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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 |  |  |  |

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- Definition Junior Surplus Notes issued by the Segregated Account of Ambac Assurance to Ambac upon Ambac's emergence from bankruptcy. These Junior Surplus notes will be includable in surplus for statutory accounting purposes as prescribed or permitted by state laws and regulations. + References No definition available. + Details
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