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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
Sep. 30, 2023
Dec. 31, 2022
Debt Securities, Available-for-sale [Line Items]    
Debt Securities, Available-for-sale, Amortized Cost $ 2,100 $ 2,041
Fixed maturity securities, at fair value 2,004  
Amortized Cost, Due in one year or less 514  
Amortized Cost, Due after one year through five years 562  
Amortized Cost, Due after five years through ten years 292  
Amortized Cost, Due after ten years 65  
Amortized Cost, Total 1,432  
Estimated Fair Value, Due in one year or less 512  
Estimated Fair Value, Due after one year through five years 529  
Estimated Fair Value, Due after five years through ten years 255  
Estimated Fair Value, Due after ten years 54  
Estimated Fair Value due, Total 1,351  
Fixed Income Securities [Member]    
Debt Securities, Available-for-sale [Line Items]    
Debt Securities, Available-for-sale, Amortized Cost 1,653 1,469
Fixed maturity securities, at fair value 1,556 1,395
Fixed Income Securities [Member] | Residential Mortgage-Backed Securities [Member]    
Debt Securities, Available-for-sale [Line Items]    
Debt Securities, Available-for-sale, Amortized Cost 234 230
Fixed maturity securities, at fair value 227 238
Fixed Income Securities [Member] | Commercial Mortgage Backed Securities [Member]    
Debt Securities, Available-for-sale [Line Items]    
Debt Securities, Available-for-sale, Amortized Cost 18  
Fixed maturity securities, at fair value 18  
Fixed Income Securities [Member] | Collateralized Debt Obligations [Member]    
Debt Securities, Available-for-sale [Line Items]    
Debt Securities, Available-for-sale, Amortized Cost 143 141
Fixed maturity securities, at fair value 142 137
Fixed Income Securities [Member] | Asset-backed Securities [Member]    
Debt Securities, Available-for-sale [Line Items]    
Debt Securities, Available-for-sale, Amortized Cost 273 227
Fixed maturity securities, at fair value $ 265 $ 224