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	Jun. 30, 2018	Dec. 31, 2017	Jun. 30, 2017	Dec. 31, 2016
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Loss Reserves Ceded To Reinsurers	$ 38,293	$ 40,658	$ 47,034	$ 30,767
Loss and loss expense reserves	2,057,334	4,745,015
Subrogation recoverable	(1,876,188)	(631,213)
Liability for Claims	181,146	4,113,802	3,923,278	$ 3,696,038
Ceded Loss And Loss Expenses Paid Not Yet Recovered	$ 778	$ 339	$ (568)