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 Thousands	Jun. 30, 2019	Dec. 31, 2018	Feb. 12, 2018
Variable Interest Entities [Line Items]
Maximum Exposure To Loss	$ 32,478,140	$ 35,566,155
Insurance Assets	2,263,317	2,257,824
Insurance Liabilities	1,100,843	1,433,877
Derivative Liabilities	7,376
satisfaction and discharge of segregated account deferred interest amounts			$ 856,834
Variable Interest Entity, Primary Beneficiary [Member]
Variable Interest Entities [Line Items]
Derivative Liabilities		5,711
Global Public Finance [Member]
Variable Interest Entities [Line Items]
Maximum Exposure To Loss	23,631,977	24,145,956
Insurance Assets	310,868	309,071
Insurance Liabilities	338,416	335,437
Derivative Liabilities	(861)	(1,457)
Global Structured Finance [Member]
Variable Interest Entities [Line Items]
Maximum Exposure To Loss	8,846,163	11,420,199
Insurance Assets	1,952,449	1,948,753
Insurance Liabilities	762,427	1,098,440
Derivative Liabilities	8,237	7,168
Global Structured Finance [Member] | Collateralized Debt Obligations [Member]
Variable Interest Entities [Line Items]
Maximum Exposure To Loss	0	9,787
Insurance Assets	0	0
Insurance Liabilities	0	0
Derivative Liabilities	0	(2)
Global Structured Finance [Member] | Residential Mortgage-Backed Securities [Member]
Variable Interest Entities [Line Items]
Maximum Exposure To Loss	6,114,419	6,713,437
Insurance Assets	1,910,305	1,859,121
Insurance Liabilities	507,709	546,682
Derivative Liabilities	0	0
Global Structured Finance [Member] | Other Consumer Asset-Backed [Member]
Variable Interest Entities [Line Items]
Maximum Exposure To Loss	1,530,894	1,700,984
Insurance Assets	14,925	15,435
Insurance Liabilities	230,914	238,234
Derivative Liabilities	0	0
Global Structured Finance [Member] | Other Commercial Asset-Backed [Member]
Variable Interest Entities [Line Items]
Maximum Exposure To Loss	382,244	873,343
Insurance Assets	6,811	20,735
Insurance Liabilities	6,627	12,264
Derivative Liabilities	0	0
Global Structured Finance [Member] | Other [Member]
Variable Interest Entities [Line Items]
Maximum Exposure To Loss	818,606	2,122,648
Insurance Assets	20,408	53,462
Insurance Liabilities	17,177	301,260
Derivative Liabilities	$ 8,237	$ 7,170