Fair Value Measurements - Additional Information (Details)	3 Months Ended
Jun. 30, 2021 USD ($) level3FinancialInstruments	Dec. 31, 2020 USD ($)
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Derivative Credit Risk Valuation Adjustment, Derivative Liabilities	$ 283,000	$ 19,000
Derivative liabilities	$ 99,000,000	$ 114,000,000
Weighted average discounted rate of estimated future premium payments to be paid by the VIEs	2.20%	2.20%
Number of Credit Risk Derivatives Held \| level3FinancialInstruments	1
Fair Value Measured at Net Asset Value Per Share [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Other investments	$ 540,000,000	$ 453,000,000
Fixed Income Securities [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Percentage of investment portfolio valued using dealer quotes	2.00%	2.00%
Minimum [Member] \| Fixed Income Securities [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Percentage of investment portfolio valued using internal valuation models	3.00%	3.00%
Maximum [Member] \| Fixed Income Securities [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Percentage of investment portfolio valued using external pricing services	95.00%	95.00%
Ambac Assurance Corporation [Member] \| 5.1% Junior Surplus Notes, General Account Due 2020
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Note Holders	1
Ambac Assurance Corporation [Member] \| 5.1% Junior Surplus Notes, General Account Due 2020 \| Corolla Notes
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Note Holders	1
Ambac Assurance Corporation [Member] \| 5.1 Junior Surplus Notes General Account Due 2020 \| Corolla Notes
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Note Holders	1
Variable Interest Entity, Primary Beneficiary [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Derivative liabilities	$ 1,800,000,000	$ 1,835,000,000
Weighted average discounted rate of estimated future premium payments to be paid by the VIEs	2.60%	2.40%

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Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]

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Investment portfolio valued using dealer quotes.

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Percentage of investment portfolio valued using external pricing services.

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Percentage of investment portfolio valued using internal valuation models.

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Amount of credit risk valuation adjustment to derivative liabilities to properly reflect the credit quality of the entity.

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Fair value, before effects of master netting arrangements, of a financial liability or contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes liabilities elected not to be offset. Excludes liabilities not subject to a master netting arrangement.

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Number of credit risk derivative instruments held.

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Amount of investments classified as other.

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