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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 |  |

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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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