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Financial Guarantees in Force - Additional Information (Detail) - USD ($) $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018
Guarantor Obligations [Line Items] |  |  |
Financial Guarantee Insurance Contracts, Accelerated Premium Revenue, Amount | $ 12 | $ 10 | $ 32
Gross par amount of financial guarantees outstanding | 39,070 | 43,908 |
Net par amount of financial guarantees | 33,888 | 38,018 |
Gross financial guarantees in force | 61,895 | 69,826 |
Net financial guarantees in force | $ 51,603 | 58,245 |
Highest single insured risk of aggregate net par amount guaranteed | 2.90% |  |
Ceded Principal Outstanding Major Reinsurer Percentage | 6.10% |  |
California [Member] |  |  |
Guarantor Obligations [Line Items] |  |  |
Aggregate net par amounts in force | 6.20% |  |
NEW JERSEY |  |  |
Guarantor Obligations [Line Items] |  |  |
Aggregate net par amounts in force | 7.00% |  |
NEW YORK |  |  |
Guarantor Obligations [Line Items] |  |  |
Aggregate net par amounts in force | 5.40% |  |
No other state [Member] | Maximum [Member] |  |  |
Guarantor Obligations [Line Items] |  |  |
Aggregate net par amounts in force | 5.00% |  |
Public Finance [Member] |  |  |
Guarantor Obligations [Line Items] |  |  |
Net par amount of financial guarantees | $ 15,497 | 17,653 |
Public Finance [Member] | Housing Revenue [Member] |  |  |
Guarantor Obligations [Line Items] |  |  |
Net par amount of financial guarantees | 5,855 | 5,991 |
Military Housing [Member] | Public Finance [Member] | Housing Revenue [Member] |  |  |
Guarantor Obligations [Line Items] |  |  |
Net par amount of financial guarantees | $ 5,575 | $ 5,654 |

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- Definition Ceded Principal Outstanding Major Reinsurer Percentage. + References No definition available. + Details
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Period Type: | duration

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- Definition Financial guarantee insurance gross outstanding principal. + References No definition available. + Details
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Data Type: | xbrli:monetaryItemType
Balance Type: | credit
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- Definition Financial guarantee insurance gross outstanding principal and interest. + References No definition available. + Details
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Namespace Prefix: | ambc_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- Definition Financial guarantee insurance outstanding principal and interest, net of reinsurance. + References No definition available. + Details
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Namespace Prefix: | ambc_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- Definition Percentage of related member outstanding par, net of reinsurance, as compared to the Company's total outstanding financial guarantee par, net of reinsurance. + References No definition available. + Details
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Namespace Prefix: | ambc_
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Period Type: | duration

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- Definition Percentage of highest single insured risk of aggregate net par amount guaranteed + References No definition available. + Details
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Namespace Prefix: | ambc_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

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- Definition This element represents the amount of premium revenue on financial guarantee insurance contracts recognized during the period that was accelerated for revenue recognition purposes. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 605 -Section 50 -Paragraph 4 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=6933581&loc=SL5749298-161291 + Details
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Period Type: | duration

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- Definition Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. + References No definition available. + Details
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- Definition Amount of participating insurance in force. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 50 -Section 50 -Paragraph 1 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=6483535&loc=d3e16392-158451 + Details
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- Details
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- Details
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- Details
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- Details
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- Details
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- Details
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- Details
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- Details
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Period Type: |