Background and Business Description - Additional Information (Detail) - USD ($)		12 Months Ended
	Apr. 29, 1991	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	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
Tier 2 Notes						$ 240,000,000
Tier 2 Note Security in RMBS Litigation Settlement Proceeds						1,600,000,000
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		45,865,081	45,571,743
Class of Warrant or Right, Exercise Price of Warrants or Rights							$ 16.67
Junior surplus note of Ambac Assurance Segregated Account							$ 350,000,000
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		$ 12,000,000	$ 10,000,000	$ 32,000,000
Business Acquisition, Percentage of Voting Interests Acquired		80.00%
Ambac [Member] | Deferred Payment Obligation [Member]
Background And Basis Of Presentation [Line Items]
Discount Amount						0.09
One Dollar Outstanding						1.00
Payment Amount in New Secured Notes						$ 0.91