Background and Business Description - Additional Information (Detail) $ / shares in Units, $ in Thousands	3 Months Ended	4 Months Ended	12 Months Ended
	Mar. 31, 2017 Segment	Apr. 30, 2013 $ / shares shares	Dec. 31, 2017 USD ($) $ / shares shares	Dec. 31, 2016 USD ($) $ / shares shares	Dec. 31, 2015 USD ($)	Dec. 31, 2014	May 01, 2013 USD ($) $ / shares shares
Background And Basis Of Presentation [Line Items]
Business Acquisition, Percentage of Voting Interests Acquired		50.00%
Entity Incorporation, Date of Incorporation			Apr. 29, 1991
Investment Warrants Expiration Date		Apr. 30, 2023
Gain Contingency, Unrecorded Amount			$ 1,400,000
AAC Note Secured by RMBS Securities Value Not Less Than			$ 350,000
Amount of Senior Surplus Notes Outstanding on June 8, 2020			25.00%
OCIApprovalLimmitwithNonAffiliatedCounterparties			$ 100,000
MatieralAdverseEffectThreshold			10.00%
ChangeinLossReserveAssumptionsThresholdDollars			$ 200,000
ChangeinLossReserveAssumptionsThresholdPercent			10.00%
Accrued rate of interest on outstanding policy obligations (percent)			5.10%
Capital Stock Authorized | shares		150,000,000
Common stock, shares authorized | shares		130,000,000
Common Stock, Par or Stated Value Per Share | $ / shares		$ 0.01
Preferred stock, shares authorized | shares		20,000,000
Preferred Stock, Par or Stated Value Per Share | $ / shares		$ 0.01
Common stock, shares issued | shares		45,000,000
Class of Warrant or Right, Exercise Price of Warrants or Rights | $ / shares		$ 16.67
Junior surplus note of Ambac Assurance Segregated Account							$ 350,000
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%
Litigation Settlement, Amount Awarded from Other Party				$ 995,000
Possible Increase in Loss Reserves Related to Puerto Rico			$ 1,500,000
Ambac Assurance [Member]
Background And Basis Of Presentation [Line Items]
Net par exposure for policies allocated to the Segregated Account			$ 9,246,357
Successor [Member]
Background And Basis Of Presentation [Line Items]
Number of reportable segments | Segment	2
Common stock, shares authorized | shares			130,000,000	130,000,000
Common Stock, Par or Stated Value Per Share | $ / shares			$ 0.01	$ 0.01			$ 0.01
Preferred stock, shares authorized | shares			20,000,000	20,000,000
Preferred Stock, Par or Stated Value Per Share | $ / shares			$ 0.01	$ 0.01
Common stock, shares issued | shares							45,000,000
Warrants Distributed In Bankruptcy | shares		5,047,138					5,047,138
Class of Warrant or Right, Exercise Price of Warrants or Rights | $ / shares							$ 16.67
Financial Guarantee Insurance Contracts, Accelerated Premium Revenue, Amount			$ 64,494	$ 52,416	$ 137,400
Successor [Member] | Ambac UK [Member]
Background And Basis Of Presentation [Line Items]
Ballantyne Settlement with JPMIM			325,600
Ambac UK Benefit Through Loss Reduction			91,600
Previous Benefit Through Reduction of Loss and LAE			53,000
Total Benefit Related to Ballantyne Settlement			$ 144,600