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 [Member] - Successor [Member] - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Variable Interest Entities [Line Items]
Maximum Exposure To Loss	$ 44,199,767	$ 48,271,212
Insurance Assets	1,069,268	1,223,037
Insurance Liabilities	4,260,578	4,261,578
Derivative Liabilities	9,745	(140,882)
Global Structured Finance [Member]
Variable Interest Entities [Line Items]
Maximum Exposure To Loss	18,569,951	22,662,741
Insurance Assets	733,921	884,450
Insurance Liabilities	3,889,522	3,902,436
Derivative Liabilities	10,296	(132,055)
Global Public Finance [Member]
Variable Interest Entities [Line Items]
Maximum Exposure To Loss	25,629,816	25,608,471
Insurance Assets	335,347	338,587
Insurance Liabilities	371,056	359,142
Derivative Liabilities	(551)	(8,827)
Collateralized Debt Obligations [Member] | Global Structured Finance [Member]
Variable Interest Entities [Line Items]
Maximum Exposure To Loss	35,555	761,451
Insurance Assets	169	218
Insurance Liabilities	1	3,319
Derivative Liabilities	(15)	(145,402)
Residential Mortgage-Backed Securities [Member] | Global Structured Finance [Member]
Variable Interest Entities [Line Items]
Maximum Exposure To Loss	12,766,685	14,859,909
Insurance Assets	619,848	725,106
Insurance Liabilities	3,218,356	3,118,892
Other Consumer Asset-Backed [Member] | Global Structured Finance [Member]
Variable Interest Entities [Line Items]
Maximum Exposure To Loss	2,266,610	2,391,604
Insurance Assets	23,405	26,758
Insurance Liabilities	328,732	302,335
Derivative Liabilities	0	0
Other Commercial Asset-Backed [Member] | Global Structured Finance [Member]
Variable Interest Entities [Line Items]
Maximum Exposure To Loss	987,797	1,686,256
Insurance Assets	30,413	66,277
Insurance Liabilities	35,976	64,961
Derivative Liabilities	0	0
Other [Member] | Global Structured Finance [Member]
Variable Interest Entities [Line Items]
Maximum Exposure To Loss	2,513,304	2,963,521
Insurance Assets	60,086	66,091
Insurance Liabilities	306,457	412,929
Derivative Liabilities	$ 10,311	$ 13,347