Financial Guarantee Insurance Contracts - Summary of Information Related to Policies Currently Included in Ambac's Loss Reserves or Subrogation Recoverable (Phantom) (Detail) - Successor [Member] - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017	Dec. 31, 2016
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Loss Reserves Ceded To Reinsurers	$ 38,735	$ 40,658	$ 34,691	$ 30,767
Loss and loss expense reserves	2,139,101	4,745,015
Subrogation recoverable	(1,894,778)	(631,213)
Liability for Claims	244,323	4,113,802	3,830,355	$ 3,696,038
Ceded Loss And Loss Expenses Paid Not Yet Recovered	$ 90	$ 339	$ (418)