss	$ 31,506	$ 35,566
Insurance Assets	2,247	2,258
Insurance Liabilities	1,083	1,434
Derivative Liabilities	7
Variable Interest Entity [Member]
Variable Interest Entities [Line Items]
Derivative Liabilities		6
Global Structured Finance [Member]
Variable Interest Entities [Line Items]
Maximum Exposure To Loss	8,165	11,420
Insurance Assets	1,961	1,949
Insurance Liabilities	762	1,098
Derivative Liabilities	8	7
Global Structured Finance [Member] | Collateralized Debt Obligations [Member]
Variable Interest Entities [Line Items]
Maximum Exposure To Loss		10
Insurance Assets		0
Insurance Liabilities		0
Derivative Liabilities		0
Global Structured Finance [Member] | Residential Mortgage-Backed Securities [Member]
Variable Interest Entities [Line Items]
Maximum Exposure To Loss	5,373	6,713
Insurance Assets	1,913	1,859
Insurance Liabilities	523	547
Derivative Liabilities	0	0
Global Structured Finance [Member] | Other Consumer Asset-Backed [Member]
Variable Interest Entities [Line Items]
Maximum Exposure To Loss	1,373	1,701
Insurance Assets	31	15
Insurance Liabilities	216	238
Derivative Liabilities	0	0
Global Structured Finance [Member] | Other Commercial Asset-Backed [Member]
Variable Interest Entities [Line Items]
Maximum Exposure To Loss	314	873
Insurance Assets	9	21
Insurance Liabilities	6	12
Derivative Liabilities	0	0
Global Structured Finance [Member] | Other Debt Obligations [Member]
Variable Interest Entities [Line Items]
Maximum Exposure To Loss	1,107	2,123
Insurance Assets	7	53
Insurance Liabilities	18	301
Derivative Liabilities	8	7
Global Public Finance [Member]
Variable Interest Entities [Line Items]
Maximum Exposure To Loss	23,341	24,146
Insurance Assets	287	309
Insurance Liabilities	321	335
Derivative Liabilities	$ 0	$ (1)