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Investments - Summary of Amortized Cost and Estimated Fair Value of Available-for-Sale Investments, Excluding VIE Investments Held by Successor Ambac, by Contractual Maturity (Details) - USD ($)
$ in Millions
Mar. 31, 2024
Dec. 31, 2023
Debt Securities, Available-for-sale [Line Items]    
Debt Securities, Available-for-sale, Amortized Cost $ 2,135 $ 2,197
Fixed maturity securities, at fair value 2,095  
Amortized Cost, Due in one year or less 515  
Amortized Cost, Due after one year through five years 538  
Amortized Cost, Due after five years through ten years 308  
Amortized Cost, Due after ten years 134  
Amortized Cost, Total 1,494  
Estimated Fair Value, Due in one year or less 513  
Estimated Fair Value, Due after one year through five years 517  
Estimated Fair Value, Due after five years through ten years 284  
Estimated Fair Value, Due after ten years 128  
Estimated Fair Value due, Total 1,441  
Fixed Income Securities [Member]    
Debt Securities, Available-for-sale [Line Items]    
Debt Securities, Available-for-sale, Amortized Cost 1,726 1,744
Fixed maturity securities, at fair value 1,687 1,710
Fixed Income Securities [Member] | Residential Mortgage-Backed Securities [Member]    
Debt Securities, Available-for-sale [Line Items]    
Debt Securities, Available-for-sale, Amortized Cost 239 239
Fixed maturity securities, at fair value 253 250
Fixed Income Securities [Member] | Commercial Mortgage Backed Securities [Member]    
Debt Securities, Available-for-sale [Line Items]    
Debt Securities, Available-for-sale, Amortized Cost 27  
Fixed maturity securities, at fair value 27  
Fixed Income Securities [Member] | Collateralized Debt Obligations [Member]    
Debt Securities, Available-for-sale [Line Items]    
Debt Securities, Available-for-sale, Amortized Cost 143 139
Fixed maturity securities, at fair value 143 139
Fixed Income Securities [Member] | Asset-backed Securities [Member]    
Debt Securities, Available-for-sale [Line Items]    
Debt Securities, Available-for-sale, Amortized Cost 232 301
Fixed maturity securities, at fair value $ 231 $ 303