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	Sep. 30, 2019	Dec. 31, 2018	Feb. 12, 2018
Variable Interest Entities [Line Items]
Maximum Exposure To Loss	$ 31,500,963	$ 35,566,155
Insurance Assets	2,295,032	2,257,824
Insurance Liabilities	1,103,793	1,433,877
Derivative Liabilities	7,512
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	23,058,272	24,145,956
Insurance Assets	317,692	309,071
Insurance Liabilities	320,883	335,437
Derivative Liabilities	(422)	(1,457)
Global Structured Finance [Member]
Variable Interest Entities [Line Items]
Maximum Exposure To Loss	8,442,691	11,420,199
Insurance Assets	1,977,340	1,948,753
Insurance Liabilities	782,910	1,098,440
Derivative Liabilities	7,934	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	5,641,415	6,713,437
Insurance Assets	1,943,385	1,859,121
Insurance Liabilities	547,002	546,682
Derivative Liabilities	0	0
Global Structured Finance [Member] | Other Consumer Asset-Backed [Member]
Variable Interest Entities [Line Items]
Maximum Exposure To Loss	1,450,873	1,700,984
Insurance Assets	19,829	15,435
Insurance Liabilities	210,512	238,234
Derivative Liabilities	0	0
Global Structured Finance [Member] | Other Commercial Asset-Backed [Member]
Variable Interest Entities [Line Items]
Maximum Exposure To Loss	352,385	873,343
Insurance Assets	6,387	20,735
Insurance Liabilities	6,209	12,264
Derivative Liabilities	0	0
Global Structured Finance [Member] | Other [Member]
Variable Interest Entities [Line Items]
Maximum Exposure To Loss	998,018	2,122,648
Insurance Assets	7,739	53,462
Insurance Liabilities	19,187	301,260
Derivative Liabilities	$ 7,934	$ 7,170