Investments - Summary of Sources of Collateral Received and Various Investment Agreement in which Collateral Pledged (Detail) - USD ($) $ in Millions	Mar. 31, 2020	Dec. 31, 2019
Investment [Line Items]
Fair Value Of Securities Deposited With Governmental Authorities	$ 7	$ 6
Fair Value of Securities Deposited in Connection wtih Letter of Credit	1
Market Value of Secured Note Collateral	174	197
Investment Portfolio [Member]
Investment [Line Items]
Fair Value of Cash and Securities Pledged to Derivative Counterparties	85	85
Ambac Assurance [Member]
Investment [Line Items]
Unsettled Amounts Secured Notes from LSNI	31	55
Standard & Poor's, B Rating [Member]
Investment [Line Items]
Investment Owned, at Fair Value	1,081
Standard & Poor's, B Rating [Member] \| Municipal Bonds [Member]
Investment [Line Items]
Investment Owned, at Fair Value	189
Standard & Poor's, B Rating [Member] \| Corporate Debt Securities [Member]
Investment [Line Items]
Investment Owned, at Fair Value	495
Standard & Poor's, B Rating [Member] \| Mortgage And Other Asset Backed Securities [Member]
Investment [Line Items]
Investment Owned, at Fair Value	396
Standard & Poor's, B- Rating [Member]
Investment [Line Items]
Investment Owned, at Fair Value		1,164
Standard & Poor's, B- Rating [Member] \| Municipal Bonds [Member]
Investment [Line Items]
Investment Owned, at Fair Value		187
Standard & Poor's, B- Rating [Member] \| Corporate Debt Securities [Member]
Investment [Line Items]
Investment Owned, at Fair Value		535
Standard & Poor's, B- Rating [Member] \| Mortgage And Other Asset Backed Securities [Member]
Investment [Line Items]
Investment Owned, at Fair Value		442
Ambac Assurance Corporation [Member] \| Standard & Poor's, B Rating [Member]
Investment [Line Items]
Investment Owned, at Fair Value	1,072	1,153
Ambac Assurance Corporation [Member] \| Standard & Poor's, B Rating [Member] \| Municipal Bonds [Member]
Investment [Line Items]
Investment Owned, at Fair Value	181	176
Ambac Assurance Corporation [Member] \| Standard & Poor's, B Rating [Member] \| Corporate Debt Securities [Member]
Investment [Line Items]
Investment Owned, at Fair Value	495	535
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	396	442
National Public Finance Guarantee Corporation [Member] \| Standard & Poor's, BBB- Rating [Member]
Investment [Line Items]
Investment Owned, at Fair Value	9	11
National Public Finance Guarantee Corporation [Member] \| Standard & Poor's, BBB- Rating [Member] \| Municipal Bonds [Member]
Investment [Line Items]
Investment Owned, at Fair Value	9	11
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.

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- Definition

Fair value of securities deposited with governmental authorities.

+ References

No definition available.

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- Definition

Fair Value of Securities Deposited in Connection wtih Letter of Credit

+ References

No definition available.

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- Definition

Market Value of Secured Note Collateral

+ References

No definition available.

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- Definition

Unsettled Amounts Secured Notes from LSNI

+ References

No definition available.

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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
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Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
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Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
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-Topic 946
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Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef
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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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