Background and Business Description - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2014	Feb. 12, 2018	May 01, 2013	Apr. 30, 2013
Background And Basis Of Presentation [Line Items]
Entity Incorporation, Date of Incorporation	Apr. 29, 1991
Minimum surplus amount	$ 100
Amount of Senior Surplus Notes Outstanding on June 8, 2020			25.00%
Tier 2 Notes					$ 240
Tier 2 Note Security in RMBS Litigation Settlement Proceeds					$ 1,600
Common stock, shares authorized	130,000,000	130,000,000
Common Stock, Par or Stated Value Per Share	$ 0.01	$ 0.01					$ 0.01
Preferred stock, shares authorized	20,000,000	20,000,000
Preferred Stock, Par or Stated Value Per Share	$ 0.01	$ 0.01
Common stock, shares issued	45,571,743	45,365,170
Class of Warrant or Right, Exercise Price of Warrants or Rights						$ 16.67
Junior surplus note of Ambac Assurance Segregated Account						$ 350
Common Stock Voting Restrictions				10.00%
Common Stock Voting Restriction Less One Vote				10.00%
Common Stock Transfer Restrictions				5.00%
Common Stock Additional Transfer Restrictions				5.00%
Financial Guarantee Insurance Contracts, Accelerated Premium Revenue, Amount	$ 10	$ 32	$ 64
Ambac UK [Member]
Background And Basis Of Presentation [Line Items]
Total Benefit Related to Ballantyne Settlement			$ 145