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	Mar. 31, 2019	Dec. 31, 2018	Feb. 12, 2018
Variable Interest Entities [Line Items]
Maximum Exposure To Loss	$ 34,528,319	$ 35,566,155
Insurance Assets	2,235,297	2,257,824
Insurance Liabilities	1,377,733	1,433,877
Derivative Liabilities	5,944
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	23,927,824	24,145,956
Insurance Assets	315,807	309,071
Insurance Liabilities	343,946	335,437
Derivative Liabilities	(1,077)	(1,457)
Global Structured Finance [Member]
Variable Interest Entities [Line Items]
Maximum Exposure To Loss	10,600,495	11,420,199
Insurance Assets	1,919,490	1,948,753
Insurance Liabilities	1,033,787	1,098,440
Derivative Liabilities	7,021	7,168
Global Structured Finance [Member] \| Collateralized Debt Obligations [Member]
Variable Interest Entities [Line Items]
Maximum Exposure To Loss	2,437	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	6,418,261	6,713,437
Insurance Assets	1,842,283	1,859,121
Insurance Liabilities	492,435	546,682
Derivative Liabilities	0	0
Global Structured Finance [Member] \| Other Consumer Asset-Backed [Member]
Variable Interest Entities [Line Items]
Maximum Exposure To Loss	1,615,109	1,700,984
Insurance Assets	14,754	15,435
Insurance Liabilities	234,639	238,234
Derivative Liabilities	0	0
Global Structured Finance [Member] \| Other Commercial Asset-Backed [Member]
Variable Interest Entities [Line Items]
Maximum Exposure To Loss	481,633	873,343
Insurance Assets	9,247	20,735
Insurance Liabilities	6,774	12,264
Derivative Liabilities	0	0
Global Structured Finance [Member] \| Other [Member]
Variable Interest Entities [Line Items]
Maximum Exposure To Loss	2,083,055	2,122,648
Insurance Assets	53,206	53,462
Insurance Liabilities	299,939	301,260
Derivative Liabilities	$ 7,021	$ 7,170

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

Mar. 31, 2019

Dec. 31, 2018

Feb. 12, 2018

Variable Interest Entities [Line Items]

Maximum Exposure To Loss

$ 34,528,319

$ 35,566,155

Insurance Assets

2,235,297

2,257,824

Insurance Liabilities

1,377,733