Financial Guarantee Insurance Contracts - Summary of Information Related to Policies Currently Included in Ambac's Loss Reserves or Subrogation Recoverable (Parenthetical) (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Loss Reserves Ceded To Reinsurers	$ 40,658	$ 30,767
Successor [Member]
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Loss Reserves Ceded To Reinsurers	40,658	30,767	$ 44,059	$ 100,355
Loss and loss expense reserves	4,745,015	4,380,769
Subrogation recoverable	(631,213)	(684,731)
Liability for Claims	4,113,802	3,696,038	2,858,813	$ 3,798,733
Ceded Loss And Loss Expenses Paid Not Yet Recovered	$ 339	$ (349)	$ (60)