Investments - Summary of Sources of Collateral Received and Various Investment Agreement in which Collateral Pledged (Details) - USD ($) $ in Millions	Sep. 30, 2021	Dec. 31, 2020
Investment [Line Items]
Fair Value Of Securities Deposited With Governmental Authorities	$ 9	$ 8
Fair Value of Securities Deposited in Connection wtih Letter of Credit	1	1
Market Value of Secured Note Collateral	0	178
Fair Value of Securities Held by AmbacUK Where Capital Stock of AUK is Pledged as Collateral for Sitka AAC Note	655
Investment Portfolio [Member]
Investment [Line Items]
Fair Value of Cash and Securities Pledged to Derivative Counterparties	120	140
Ambac Assurance [Member]
Investment [Line Items]
Unsettled Amounts Secured Notes from LSNI	0	9
Standard & Poor's, B Rating [Member]
Investment [Line Items]
Investment Owned, at Fair Value	785
Standard & Poor's, B Rating [Member] \| Municipal Bonds [Member]
Investment [Line Items]
Investment Owned, at Fair Value	320
Standard & Poor's, B Rating [Member] \| Corporate Debt Securities [Member]
Investment [Line Items]
Investment Owned, at Fair Value	0
Standard & Poor's, B Rating [Member] \| Mortgage And Other Asset Backed Securities [Member]
Investment [Line Items]
Investment Owned, at Fair Value	466
Ambac Assurance Corporation [Member] \| Standard & Poor's, B Rating [Member]
Investment [Line Items]
Investment Owned, at Fair Value	783
Ambac Assurance Corporation [Member] \| Standard & Poor's, B Rating [Member] \| Municipal Bonds [Member]
Investment [Line Items]
Investment Owned, at Fair Value	317
Ambac Assurance Corporation [Member] \| Standard & Poor's, B Rating [Member] \| Mortgage And Other Asset Backed Securities [Member]
Investment [Line Items]
Investment Owned, at Fair Value	466
National Public Finance Guarantee Corporation [Member] \| Standard & Poor's, BBB- Rating [Member]
Investment [Line Items]
Investment Owned, at Fair Value	2	6
National Public Finance Guarantee Corporation [Member] \| Standard & Poor's, BBB- Rating [Member] \| Municipal Bonds [Member]
Investment [Line Items]
Investment Owned, at Fair Value	2	6
National Public Finance Guarantee Corporation [Member] \| Standard & Poor's, BBB- Rating [Member] \| Corporate Debt Securities [Member]
Investment [Line Items]
Investment Owned, at Fair Value	$ 0	$ 0

Fair value of cash and securities pledged to derivative counterparties.

No definition available.

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- Definition

Fair value of securities deposited with governmental authorities.

+ References

No definition available.

+ Details

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- Definition

Fair Value of Securities Held by AmbacUK Where Capital Stock of AUK is Pledged as Collateral for Sitka AAC Note

+ References

No definition available.

+ Details

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- Definition

Fair Value of Securities Deposited in Connection wtih Letter of Credit

+ References

No definition available.

+ Details

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- Definition

Market Value of Secured Note Collateral

+ References

No definition available.

+ Details

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- Definition

Unsettled Amounts Secured Notes from LSNI

+ References

No definition available.

+ Details

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- Definition

Value of the investment at close of period. For schedules of investments that are categorized, the value would be aggregated by category. For investment in and advances to affiliates, if operations of any controlled companies are different in character from those of the company, group such affiliates within divisions and by type of activities.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 946
-SubTopic 210
-Section 50
-Paragraph 6
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=99383244&loc=d3e12121-115841

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 946
-SubTopic 320
-Section S99
-Paragraph 5D
-Subparagraph (SX 210.12-13D(Column C))
-URI http://asc.fasb.org/extlink&oid=120401592&loc=SL120429264-123010

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 946
-SubTopic 320
-Section S99
-Paragraph 3
-Subparagraph (SX 210.12-12B(Column C))
-URI http://asc.fasb.org/extlink&oid=120401592&loc=d3e611197-123010

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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.

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