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Financial Guarantee Insurance Contracts Financial Guarantee Insurance Contracts - Summary of Percentage Ceded to Reinsurers and Reinsurance Recoverable and Rating Levels (Details) - USD ($) $ in Millions | Sep. 30, 2021 | Dec. 31, 2020 | Sep. 30, 2020 | Dec. 31, 2019
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Insurance [Abstract] |  |  |  |
Loss Reserves Ceded To Reinsurers | $ 29 | $ 33 | $ 36 | $ 26
Ceded Loss And Loss Expenses Paid Not Yet Recovered | $ 1 | $ 1 | $ 1 |

X
- Definition Ceded Loss And Loss Expenses Paid Not Yet Recovered + References No definition available. + Details
Name: | ambc_CededLossAndLossExpensesPaidNotYetRecovered
Namespace Prefix: | ambc_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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- Definition The known and estimated amount recoverable as of the balance sheet date from reinsurers for claims expected to be paid or incurred by the ceding insurer. + References No definition available. + Details
Name: | ambc_LossReservesCededToReinsurers
Namespace Prefix: | ambc_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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- References No definition available. + Details
Name: | us-gaap_InsuranceAbstract
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration