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) - Successor [Member] - USD ($) $ in Thousands	Jun. 30, 2018	Feb. 12, 2018	Dec. 31, 2017
Variable Interest Entities [Line Items]
SatisfactionAndDischargeOfSegregatedAccountDeferredPrincipalAmounts		$ 3,000,158
SatisfactionAndDischargeOfSegregatedAccountDeferredInterestAmounts		$ 856,834
Variable Interest Entity, Primary Beneficiary [Member]
Variable Interest Entities [Line Items]
Maximum Exposure To Loss	$ 37,507,047		$ 44,199,767
Insurance Assets	2,298,884		1,069,268
Insurance Liabilities	1,617,125		4,260,578
Derivative Liabilities	6,172		9,745
Variable Interest Entity, Primary Beneficiary [Member] | Global Structured Finance [Member]
Variable Interest Entities [Line Items]
Maximum Exposure To Loss	12,611,489		18,569,951
Insurance Assets	1,966,733		733,921
Insurance Liabilities	1,253,527		3,889,522
Derivative Liabilities	7,490		10,296
Variable Interest Entity, Primary Beneficiary [Member] | Global Public Finance [Member]
Variable Interest Entities [Line Items]
Maximum Exposure To Loss	24,895,558		25,629,816
Insurance Assets	332,151		335,347
Insurance Liabilities	363,598		371,056
Derivative Liabilities	(1,318)		(551)
Variable Interest Entity, Primary Beneficiary [Member] | Collateralized Debt Obligations [Member] | Global Structured Finance [Member]
Variable Interest Entities [Line Items]
Maximum Exposure To Loss	26,882		35,555
Insurance Assets	108		169
Insurance Liabilities	1		1
Derivative Liabilities	(8)		(15)
Variable Interest Entity, Primary Beneficiary [Member] | Residential Mortgage-Backed Securities [Member] | Global Structured Finance [Member]
Variable Interest Entities [Line Items]
Maximum Exposure To Loss	7,355,039		12,766,685
Insurance Assets	1,862,269		619,848
Insurance Liabilities	598,722		3,218,356
Variable Interest Entity, Primary Beneficiary [Member] | Other Consumer Asset-Backed [Member] | Global Structured Finance [Member]
Variable Interest Entities [Line Items]
Maximum Exposure To Loss	2,144,354		2,266,610
Insurance Assets	21,150		23,405
Insurance Liabilities	326,476		328,732
Derivative Liabilities	0		0
Variable Interest Entity, Primary Beneficiary [Member] | Other Commercial Asset-Backed [Member] | Global Structured Finance [Member]
Variable Interest Entities [Line Items]
Maximum Exposure To Loss	893,643		987,797
Insurance Assets	25,768		30,413
Insurance Liabilities	23,508		35,976
Derivative Liabilities	0		0
Variable Interest Entity, Primary Beneficiary [Member] | Other [Member] | Global Structured Finance [Member]
Variable Interest Entities [Line Items]
Maximum Exposure To Loss	2,191,571		2,513,304
Insurance Assets	57,438		60,086
Insurance Liabilities	304,820		306,457
Derivative Liabilities	$ 7,498		$ 10,311