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Variable Interest Entities - Summary of Fair Value of Fixed Income Securities, by Asset-Type, Held by Consolidated Variable Interest Entities (Detail) - USD ($) $ in Millions | Dec. 31, 2020 | Dec. 31, 2019
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Variable Interest Entities [Line Items] |  |
Variable Interest Entity Nonconsolidated Carrying Amount Insurance Assets | $ 2,314 | $ 2,247
Variable Interest Entity Nonconsolidated Carrying Amount Insurance Liabilities | 1,122 | 1,083
Variable Interest Entity, Reporting Entity Involvement, Maximum Loss Exposure, Amount | 27,998 | 31,506
Investments: |  |
Variable Interest Entity Nonconsolidated Carrying Amount Derivative Net Assets Or Liabilities | 8 |
Fixed income securities, amortized cost | 2,807 | 3,187
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax | 149 | 132
Variable Interest Entity [Member] |  |
Investments: |  |
Variable Interest Entity Nonconsolidated Carrying Amount Derivative Net Assets Or Liabilities |  | 7
Municipal Bonds [Member] | Variable Interest Entity [Member] |  |
Investments: |  |
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax | $ 27 | $ 25

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- Definition The net carrying amount of derivative assets or liabilities in the reporting entity's statement of financial position that relate to the entity's variable interests in Variable Interest Entities (VIEs) where the VIEs are not consolidated. + References No definition available. + Details
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Balance Type: | debit
Period Type: | instant

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- Definition The carrying amount of insurance assets in the reporting entity's statement of financial position that relate to the entity's variable interests in Variable Interest Entities (VIEs) where the VIEs are not consolidated. + References No definition available. + Details
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Balance Type: | debit
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- Definition The carrying amount of insurance liabilities in the reporting entity's statement of financial position that relate to the entity's variable interests in Variable Interest Entities (VIEs) where the VIEs are not consolidated. + References No definition available. + Details
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Namespace Prefix: | ambc_
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Balance Type: | credit
Period Type: | instant

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- Definition Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale). + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=118939692&loc=d3e27161-111563 + Details
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- Definition Amortized cost of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale). + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=118939692&loc=d3e27161-111563 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 326 -SubTopic 30 -Section 45 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=118942338&loc=SL82898722-210454 + Details
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- References No definition available. + Details
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- Definition The reporting entity's maximum amount of exposure to loss as a result of its involvement with the Variable Interest Entity (VIE). + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 810 -SubTopic 10 -Section 50 -Paragraph 6 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=118582676&loc=d3e5747-111685 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 810 -SubTopic 10 -Section 50 -Paragraph 4 -Subparagraph b -URI http://asc.fasb.org/extlink&oid=118582676&loc=d3e5728-111685 + Details
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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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- Details
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- Details
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Period Type: |