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) - Variable Interest Entity, Primary Beneficiary [Member] - Successor [Member] - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Variable Interest Entities [Line Items]
Maximum Exposure To Loss	$ 39,387,600	$ 44,199,767
Insurance Assets	2,195,593	1,069,268
Insurance Liabilities	1,713,791	4,260,578
Derivative Liabilities	6,733	9,745
Global Structured Finance [Member]
Variable Interest Entities [Line Items]
Maximum Exposure To Loss	13,692,404	18,569,951
Insurance Assets	1,849,915	733,921
Insurance Liabilities	1,334,270	3,889,522
Derivative Liabilities	7,738	10,296
Global Public Finance [Member]
Variable Interest Entities [Line Items]
Maximum Exposure To Loss	25,695,196	25,629,816
Insurance Assets	345,678	335,347
Insurance Liabilities	379,521	371,056
Derivative Liabilities	(1,005)	(551)
Collateralized Debt Obligations [Member] | Global Structured Finance [Member]
Variable Interest Entities [Line Items]
Maximum Exposure To Loss	31,644	35,555
Insurance Assets	157	169
Insurance Liabilities	2	1
Derivative Liabilities	(13)	(15)
Residential Mortgage-Backed Securities [Member] | Global Structured Finance [Member]
Variable Interest Entities [Line Items]
Maximum Exposure To Loss	8,033,584	12,766,685
Insurance Assets	1,742,097	619,848
Insurance Liabilities	670,052	3,218,356
Other Consumer Asset-Backed [Member] | Global Structured Finance [Member]
Variable Interest Entities [Line Items]
Maximum Exposure To Loss	2,209,778	2,266,610
Insurance Assets	22,416	23,405
Insurance Liabilities	331,147	328,732
Derivative Liabilities	0	0
Other Commercial Asset-Backed [Member] | Global Structured Finance [Member]
Variable Interest Entities [Line Items]
Maximum Exposure To Loss	934,919	987,797
Insurance Assets	27,527	30,413
Insurance Liabilities	30,837	35,976
Derivative Liabilities	0	0
Other [Member] | Global Structured Finance [Member]
Variable Interest Entities [Line Items]
Maximum Exposure To Loss	2,482,479	2,513,304
Insurance Assets	57,718	60,086
Insurance Liabilities	302,232	306,457
Derivative Liabilities	$ 7,751	$ 10,311